Rocks And Minerals

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Geology Rocks and minerals

Pictures of Igneous Rocks Photos of Common Intrusive and Extrusive Igneous Rock Types

Andesite is a fine-grained, extrusive igneous rock composed mainly of plagioclase with other minerals such as hornblende, pyroxene, and biotite. The specimen shown is about two inches (five centimeters) across.

What are Igneous Rocks? Igneous rocks are formed from the solidification of molten rock material. There are two basic types. Intrusive igneous rocks crystallize below Earth's surface, and the slow cooling (Refroidissement) that occurs there allows (Qui se produit là permet) large crystals to form. Examples of intrusive igneous rocks are diorite, gabbro, granite, pegmatite, and peridotite. Extrusive igneous rocks erupt onto (sur) the surface, where they cool quickly (Où ils se refroidissent rapidement) to form small crystals. Some cool so quickly that they form an amorphous glass. These rocks include andesite, basalt, obsidian, pumice, rhyolite, scoria, and tuff.

Diorite is a coarse-grained, intrusive igneous rock that contains a mixture of feldspar, pyroxene, hornblende, and sometimes quartz. The specimen shown above is about two inches (five centimeters) across.

Basalt is a fine-grained, dark-colored extrusive igneous rock composed mainly of plagioclase and pyroxene. The specimen shown is about two inches (five centimeters) across.

Gabbro is a coarse-grained (grain grossier), dark-colored, intrusive igneous rock that contains feldspar, pyroxene, and sometimes olivine. The specimen shown above is about two inches (five centimeters) across. (à peu près)

Obsidian is a dark-colored volcanic glass that forms from the very rapid cooling of molten rock material. It cools so rapidly that crystals do not form. The specimen shown above is about two inches (five centimeters) across.

Granite is a coarse-grained, light-colored, intrusive igneous rock that contains mainly quartz, feldspar, and mica minerals. The specimen above is about two inches (five centimeters) across.

Peridotite is a coarse-grained intrusive igneous rock that is composed almost entirely of olivine. It may contain small amounts of amphibole, feldspar, quartz, or pyroxene. The specimen shown above is about two inches (five centimeters) across.

Pegmatite is a light-colored, extremely coarse-grained intrusive igneous rock. It forms near the margins of a magma chamber during the final phases of magma chamber crystallization. It often contains rare minerals that are not found in other parts of the magma chamber. The specimen shown above is about two inches (five centimeters) across.

Rhyolite is a light-colored, fine-grained, extrusive igneous rock that typically contains quartz and feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

Pumice is a light-colored vesicular igneous rock. It forms through very rapid solidification of a melt. The vesicular texture is a result of gas trapped in the melt at the time of solidification. The specimen shown above is about two inches (five centimeters) across.

Scoria is a dark-colored, vesicular, extrusive igneous rock. The vesicles are a result of trapped gas within the melt (dans la partie fusionnée) at the time of solidification. It often forms as a frothy crust on the top of a lava flow or as material ejected from a volcanic vent and solidifying while airborne. The specimen shown above is about two inches (five centimeters) across.

Fire Opal is sometimes found filling cavities in rhyolite. Long after the rhyolite has cooled, silica-rich ground water moves through the rock, sometimes depositing gems like opal, red beryl, topaz, jasper, or agate in the cavities of the rock. This is one of many excellent geological photographs generously shared through a Creative Commons License by Didier Descouens.

Welded Tuff is a rock that is composed of materials that were ejected from a volcano, fell to Earth, and then lithified into a rock. It is usually composed mainly of volcanic ash and sometimes contains larger size particles such as cinders. The specimen shown above is about two inches (five centimeters) across.

Andesite

Andesite: The specimen shown is about two inches (five centimeters) across and has a porphyritic texture.

Igneous rock composition chart: This chart shows that andesite is typically composed of plagioclase, amphiboles, and micas; sometimes with minor amounts of pyroxenes, quartz, or orthoclase.

What is Andesite? Andesite is the name used for a family of fine-grained, extrusive igneous rocks that are usually light to dark gray in color. They often weather to various shades of brown, and these specimens must be broken for proper examination. Andesite is rich in plagioclase feldspar minerals and may contain biotite, pyroxene, or amphibole. Andesite usually does not contain quartz or olivine. Andesite is typically found in lava flows produced by stratovolcanoes. Because these lavas cooled rapidly at the surface, they are generally composed of small crystals. The mineral grains are usually so small that they cannot be seen without the use of a magnifying device (Dispositif de grossissement). Some specimens that cooled rapidly contain a significant amount of glass, while others that formed from gas-charged lavas have a vesicular or amygdaloidal texture.

Stratovolcanoes: Pavlof Volcano (right) and Pavlof Sister Volcano (left) are a pair of symmetrical stratovolcanoes built of andesite flows and tephra on the Alaska Peninsula. Pavlof Volcano is one of the most active volcanoes in Alaska. Photo by T. Miller, United States Geological Survey.

Where Does Andesite Form? Andesite and diorite are common rocks of the continental crust above subduction zones. They generally form after an oceanic plate melts (plaque océanique fond) during its descent into the subduction zone to produce a source of magma. Diorite is a coarse-grained igneous rock that forms when the magma did not erupt, but instead slowly crystallized within Earth's crust (Mais lentement cristallisé dans la croute terrestre). Andesite is a fine-grained rock that formed when the magma erupted onto (sur) the surface and crystallized quickly. Andesite and diorite have a composition that is intermediate between basalt and granite. This is because their parent magmas formed from the partial melting (fusion partielle) of a basaltic oceanic plate. This magma may have received a granitic contribution by melting granitic rocks as it ascended or mixed with granitic magma. Andesite derives its name from the Andes Mountains of South America. In the Andes it occurs (il arrive) as lava flows interbedded with ash (cendre) and tuff deposits on the steep flanks (Flancs escarpés) of stratovolcanoes. Andesite stratovolcanoes are found above subduction zones in Central America, Mexico, Washington, Oregon, the Aleutian Arc, Japan, Indonesia, the Philippines, the Caribbean, and New Zealand, among other locations. Andesite can also form away from the subduction zone environment. For example, it can form at ocean ridges (Crètes océaniques) and oceanic hot spots from partial melting of basaltic rocks. It can also form during eruptions at continental plate interiors where deep-source magma melts continental crust or mixes with continental magmas. There are many other environments where andesite might form.

Pavlof Volcano - plate tectonics: Simplified plate tectonics cross-section showing how Pavlof Volcano is located above a subduction zone where basaltic crust of the Pacific Plate is being partially melted at depth. The ascending magma then passes through continental crust, where it might mix with other magmas or be altered by melting rocks of different composition.

Andesite Porphyry Occasionally, andesites contain large, visible grains of plagioclase, amphibole, or pyroxene. These large crystals are known as "phenocrysts." They begin forming when a magma, which is cooling at depth, approaches the crystallization temperature of some of its minerals. These high-crystallization-temperature minerals begin forming below the surface and grow to visible sizes before the magma erupts. When the magma erupts onto the Earth's surface, the rest of the melt crystallizes quickly. This produces a rock with two different crystal sizes: large crystals that formed slowly at depth (known as "phenocrysts"), and small crystals that formed quickly at the surface (known as "groundmass"). "Andesite porphyry" is the name used for these rocks with two crystal sizes.

Andesite outcrop: Close view of an andesite lava flow at Brokeoff Volcano in California. Photo by the United States Geological Survey.

Hornblende Andesite Porphyry: A specimen of andesite with large visible hornblende phenocrysts. This type of rock could be called an "andesite porphyry" because of its texture. It could also be called a "hornblende andesite" because of its composition. Photo by NASA.

Dissolved Gas and Explosive Eruptions Some magmas that produce volcanic eruptions above (au-dessus) subduction zones contain enormous amounts of dissolved gas. These magmas can contain several percent dissolved gas by weight. This gas can have several origins, examples of which include the following:    

Water vapor produced when ocean-floor sediments on an oceanic plate are heated in a subduction zone. Water vapor produced when hydrous minerals dehydrate in the heat of a subduction zone. Carbon dioxide produced when rising magma encounters carbonate rocks, such as limestone, marble, or dolomite. Water vapor produced when a rising magma chamber encounters groundwater.

At depth, these gases can be dissolved in the magma like carbon dioxide dissolved in a can of cold beer. If that can of beer is shaken and suddenly depressurized by opening the can, the gas and the beer will erupt from the opening. A volcano behaves in a similar manner. A rising magma chamber instantly depressurized by a landslide, faulting, or other event can produce a similar but much larger explosive eruption. Many volcanic plumes and ash eruptions occur when gas-charged andesitic magmas erupt. The gas pressure that causes the eruption blows large amounts of tiny rock and magma particles into the atmosphere. These particles can be blown high into the atmosphere and carried long distances by the wind. They often cause problems for aircraft operating downwind from the volcano. Catastrophic eruptions like Mount St. Helens, Pinatubo, Redoubt, and Novarupta were produced by andesitic magmas with enormous amounts of dissolved gas under high pressure. It is difficult to imagine how a magma can contain enough dissolved gas to produce one of these eruptions.

Andesite Flow: One of numerous massive andesite flows from the Zarembo Island area of southeastern Alaska. They are gray pyroxene and feldspar porphyrys that weather to maroon or green. Photo by USGS.

The Elusive Definition of Andesite The formal definition of andesite is problematic. Many authors have classified igneous rocks based upon their chemical and mineralogical compositions. However, none of these classifications are in perfect agreement. For a fine-grained rock like andesite, these classifications are impossible to use precisely when in the field or the classroom. They require chemical or mineralogical analyses that are usually not available, affordable, or practical. If you examine a rock that appears to be andesite, but you are not confident that it meets the mineralogical or chemical classification of andesite, you can properly call it an "andesitoid" rock. That means that while the rock looks like andesite, a microscopic examination or chemical testing might prove you wrong! Contributor: Hobart King

Diorite

Diorite: This specimen clearly shows the familiar "salt and pepper" appearance of diorite, produced by white plagioclase contrasting with black hornblende and biotite. This specimen is about two inches across.

What is Diorite? Diorite is the name used for a group of coarse-grained igneous rocks with a composition between that of graniteand basalt. It usually occurs as large intrusions, dikes, and sills within continental crust. These often form above a convergent plate boundary where an oceanic plate subducts beneath a continental plate. Partial melting of the oceanic plate produces a basaltic magma that rises and intrudes the granitic rock of the continental plate. There, the basaltic magma mixes with granitic magmas or melts granitic rock as it ascends through the continental plate. This produces a melt that is intermediate in composition between basalt and granite. Diorite forms if this type of melt crystallizes below the surface. Diorite is usually composed of sodium-rich plagioclasewith lesser amounts of hornblende and biotite. It usually contains little if any quartz. This makes diorite a coarse-grained rock with a contrasting mix of black and white mineral grains. Students often use this "salt and pepper" appearance as a clue to the identification of diorite.

Igneous rock compositions: This chart illustrates the generalized mineral composition of igneous rocks. It shows that diorites and andesites are composed mainly of plagioclase feldspar, amphiboles, and micas; sometimes with minor amounts of orthoclase, quartz, or pyroxene.

Diorite and Andesite Diorite and andesite are similar rocks. They have the same mineral composition and occur in the same geographic areas. The differences are in their grain sizes and their rates of cooling. Diorite crystallized slowly within the Earth. That slow cooling produced a coarse grain size. Andesite forms when a similar magma crystallizes quickly at Earth's surface. That rapid cooling produces a rock with small crystals.

Polished diorite: This photo shows a sample of diorite as it might appear in a polished countertop, facing stone, or floor tile. It would probably be marketed as "white granite" at a cabinet shop or building supply store. Photo © iStockphoto / jskiba.

Diorite Ax: Photographs of a Neolithic ax made of diorite that was found in the surroundings of Reims, France. It is in the Alexis Damour Collection at the Museum of Toulouse. Creative Commons photographs by Didier Descouens.

Uses of Diorite In areas where diorite occurs near the surface, it is sometimes mined for use as a crushed stone. It has a durability that compares favorably to granite and trap rock. It is used as a base material in the construction of roads, buildings, and parking areas. It is also used as a drainage stone and for erosion control. In the dimension stone industry, diorite is often cut into facing stone, tile, ashlars, blocking, pavers, curbing, and a variety of dimension stone products. These are used as construction stone, or polished and used as architectural stone. Diorite was used as a structural stone by the Inca and Mayan civilizations of South America and by many ancient civilizations in the Middle East. In the dimension stone industry, diorite is sold as a "granite." The dimension stone industry uses the name "granite" for any rock with visible, interlocking grains of feldspar. This simplifies discussions with customers who do not know how to identify igneous and metamorphic rocks.

Diorite Sculptures: The sculpture on the left is a diorite statue of Gudea, a Mesopotamian ruler, made in about 2090 BC. It is about 19 inches tall and is currently displayed at the Metropolitan Museum of Art. A public domain image. The vase on the right was made in ancient Egypt from diorite with spectacular feldspar phenocrysts. It is in the collection of the Field Museum. A GNU Free Documentation image by Madman2001.

Diorite in Art Diorite is difficult to sculpt because of its hardness, variable composition, and coarse grain size. For those reasons, it is not a favored stone of sculptors, although it was popular among ancient sculptors of the Middle East. The most famous diorite sculpture is the Code of Hammurabi, a black diorite pillar about seven feet tall, inscribed with Babylonian laws in about 1750 BC. Diorite has the ability to accept a bright polish, and it has occasionally been cut into cabochons or used as a gemstone. In Australia, a diorite with beautiful pink feldspar phenocrysts has been cut into cabochons and called "pink marshmallow stone."

Diorite Cabochon: A diorite in Australia contains large, beautiful pink feldspar crystals. It is often cut into cabochons for use as a novelty gem. It has been given the name "pink marshmallow stone."

Contributor: Hobart King

Basalt What Is Basalt, How Does It Form, and How Is It Used?

Basalt: A fine-grained igneous rock that is usually black in color. The specimen shown is about two inches (five centimeters) across.

What is Basalt? Basalt is a dark-colored, fine-grained, igneous rockcomposed mainly of plagioclase and pyroxene minerals. It most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt is a fine-grained rock while gabbro is a coarse-grained rock.

Olympus Mons Volcano: This shield volcano is composed of basalt and has enormous calderas at the summit. Olympus Mons is the highest topographic feature on Mars and is the largest known volcano in our solar system. It is about 375 miles (600 kilometers) in diameter and 15 miles (25 kilometers) high. NASA Mars Orbiter Camera image.

Earth's Most Abundant Bedrock Basalt underlies more of Earth's surface than any other rock type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents, lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock.

Basalt on Moon and Mars Basalt is also an abundant rock on the Moon. Much of the Moon's surface is underlain by basaltic lava flows and flood basalts. These areas of the Moon are known as "lunar maria." Large areas of the Moon have been resurfaced by extensive basalt flows which may have been triggered by major impact events. The ages of lunar maria can be estimated by observing the density of impact craters on their surface. Younger basalt flows will have fewer craters. Olympus Mons is a shield volcano on Mars. It, like most other volcanic features on Mars, was formed from basaltic lava flows. It is the highest mountain on Mars and is the largest known volcano in our solar system.

Basalt-Forming Environments: This map shows the location of oceanic divergent boundaries and hotspots. These are locations where large volumes of basalt have been formed. Map copyright by Geology.com and MapResources. Locations generalized after United States Geological Survey, Geologic Investigations Map I-2800: This Dynamic Planet.

Igneous rock composition chart: This chart shows that basalt is typically composed of pyroxenes, plagioclase, micas, and amphiboles.

Basalt-Forming Environments Most of the basalt found on Earth was produced in just three rock-forming environments: 1) oceanic divergent boundaries, 2) oceanic hotspots, and 3) mantle plumes and hotspots beneath continents. The images on this page feature some of these basalt-forming environments.

Sea floor pillow basalts on the Juan de Fuca Ridge, a divergent plate boundary located about 150 miles (240 kilometers) west of the Washington-Oregon coast. This lava flow, produced by a fissure eruption, was about five years old when the photograph was taken. NOAA Ocean Explorer image.

Hawaii Basalt Flows: Lava flows dump into the Pacific Ocean on the coast of Hawaii. Multiple locations where hot lava streams into the ocean can be seen in this image along with a red-hot lava flow traversing the lava field. This photo shows the enormous extent of the flows. They extend from the shoreline up to the horizon. A volcanic plume from the Pu`u `O`o vent can be seen over the horizon near the center of the image. The lava in these flows originated from the Pu`u `O`o vent. USGS image.

Basalts at Oceanic Divergent Boundaries Most of Earth's basalt is produced at divergent plate boundaries on the mid-ocean ridge system (see map). Here convection currents deliver hot rock from deep in the mantle. This hot rock melts as the divergent boundary pulls apart, and the molten rock erupts onto the sea floor. These submarine fissure eruptions often produce pillow basalts as shown in the image on this page.

The active mid-ocean ridges host repeated fissure eruptions. Most of this activity is unnoticed because these boundaries are under great depths of water. At these deep locations, any steam, ash, or gas produced is absorbed by the water column and does not reach the surface. Earthquake activity is the only signal to humans that many of these deep ocean ridge eruptions provide. However, Iceland is a location where a mid-ocean ridge has been lifted above sea level. There, people can directly observe this volcanic activity.

Thermal image of a hot basalt flow on the flank of Hawaii's Kilauea volcano. Hot lava at the front of the flow is revealed in yellow, orange and red colors. The channel that it flowed through on the previous day appears as a purple and blue track. United States Geological Survey image.

Oceanic Hotspots Another location where significant amounts of basalt are produced is above oceanic hotspots. These are locations (see map above) where a small plume of hot rock rises up through the mantle from a hotspot on Earth's core. The Hawaiian Islands are an example of where basaltic volcanoes have been built above an oceanic hotspot. Basalt production at these locations begins with an eruption on the ocean floor. If the hotspot is sustained, repeated eruptions can build the volcanic cone larger and larger until it becomes high enough to become an island. All of the islands in the Hawaiian Island chain were built up from basalt eruptions on the sea floor. The island that we know today as "Hawaii" is thought to be between 300,000 and 600,000 years old. It began as an eruption on the floor of the Pacific Ocean. The volcanic cone grew as recurrent eruptions built up layer after layer of basalt flows. About 100,000 years ago it is thought to have grown tall enough to emerge from the ocean as an island. Today it consists of five overlapping volcanoes. Kilauea is the most active of these volcanoes. It has been in amost continuous eruption since January, 1983. Basalt flows from Kilauea have extruded over one cubic mile of lava, which currently covers about 48 square miles of land. These flows have travelled over seven miles to reach the ocean, covering highways, homes and entire subdivisions that were in their path.

Columbia River Flood Basalts: The Columbia River Flood Basalts are an extensive sequence of stacked lava flows that reach a cumulative thickness of up to 6000 feet. The outcrops in the foreground and in the distance of this photo are all made up of layered basalt flows. Although basalt is typically a dark black rock, it often weathers to a yellow-brown color similar to the rocks shown here. Public domain image by Williamborg.

Columbia River Flood Basalts Map: A map of the area underlain by the Columbia River Flood Basalts in Washington, Oregon, and Idaho. The area shown is what has not yet been eroded away - the original extent of these basalt flows was much greater. Over 300 individual flows have been identified, and several hundred meters of basalt underlies much of the area shown in the map above. Map © by Geology.com and MapResources.com.

Plumes & Hotspots Below Continents The third basalt-forming environment is a continental environment where a mantle plume or hotspot delivers enormous amounts of basaltic lava through the continental crust and up to Earth's surface. These eruptions can be from either vents or fissures. They have produced the largest basalt flows on the continents. The eruptions can occur repeatedly over millions of years, producing layer after layer of basalt stacked in a vertical sequence (see outcrop photo). The Columbia River Flood Basalts in Washington, Oregon, and Idaho are an example of extensive flood basalts on land (see map below). Other examples include the Emeishan Traps of China, the Deccan Traps of India, the Keweenawan Lavas of the Lake Superior region, the Etendeka Basalts of Namibia, the Karroo

Basalts of South Africa, and the Siberian Traps of Russia. (The word "traps" is derived from the Swedish word for "stairs," which describes the outcrop profile of these layered basalt deposits, as shown in the outcrop photo.)

The Roman theatre: (left) in Bosra, Syria. The dark building stone is basalt. Image © iStockphoto / Steve Estvanik. Basalt paving stones: (right) on a city street in Rome, Italy. Basalt pavers were often used in areas close to volcanoes. Image © iStockphoto / Giovanni Rinaldi.

Uses of Basalt Basalt is used for a wide variety of purposes. It is most commonly crushed for use as an aggregate in construction projects. Crushed basalt is used for road base, concrete aggregate, asphalt pavement aggregate, railroad ballast, filter stone in drain fields, and may other purposes. Basalt is also cut into dimension stone. Thin slabs of basalt are cut and sometimes polished for use as floor tiles, building veneer, monuments, and other stone objects. Contributor: Hobart King

Gabbro What Is Gabbro, What Minerals Are In Gabbro, and What Is It Used For?

Gabbro is a dark-colored coarse-grained intrusive igneous rock. The specimen shown above is about two inches (five centimeters) across.

What is Gabbro? Gabbro is a coarse-grained, dark-colored, intrusive igneous rock. It is usually black or dark green in color and composed mainly of the minerals plagioclase and augite. It is the most abundant rock in the deep oceanic crust. Gabbro has a variety of uses in the construction industry. It is used for everything from crushed stone base materials at construction sites to polished stone counter tops and floor tiles.

Igneous rock composition chart: A chart that illustrates the generalized mineral composition of igneous rocks. By studying this chart, you can see that gabbros and basalts are composed mainly of plagioclase feldspar, micas, amphiboles, and olivine.

What Minerals are in Gabbro?

Gabbro is composed mainly of calcium-rich plagioclasefeldspar (usually labradorite or bytownite) and clinopyroxene (augite). Minor amounts of olivine and orthopyroxene might also be present in the rock. (See composition chart on this page.) This mineral composition usually gives gabbro a black to very dark green color. A minor amount of lightcolored mineral grains may also be present. Unlike many other igneous rocks, gabbro usually contains very little quartz. You can see a close-up view of gabbro toward the bottom of this page.

Gabbro and Basalt are Related Gabbros are equivalent in composition to basalts. The difference between the two rock types is their grain size. Basalts are extrusive igneous rocks that cool quickly and have fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals.

Divergent boundary: In the oceanic crust, basalt forms near the surface at a divergent boundary, but gabbro forms at depth from slow crystallization. Learn about teaching plate tectonics.

Gabbro in Oceanic Crust It is often stated that Earth's oceanic crust is made up of basalt. The word "basalt" is used because the rocks of the oceanic crust have a "basaltic" composition. However, only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust are generally coarser-grained gabbro. Basalt occurs at the surface of the crust because the rocks there have cooled quickly. At greater depth the cooling rate is slower, and large crystals have time to develop. (See illustration.)

Black granite: A view of polished gabbro (labradorite). Polished gabbro is sold under the name "black granite" and is used for cemetery markers, floor tile, kitchen counter tops, facing stone, and other dimension stone uses.

Gabbro in Continental Crust On the continents, gabbro can be found within thick lava flows of basaltic composition, where slow cooling allows large crystals to form. Gabbro will also be present in the deep plutons that form when magma chambers that feed basaltic eruptions crystallize. Large volumes of gabbro are present beneath extensive flood basalts such as the Columbia River flood basalts of Washington and Oregon and the Deccan Traps of India.

Close-up view of gabbro: Magnified view of the gabbro shown in the photograph at the top of the page. The area shown in this image is about 1/2 inch across.

Uses of Gabbro Gabbro can be polished to a brilliant black luster. Brightly polished gabbro is used to make cemetery markers, kitchen counter tops, floor tiles, facing stone, and other dimension stone products. It is a highly desirable rock that stands up to weathering and wear. In the dimension stone industry, gabbro is sold under the name "black granite." Gabbro is also used to make a number of rough-cut products such as curbing, ashlars, paving stones, and other products. The most common use of gabbro is as a crushed stone or aggregate. Crushed gabbro is used as a base material in construction projects, as a crushed stone for road construction, as railroad ballast, and anywhere that a durable crushed stone is needed as fill.

Gabbro as an Ore Gabbro sometimes contains economic amounts of some relatively rare metals. Gabbros containing significant amounts of the mineral ilmenite are mined for their titanium content. Other gabbros are mined to yield nickel, chromium, or platinum. Contributor: Hobart King

Obsidian What is Obsidian, How Does it Form, and What is it Used For?

Obsidian: The specimen shown above is about two inches (five centimeters) across. The curved semi-concentric ridges are breakage marks associated with obsidian's conchoidal fracture. The rock has very sharp edges.

What is Obsidian? Obsidian is an igneous rock that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. It is an amorphous material known as a "mineraloid." The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo).

Where Does Obsidian Form? Obsidian is usually an extrusive rock - one that solidifies above Earth's surface. However, it can form in a variety of cooling environments:     

along the edges of a lava flow (extrusive) along the edges of a volcanic dome (extrusive) around the edges of a sill or a dike (intrusive) where lava contacts water (extrusive) where lava cools while airborne (extrusive)

Types of Obsidian: The specimens shown above are from Glass Butte rockhounding site in central Oregon. It shows the diversity of obsidian types that can be found in a small geographic area. Clockwise from upper left are: double flow obsidian, rainbow obsidian, black obsidian, pumpkin obsidian, mahogany obsidian, gold sheen obsidian, and the piece in the center is gold sheen. The nice photo above is from the Glass Butte Rockhounding Site page on the Deschutes National Forest website.

Mahogany obsidian: A tumble-polished specimen of "mahogany obsidian." Image © iStockphoto / Arpad Benedek.

What Color is Obsidian? Rainbow Obsidian: A baroque cabochon of iridescent "rainbow obsidian."

Black is the most common color of obsidian. However, it can also be brown, tan, or green. Rarely, obsidian can be blue, red, orange, or yellow. The colors are thought to be caused mainly by trace elements or inclusions. Occasionally two colors of obsidian will be swirled together in a single specimen. The most common color combination is black and brown obsidian swirled together - that's called "mahogany obsidian" (see photo).

As a "glass," obsidian is chemically unstable. With the passage of time, some obsidian begins to crystallize. This process does not happen at a uniform rate throughout the rock. Instead it begins at various locations within the rock. At these locations, the crystallization process forms radial clusters of white or gray cristobalite crystals within the obsidian. When cut and polished, these specimens are referred to as "snowflake obsidian" (see photos). Rarely, obsidian has an iridescent or metallic "sheen" caused by light reflecting from minute inclusions of mineral crystals, rock debris, or gas. These colored specimens are known as "rainbow obsidian," "golden obsidian," or "silver obsidian," depending upon the color of the sheen or iridescence. These specimens are very desirable for the manufacture of jewelry.

Snowflake obsidian: A tumble-polished specimen of "snowflake obsidian." Image © iStockphoto / Martin Novak.

What is the Composition of Obsidian? Most obsidians have a composition similar to rhyolite and granite. Granites and rhyolites can form from the same magma as obsidian and are often geographically associated with the obsidian. Rarely, volcanic glasses are found with a composition similar to basalt and gabbro. These glassy rocks are named "tachylyte."

Are There Other Glassy Igneous Rocks? Pumice, scoria, and tachylyte are other volcanic glasses formed by rapid cooling. Pumice and scoria differ from obsidian by having abundant vesicles - cavities in the rock produced when gas bubbles were trapped in a solidifying melt. Tachylyte differs in composition - it has a composition similar to basalt and gabbro.

Obsidian outcrop: Obsidian along the edge of a lava flow in central Oregon. Image © iStockphoto / Phil Augustavo.

Obsidian knife blade: A knife blade manufactured from mahogany obsidian. The craftsman who made this blade had a very high skill level and was able to produce a serrated edge. Image © iStockphoto / Al Braunworth.

Occurrence of Obsidian Obsidian is found in many locations worldwide. It is confined to areas of geologically recent volcanic activity. Obsidian older than a few million years is rare because the glassy rock is rapidly destroyed or altered by weathering, heat, or other processes. Significant deposits of obsidian are found in Argentina, Canada, Chile, Ecuador, Greece, Guatemala, Hungary, Iceland, Indonesia, Italy, Japan, Kenya, Mexico, New Zealand, Peru, Russia, United States, and many other locations. In the United States it is not found east of the Mississippi River, as there is no geologically recent volcanic activity there. In the western US it is found at many locations in Arizona, California, Idaho, Nevada, New Mexico, Oregon, Washington, and Wyoming. Most obsidian used in the jewelry trade is produced in the United States.

Obsidian spear point: A spear point fashioned from opaque black obsidian. Image © iStockphoto / Charles Butzin.

Uses of Obsidian as a Cutting Tool The conchoidal fracture of obsidian causes it to break into pieces with curved surfaces. This type of fracturing can produce rock fragments with very sharp edges. These sharp fragments may have prompted the first use of obsidian by people. The first use of obsidian by people probably occurred when a sharp piece of obsidian was used as a cutting tool. People then discovered how to skillfully break the obsidian to produce cutting tools in a variety of shapes. Obsidian was used to make knives, arrowheads, spear points, scrapers, and many other weapons and tools. Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharp object. The easy-to-recognize rock became one of the first targets of organized "mining." It

is probably a safe bet that all natural obsidian outcrops that are known today were discovered and utilized by ancient people.

Apache tears: "Apache Tears" is a name used for small obsidian nodules of about one inch or less that can be found in volcanic areas of the southwestern United States. Their name comes from a Native American legend. During a battle between Apaches and the U.S. Cavalry in 1870, the outnumbered Apaches, facing defeat, rode their horses over a cliff rather than allow themselves to be killed by their enemy. Upon hearing the story of the battle, the tears of their family members turned to stone when they hit the ground. Those stones are now found as the black obsidian nodules. People who do rock tumbling often polish Apache Tears. They are difficult to polish because the obsidian chips and bruises easily. Success occurs when they are cushioned during the tumbling with smaller pieces of rough or small ceramic media.

Stone Age Manufacturing and Trade The manufacture of obsidian tools by humans dates back to the Stone Age. At some locations, tons of obsidian flakes reveal the presence of ancient "factories." Some of these sites have enough waste debris to suggest that many people labored there for decades producing a variety of obsidian objects. Making arrowheads, spear points, knife blades, and scrapers from obsidian, chert, or flint might have been the world's first "manufacturing industry." Obsidian was so valued for these uses that ancient people mined, transported, and traded obsidian and obsidian objects over distances of up to a thousand miles. Archaeologists have been able to document the geography of this trade by matching the characteristics of obsidian in outcrops with the characteristics of obsidian in cutting tools. A study done by the Idaho National Laboratory used composition studies by X-ray fluorescence to identify the source outcrops of obsidian artifacts and map their use across the western United States.

Obsidian in Modern Surgery Although using a rock as a cutting tool might sound like "stone age equipment," obsidian continues to play an important role in modern surgery. Obsidian can be used to produce a cutting edge that is thinner and sharper than the best surgical steel. Today, thin blades of obsidian are placed in surgical scalpels used for some of the most precise surgery. In controlled studies, the performance of obsidian blades was equal to or superior to the performance of surgical steel.

Obsidian jewelry: Mahogany obsidian and snowflake obsidian cabochons set in sterling silver pendants.

Obsidian for opal triplets: A thin piece of obsidian is often used as a "backing" material for opal doublets and triplets. The black obsidian adds stability to the opal and provides a dark background color that contrasts with the opal's fire.

Uses of Obsidian in Jewelry Obsidian is a popular gemstone. It is often cut into beads and cabochons or used to manufacture tumbled stones. Obsidian is sometimes faceted and polished into highly reflective beads. Some transparent specimens are faceted to produce interesting gems. The use of obsidian in jewelry can be limited by its durability. It has a hardness of about 5.5 which makes it easy to scratch. It also lacks toughness and is easily broken or chipped upon impact. These durability concerns make obsidian an inappropriate stone for rings and bracelets. It is best suited for use in low-impact pieces such as earrings, brooches, and pendants. Obsidian is also used in making opal doublets and opal triplets. Thin slices or chips of opal are glued to a thin slice of obsidian to make a composite stone. The black obsidian provides an inexpensive and colorcontrasting background that makes opal's colorful fire much more obvious. It also adds mass and stability to the opal that facilitates cutting it into a gem.

Other Uses of Obsidian Freshly broken pieces of obsidian have a very high luster. Ancient people noticed that they could see a reflection in obsidian and used it as a mirror. Later, pieces of obsidian were ground flat and highly polished to improve their reflective abilities. Obsidian's hardness of 5.5 makes it relatively easy to carve. Artists have used obsidian to make masks, small sculptures, and figurines for thousands of years. Contributor: Hobart King

Granite What is Granite? What is Granite Used For?

Granite: The specimen above is a typical granite. It is about two inches across. The grain size is coarse enough to allow recognition of the major minerals. The pink grains are orthoclase feldspar, and the clear to smoky grains are quartz or muscovite. The black grains can be biotite or hornblende. Numerous other minerals can be present in granite.

What is Granite? Granite is a light-colored igneous rock with grains large enough to be visible with the unaided eye. It forms from the slow crystallization of magma below Earth's surface. Granite is composed mainly of quartz and feldspar with minor amounts of mica, amphiboles, and other minerals. This mineral composition usually gives granite a red, pink, gray, or white color with dark mineral grains visible throughout the rock.

Granite in Yosemite Valley: Photograph of Yosemite Valley, California, showing the steep granite cliffs that form the walls of the valley. © iStockphoto / photo75.

The Best-Known Igneous Rock Granite is the best-known igneous rock. Many people recognize granite because it is the most common igneous rock found at Earth's surface and because granite is used to make many objects that we encounter in daily life. These include counter tops, floor tiles, paving stone, curbing, stair treads, building veneer, and cemetery monuments. Granite is used all around us - especially if you live in a city. Granite is also well-known from its many world-famous natural exposures. These include: Stone Mountain, Georgia; Yosemite Valley, California; Mount Rushmore, South Dakota; Pike's Peak, Colorado; and White Mountains, New Hampshire.

Granite: Photograph of a white, fine-grained granite. This specimen is about two inches across.

Multiple Definitions of Granite The word "granite" is used in a variety of ways by different people. A simple definition is used in introductory courses; a more precise definition is used by petrologists (geologists who specialize in the study of rocks); and, the definition of granite expands wildly when used by people who sell decorative stone such as countertops, tile, and building veneer. These multiple definitions of granite can lead to communication problems. However, if you know who is using the word and who they are communicating with, you can interpret the word in its proper context. Three common usages of the word "granite" are explained below.

Granite close up: Magnified view of the white, fine-grained granite from the photograph above. The area shown in this image is about 1/4 inch across.

A) Introductory Course Definition

Granite is a coarse-grained, light-colored igneous rock composed mainly of feldspars and quartz with minor amounts of mica and amphibole minerals. This simple definition enables students to easily identify the rock based upon a visual inspection. B) Petrologist's Definition

Granite is a plutonic rock in which quartz makes up between 10 and 50 percent of the felsic components and alkali feldspar accounts for 65 to 90 percent of the total feldspar content. Applying this definition requires the mineral identification and quantification abilities of a competent geologist. Many rocks identified as "granite" using the introductory course definition will not be called "granite" by the petrologist - they might instead be alkali granites, granodiorites, pegmatites, or aplites. A petrologist might call these "granitoid rocks" rather than granites. There are other definitions of granite based upon mineral composition. The chart below illustrates the range of granite compositions. From the chart you can see that orthoclase feldspar, quartz, plagioclase feldspar, micas, and amphiboles can each have a range of abundances.

Pegmatite: Photograph of a granite with very large crystals of orthoclase feldspar. Granites with such large crystals are known as "pegmatites." This rock is about two inches across.

Granite composition chart: This chart illustrates the generalized mineral composition of igneous rocks. Granites and rhyolites (compositionally equivalent to granite but of a fine grain size) are composed mainly of orthoclase feldspar, quartz, plagioclase feldspar, mica, and amphibole.

"Granite": All of the rocks above would be called "granite" in the commercial stone industry. Clockwise from top left they are: granite, gneiss, pegmatite, and labradorite. Click on any of their names above for an enlarged view. Each of the images above represents a slab of polished rock about eight inches across.

C) Commercial Definition

The word "granite" is used by people who sell and purchase cut stone for structural and decorative use. These "granites" are used to make countertops, floor tiles, curbing, building veneer, monuments, and many other products. In the commercial stone industry, a "granite" is a rock with visible grains that is harder than a marble. Under this definition, gabbro, basalt, pegmatite, schist, gneiss, syenite, monzonite, anorthosite, granodiorite, diabase, diorite, and many other rocks will be called "granite." The collection of images here illustrates the range of rocks that might be called "granite."

Granite counter tops: Granite counter tops in a new kitchen. Image © iStockphoto / Bernardo Grijalva.

Mount Rushmore: Mount Rushmore in the Black Hills, South Dakota is a sculpture of United States presidents George Washington, Thomas Jefferson, Theodore Roosevelt, and Abraham Lincoln sculpted from a granite outcrop. Image © iStockphoto / Jonathan Larsen.

Uses of Granite Granite is the rock most often quarried as a "dimension stone" (a natural rock material that has been cut into blocks or slabs of specific length, width, and thickness). Granite is hard enough to resist most abrasion, strong enough to bear significant weight, inert enough to resist weathering, and it accepts a brilliant polish. These characteristics make it a very desirable and useful dimension stone. Most of the granite dimension stone produced in the United States comes from high-quality deposits in five states: Massachusetts, Georgia, New Hampshire, South Dakota, and Idaho. Granite has been used for thousands of years in both interior and exterior applications. Rough-cut and polished granite is used in buildings, bridges, paving, monuments, and many other exterior projects. Indoors, polished granite slabs and tiles are used in countertops, tile floors, stair treads, and many other practical and decorative features.

Related: Azurite Granite? A Gemstone?

"K2 Granite," also known as "K2 Jasper" An azurite granite found at the base of K2, the world's second-highest mountain.

K2 granite: A piece of dry K2 Granite. A wet surface would increase the intensity of the blue azurite orbs. This piece is approximately 10 centimeters across, and the largest azurite orbs are about 1 centimeter across.

What is K2 Granite? "K2 Granite," also known as "K2 Jasper" and "raindrop azurite," is an extremely interesting rock and lapidary material from the Skardu area of northern Pakistan. It is like an eye magnet for anyone who sees it for the first time. It is a bright white granite that contains sharply contrasting orbs of bright blue azurite. The azurite orbs range from a few millimeters up to about two centimeters in diameter. On a broken surface or on the surface of a slab, the blue orbs look like drops of bright blue ink that splashed onto the rock. Upon closer examination, however, you will see that they are actually spherical in shape. Although K2 Jasper is the most commonly used name for marketing this material, it is definitely not jasper. If you examine the rock with a magnifying glass, you will see cleavage faces of feldspar minerals and black flakes of biotite. The white granite is very fine-grained and composed of quartz, sodium plagioclase, muscovite, and biotite. Some specimens show strong alignment of the biotite grains and could be called "granite gneiss." Examination of the azurite spheres with a good hand lens or microscope reveals that the azurite is present along mineral grain boundaries, within tiny fractures, and as a "dye" penetrating the feldspar grains. The azurite is a secondary material that clearly formed after all of the other minerals in the granite had solidified from the parent melt.

World's second-highest mountain: A view of K2, also known as Mount Godwin Austen, in the morning sun. With a summit elevation of 8,611 meters, K2 is the second-highest mountain in the world after Mount Everest (8,848 meters), and ahead of Kangchenjunga (8,586 meters). Image © iStockphoto and PatrickPoendl.

People Don't Believe It's Azurite Many people see this material at mineral shows or lapidary shows and immediately think that the round blue dots have been produced with a dye. When they ask about the identity of the blue material and learn that it is "azurite," they usually have a hard time believing it because white granite and azurite rarely occur together. For most people, this is the first time that they have seen the two materials in such close association. Some specimens also have small areas that are stained green with malachite. In the close-up photo of K2 granite, you can see dozens of small green malachite stains. If you still doubt the “azurite in granite” identification, you might enjoy visiting a forum at mindat.org. There you will find experienced mineralogists, people from Pakistan who obtain K2 at its source, and lapidarists who cut K2 cabochons, discussing the material and sharing observations, photomicrographs, chemical analyses, and x-ray diffraction data.

Azurite granite with malachite: The photo above features a close-up of a piece of K2 Granite. Dozens of green malachite stains can be seen in this specimen.

Where Is K2 Found? K2 granite is named after a mountain in the Karakoram Range near the border between Pakistan and China. K2, also known as "Mount Godwin Austen," is the second-highest mountain in the world. The azurite granite is found in colluvium near the base of the mountain. It is in a very remote area visited by very few people.

K2 cabochon: An oval cabochon cut from K2 Granite with several bright blue azurite stains. Within each stain you can see the texture of the granite and grains of black biotite. These indicate that the stain formed after the granite solidified from its parent melt. This cabochon is about 20 x 30 millimeters in size.

Lapidary Properties K2 granite cuts, tumbles, and polishes beautifully. Due to its high feldspar content, it can be easily cut with a lapidary saw and shapes quickly on a diamond wheel. Although azurite has a Mohs hardness of 3.5 to 4, the blue dots have the same cutting and polishing properties as the surrounding white granite. This is because the azurite exists as a stain rather than as discrete mineral grains.

K2 shapes and polishes well in a rock tumbler to produce tumbled stones. It also cuts attractive spheres. Cut beads are not seen in the marketplace. This is likely because if you cut ten pounds of K2 into 1-centimeter beads, very few of them will display the blue azurite color. K2 is relatively durable in jewelry. The feldspar minerals in K2 have a hardness of about 6 on the Mohs scale and will be scratched or show signs of wear over time if subjected to abrasion or impact. K2 is therefore not a good stone for mounting in a ring or bracelet. K2 attracts a lot of attention at gem and mineral shows. The rare combination of azurite in granite starts a lot of discussions, and even the occasional argument. Thus far, K2 is not extremely expensive. Great material can be purchased for about $30 to $40 per pound. This price is similar to what is paid for nice specimens of many popular agates and jaspers. The best material has numerous, randomly spaced azurite stains on a bright white granite background. Contributor: Hobart King

High price often reduces the popularity of a construction material, and granite often costs significantly more than man-made materials in most projects. However, granite is frequently selected because it is a prestige material, used in projects to produce impressions of elegance, durability, and lasting quality. Granite is also used as a crushed stone or aggregate. In this form it is used as a base material at construction sites, as an aggregate in road construction, railroad ballast, foundations, and anywhere that a crushed stone is useful as fill.

Granitic rocks: This triangular diagram is a classification method for granitic rocks. It is based upon the relative abundance of feldspars (K-Na-Ca) and quartz. Mafic elements are not considered. It is modified after a classification chart prepared by the International Union of Geological Sciences. Image and modification by the United States Geological Survey.

Granite in the Continental Crust Most introductory geology textbooks report that granite is the most abundant rock in the continental crust. At the surface, granite is exposed in the cores of many mountain ranges within large areas known as "batholiths," and in the core areas of continents known as "shields." The large mineral crystals in granite are evidence that it cooled slowly from molten rock material. That slow cooling had to have occurred beneath Earth's surface and required a long period of time to occur. If they are today exposed at the surface, the only way that could happen is if the granite rocks were uplifted and the overlying sedimentary rockswere eroded. In areas where Earth's surface is covered with sedimentary rocks, granites, metamorphosed granites, or closely related rocks are usually present beneath the sedimentary cover. These deep granites are known as "basement rocks." Contributor: Hobart King

Peridotite A group of ultramafic rocks, including Kimberlite. They sometimes contain chromite or diamonds.

Kimberlite with diamond: Kimberlite, the rock that is found in many diamond pipes, is a variety of peridotite. The specimen above is a piece of kimberlite with numerous visible grains of phlogopite and a six millimeter octahedral diamond crystal of about 1.8 carats. This specimen is from the Finsch Diamond Mine in South Africa. Wikimedia photo by StrangerThanKindness used here under a Creative Commons License.

Types of peridotite: Peridotite is a generic name for a number of different rock types. All of them are rich in olivine and mafic minerals. They are usually green in color and have a high specific gravity for a nonmetallic material. Shown above are specimens of lherzolite, harzburgite, dunite, and wehrlite. Image by USGS.

What is Peridotite? Peridotite is a generic name used for coarse-grained, dark-colored, ultramafic igneous rocks. Peridotites usually contain olivine as their primary mineral, frequently with other mafic minerals such as pyroxenes and amphiboles. Their silica content is low compared to other igneous rocks, and they contain very little quartz and feldspar. Peridotites are economically important rocks because they often contain chromite - the only ore of chromium; they can be source rocks for diamonds; and, they have the potential to be used as a material for sequestering carbon dioxide. Much of Earth's mantle is believed to be composed of peridotite.

Peridotite: The specimen shown is about two inches (five centimeters) across.

Many Types of Peridotite The peridotite “family” contains a number of different intrusive igneous rocks. These include lherzolite, harzburgite, dunite, wehrlite, and kimberlite (see photos). Most of them have an obvious green color, attributed to their olivine content. 

Lherzolite: a peridotite composed primarily of olivine with significant amounts of orthopyroxene and clinopyroxene. Some researchers believe that much of Earth's mantle is composed of lherzolite.



Harzburgite: a peridotite composed primarily of olivine and orthopyroxene with small amounts of spinel and garnet.



Dunite: a peridotite that is composed mainly of olivine and may contain significant amounts of chromite, pyroxene, and spinel.



Wehrlite: a peridotite that is composed mainly of orthopyroxene and clinopyroxene, with olivine and hornblende.



Kimberlite: a peridotite that is composed of at least 35% olivine with significant amounts of other minerals that might include phlogopite, pyroxenes, carbonates, serpentine, diopside, monticellite, and garnet. Kimberlite sometimes contains diamonds.

Alteration of Peridotite Peridotite is a rock type that is more representative of Earth’s mantle than of the crust. The minerals that compose it are generally high-temperature minerals that are unstable at Earth’s surface. They are quickly altered by hydrothermal solutions and weathering. Those that contain magnesium-oxide-bearing minerals can alter to form carbonates, such as magnesite or calcite, which are much more stable at Earth's surface. Alteration of other peridotites forms serpentinite, chlorite, and talc.

Peridotite can sequester gaseous carbon dioxide into a geologically stable solid. This occurs when carbon dioxide combines with magnesium-rich olivine to form magnesite. This reaction happens at a geologically rapid rate. The magnesite is much more stable over time and serves as a carbon dioxide sink. Perhaps this characteristic of peridotite can be used by humans to intentionally sequester carbon dioxide and contribute to solving the climate change problem.

The Tablelands: One of the few extensive surface exposures of peridotite is an area known as "The Tablelands" in Gros Morne National Park, Newfoundland. This area is the mantle portion of a large slab of oceanic lithosphere that was overthrust onto continental lithosphere. These rocks from the mantle lack the nutrients required to support most types of plants, and the soils that form from them are usually barren. The brownish color is from iron staining. Image © iStockphoto / Wildnerdpix.

Peridotite Xenolith: This photograph is of a volcanic bomb that contains a peridotite (dunite) xenolith composed almost entirely of olivine. Photo by Woudloper, used here under a Creative Commons License.

Ophiolites, Pipes, Dikes and Sills Earth's mantle is thought to be composed mainly of peridotite. Some of the occurrences of peridotite on Earth's surface are thought to be rocks from the mantle that have been brought up from depth by deep-source magmas. Ophiolites and pipes are two structures that have brought mantle peridotite to the surface. Peridotite is also found in the igneous rocks of sills and dikes. Ophiolites: An ophiolite is a large slab of oceanic crust, including part of the mantle, that has been overthrust onto continental crust at a convergent plate boundary. These structures bring large masses of peridotite up to Earth's surface and offer a rare opportunity to examine rocks from the mantle. Studies of ophiolites have

helped geologists better understand the mantle, the process of seafloor spreading, and the formation of oceanic lithosphere. Pipes: A pipe is a vertical intrusive structure that forms when a deep-source volcanic eruption brings magma up from the mantle. The magma often breaks through the surface, producing an explosive eruption and a steepwalled crater known as a maar. These deep-source eruptions are the origin for most of the Earth's primary diamond deposits. The magma that forms the pipe is thought to ascend rapidly from the mantle, tearing rocks free from the mantle and from the walls of the pipe. These pieces of foreign rock are known as "xenoliths." The diamonds are found in the xenoliths and in the residual material produced by their weathering. Xenoliths provide the only way that diamonds can ascend from the mantle to the surface without being melted or corroded by the hot magma. Dikes and Sills: Dikes and sills are intrusive igneous rock bodies. Some of them are composed of peridotite that was sourced from deep within the Earth. When they are exposed by erosion, they provide another way that peridotite from great depth can be observed at Earth's surface.

Garnet peridotite: A specimen of garnet peridotite from Alpe Arami, near Bellinzona, Switzerland. Certain types of garnet, along with chromite and ilmenite, can be indicator minerals for diamond prospecting. Public domain image by Woudloper.

Diamonds and Peridotite

How do diamonds form? A detailed article that explains the four sources of diamonds found at Earth's surface.

The formation of diamonds requires very high temperatures and pressures that only occur on Earth at depths of 100 miles below the surface and at locations in the mantle where temperatures are at least 2000 degrees Fahrenheit. The diamonds are delivered to the surface in pieces of rock, known as xenoliths, which are torn from the mantle by deep-source volcanic eruptions. When the mantle material approaches the surface, an explosive eruption occurs that forms a pipe-shaped structure that might be several hundred yards to over a

mile in diameter. These "pipes," the rocks that are blasted from them, and the sediments and soils produced by their weathering are the source for most of Earth's natural diamonds. Peridotite Information [1] Mineral Carbonation Using Ultramafic Rocks, USGS Cooperative Research on CO2 Sequestration Using Ultramafic and Carbonate Rocks, Crustal Geophysics and Geochemistry Science Center, United States Geological Survey, last accessed June 2016. [2] Stratiform Chromite Deposit Model: Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II; Chapter E of Mineral Deposit Models for Resource Assessment; Scientific Investigations Report 2010–5070–E; 131 pages; November 2012. [3] Chromium: John F. Papp, United States Geological Survey, Mineral Commodity Summaries, January 2013. [4] Chromium: John F. Papp, United States Geological Survey, 2011 Minerals Yearbook, April 2013.

Chromite in Peridotite Some peridotites contain significant amounts of chromite. Some of these form when a subsurface magma slowly crystallizes. During the early stages of crystallization, the highest-temperature minerals such as olivine, orthopyroxene, clinopyroxene, and chromite begin to crystallize from the melt. The crystals are heavier than the melt and sink to the bottom of the melt. These high-temperature minerals can form layers of peridotite on the bottom of the magma body. This can form a layered deposit where up to 50% of the rock can be chromite. These are known as "stratiform deposits." Most of the world's chromite is contained in two stratiform deposits: the Bushveld Complex in South Africa and the Great Dyke in Zimbabwe. Another type of chromite deposit occurs where tectonic forces push large masses of oceanic lithosphere up onto a continental plate in a structure that is known as an "ophiolite." These ophiolites contain significant amounts of chromite and are called "podiform deposits."

Aeromagnetic prospecting: Finding small bodies of peridotite such as a kimberlite pipe can be very difficult because they are so small. Aeromagnetic surveys are sometimes employed to find them. The geographic areas underlain by peridotite will often be a magnetic anomaly in contrast to their surrounding rocks. Images by the United States Geological Survey.

Prospecting for Peridotite Peridotite bodies exposed at Earth's surface are rapidly attacked by weathering. They can then be obscured by soil, sediment, glacial till, and vegetation. Finding a peridotite body as small as a kimberlite pipe, which might be only a few hundred yards across, can be very difficult. Because peridotite often has magnetic properties

that are distinctly different from the surrounding rocks, a magnetic survey can sometimes be used to locate them. The survey can be conducted using an aircraft that slowly tows a magnetometer at low altitudes, recording the magnetic intensity as it travels. The magnetic data can be plotted on a map, often revealing the location of the pipe as an anomaly. (See map and photo.) Peridotite bodies are also found by prospecting for some of the rare minerals that they contain. When a peridotite weathers, the olivine breaks down, quickly leaving the more resistant minerals behind. Geologists have located peridotite bodies by prospecting for chromite, garnet, and other resistant indicator minerals. When scattered by the action of water, wind, or ice, they will be most highly concentrated near the pipe and be diluted by local rock debris with distance. The grains of these minerals might also be more rounded with distance of transport. This allows geologists to use the "trail-to-lode" prospecting method to find them. Contributor: Hobart King

Pegmatite An extreme igneous rock with large crystals and rare minerals

Pegmatite: Pegmatite is an igneous rock composed almost entirely of crystals that are over one centimeter in diameter. The specimen shown here is about two inches (five centimeters) across.

Topaz on albite: A crystal of imperial topaz on an albite matrix from a pocket in the Katlang Pegmatite of Pakistan. Specimen is about 4.5 x 3.5 x 3.5 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

What is Pegmatite? Pegmatites are extreme igneous rocks that form during the final stage of a magma’s crystallization. They are extreme because they contain exceptionally large crystals and they sometimes contain minerals that are rarely found in other types of rocks. To be called a “pegmatite,” a rock should be composed almost entirely of crystals that are at least one centimeter in diameter. The name “pegmatite” has nothing to do with the mineral composition of the rock. Most pegmatites have a composition that is similar to granite with abundant quartz, feldspar, and mica. These are sometimes called “granite pegmatites” to indicate their mineralogical composition. However, compositions such as “gabbro pegmatite,” “syenite pegmatite,” and any other plutonic rock name combined with “pegmatite” are possible. Pegmatites are sometimes sources of valuable minerals such as spodumene (an ore of lithium) and beryl (an ore of beryllium) that are rarely found in economic amounts in other types of rocks. They also can be a source of gemstones. Some of the world’s best tourmaline, aquamarine, and topaz deposits have been found in pegmatites.

Giant spodumene crystals: Molds of giant spodumene crystals at the Etta Mines, Black Hills, Pennington County, South Dakota. Note miner at right center for scale. USGS photo.

Himalaya pegmatite: A specimen of the Himalaya Pegmatite of San Diego County, California, that is famous for yielding gemand mineral-specimen-quality tourmaline and other fine crystals. This is a pocket piece with feldspar, smoky quartz, cleavelandite, and a fantastic multicolor tourmaline crystal. Specimen is about 12.7 x 7.7 x 7.5 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

The Rock with Large Crystals Large crystals in igneous rocks are usually attributed to a slow rate of crystallization. However, with pegmatites, large crystals are attributed to low-viscosity fluids that allow ions to be very mobile. During the early states of a magma’s crystallization, the melt usually contains a significant amount of dissolved water and other volatiles such as chlorine, fluorine, and carbon dioxide. Water is not removed from the melt during the early crystallization process, so its concentration in the melt grows as crystallization progresses. Eventually there is an overabundance of water, and pockets of water separate from the melt. These pockets of superheated water are extremely rich in dissolved ions. The ions in the water are much more mobile than ions in the melt. This allows them to move about freely and form crystals rapidly. This is why crystals of a pegmatite grow so large. The extreme conditions of crystallization sometimes produce crystals that are several meters in length and weigh over one ton. For example: a large crystal of spodumene at the Etta Mine in South Dakota was 42 feet long, 5 feet in diameter and yielded 90 tons of spodumene!

Crabtree pegmatite: One of the most interesting pegmatites is the Crabtree Pegmatite of western North Carolina. It is a granitic pegmatite that intrudes the boundary between two rock units in a dike that is up to two meters wide. It was mined for emeralds by a series of owners, which included Tiffany and Company, between 1894 and the 1990s. Many fine clear emeralds were produced, and much of the rock was sold as "emerald matrix" for slabbing and cabochon cutting. This specimen is about 7 x 7 x 7 centimeters in size and contains numerous small emerald crystals that are several millimeters in length.

Activity at the Margins of a Batholith Pegmatites form from waters that separate from a magma in the late stages of crystallization; this activity often occurs in small pockets along the margins of a batholith. Pegmatite can also form in fractures that develop on the margins of the batholith. This is how “pegmatite dikes” are formed. Because these dikes and pockets are small in size, the mining operations that exploit them are also small. The mining of pegmatites might be done in an underground operation that follows a dike or exploits a small pocket. It can also be done at an outcrop where the pegmatite is easily discovered by people. Pegmatites usually do not support large mining operations that employ dozens of workers and have continuous activity of many years.

Polished pegmatite countertop: A portion of a countertop made from polished pegmatite. Large crystals of feldspar, smoky quartz, and hornblende are visible. The view seen here is about 12 inches across.

Rare Minerals in Large Crystals In the early stages of crystallization, the ions that form high-temperature minerals are depleted from the melt. Rare ions that do not participate in the crystallization of common rock-forming minerals become concentrated in the melt and in the excluded water. These ions can form the rare minerals that are often found in pegmatites. Examples are small ions such as lithium and beryllium that form spodumene and beryl; or large ions such as tantalum and niobium that form minerals such as tantalite and niobite. Rare elements concentrated in large crystals make pegmatite a potential source of valuable ore.

Polished pegmatite countertop: A portion of a countertop made from polished pegmatite. Large crystals of feldspar, quartz, and hornblende are visible. The view seen here is about six inches across.

Uses of Pegmatite Pegmatite rock has very few uses. However, pegmatite deposits often contain gemstones, industrial minerals, and rare minerals. ARCHITECTURAL STONE Pegmatite rock has limited use as an architectural stone. Occasionally it is encountered in a dimension stone quarry that produces granite for architectural use. If the pegmatite is sound and attractive, it might be cut into slabs and polished for building facing, countertops, tile or other decorative stone products and sold commercially as a “granite.” GEMSTONE MINING Some of the world’s best gemstone mines are in pegmatites. Gemstones found in pegmatite include: amazonite, apatite, aquamarine, beryl, chrysoberyl, emerald, garnet, kunzite, lepidolite, spodumene, topaz, tourmaline, zircon, and many others. Large crystals of excellent-quality material are often found in pegmatite. RARE MINERALS Pegmatite is the host rock for many rare mineral deposits. These minerals can be commercial sources of: beryllium, bismuth, boron, cesium, lithium, molybdenum, niobium, tantalum, tin, titanium, tungsten, and many other elements. In most cases the mining operations are very small, employing less than a dozen people. If the mine contains nice crystals, the minerals are often more valuable as mineral specimens and faceting rough than being sold as an ore. INDUSTRIAL MINERALS Pegmatite is often mined for industrial minerals. Large sheets of mica are mined from pegmatite. These are used to make components for electronic devices, retardation plates, circuit boards, optical filters, detector windows, and many other products. Feldspar is another mineral frequently mined from pegmatite. It is used as a primary ingredient for making glass and ceramics. It is also used as a filler in many products. Contributor: Hobart King

Rhyolite An extrusive igneous rock with a very high silica content.

Rhyolite: A pink specimen of rhyolite with numerous very tiny vugs with some evidence of flow structures. The specimen shown here is about two inches across.

Igneous rock composition chart: This chart shows that rhyolite is typically composed of orthoclase, quartz, plagioclase, micas, and amphiboles.

What is Rhyolite? Rhyolite is an extrusive igneous rock with a very high silica content. It is usually pink or gray in color with grains so small that they are difficult to observe without a hand lens. Rhyolite is made up of quartz, plagioclase, and sanidine, with minor amounts of hornblende and biotite. Trapped gases often produce vugs in the rock. These often contain crystals, opal, or glassy material. Many rhyolites form from granitic magma that has partially cooled in the subsurface. When these magmas erupt, a rock with two grain sizes can form. The large crystals that formed beneath the surface are called phenocrysts, and the small crystals formed at the surface are called groundmass. Rhyolite usually forms in continental or continent-margin volcanic eruptions where granitic magma reaches the surface. Rhyolite is rarely produced at oceanic eruptions.

Rhyolite Porphyry: Several specimens of rhyolite porphyry, each about three inches across. Click the image to enlarge.

Eruptions of Granitic Magma Eruptions of granitic magma can produce rhyolite, pumice, obsidian, or tuff. These rocks have similar compositions but different cooling conditions. Explosive eruptionsproduce tuff or pumice. Effusive eruptions produce rhyolite or obsidian if the lava cools rapidly. These different rock types can all be found in the products of a single eruption. Eruptions of granitic magma are rare. Since 1900 only three are known to have occurred. These were at St. Andrew Strait Volcano in Papua New Guinea, Novarupta Volcanoin Alaska, and Chaiten Volcano in Chile. Granitic magmas are rich in silica and often contain up to several percent gas by weight. (Think about that several percent gas by weight is a LOT of gas!) As these magmas cool, the silica starts to connect into complex molecules. This gives the magma a high viscosity and causes it to move very sluggishly. The high gas content and high viscosity of these magmas are perfect for producing an explosive eruption. The viscosity can be so high that the gas can only escape by blasting the magma from the vent. Granitic magmas have produced some of the most explosive volcanic eruptions in Earth's history. Examples include Yellowstone in Wyoming, Long Valley in California, and Valles in New Mexico. The sites of their eruption are often marked by large calderas.

Lava Dome: Photo of a lava dome in the caldera of Mount St. Helens. Activity at St. Helens slowly extrudes thick lavas that gradually build domes in the caldera. This dome is composed of dacite, a rock that is intermediate in composition between rhyolite and andesite. Photo by the United States Geological Survey.

Lava Domes Sluggish rhyolitic lava can slowly exude from a volcano and pile up around the vent. This can produce a mound-shaped structure known as a "lava dome." Some lava domes have grown to a height of several hundred meters. Lava domes can be dangerous. As additional magma extrudes, the brittle dome can become highly fractured and unstable. The ground can also change slope as the volcano inflates and contracts. This activity can trigger a dome collapse. A dome collapse can lower the pressure on the extruding magma. This sudden lowering of pressure can result in an explosion. It can also result in a debris avalanche of material falling from the tall collapsing dome. Many pyroclastic flows and volcanic debris avalanches have been triggered by a lava dome collapse.

Fire Opal is sometimes found filling cavities in rhyolite. This specimen of rhyolite has multiple vugs filled with gemmy transparent orange fire opal. This material can be cut into beautiful cabochons and is sometimes faceted when it is transparent or even translucent. Famous deposits of this type of fire-opal-in-rhyolite are found in Mexico. This photo is used here through a Creative Commons license. It was produced by Didier Descouens.

Rhyolite and Gemstones Many gem deposits are hosted in rhyolite. These occur for a logical reason. The thick granitic lava that forms rhyolite often cools quickly while pockets of gas are still trapped inside of the lava. As the lava quickly cools, the trapped gas is unable to escape and forms cavities known as "vugs." Later, when the lava flow has cooled and hydrothermal gases or ground water move through, material can precipitate in the vugs. This is how some of the world's best deposits of red beryl, topaz, agate, jasper, and opal are formed. Gem hunters have learned this and are always on the lookout for vuggy rhyolite.

Rhyolite Arrowheads: Rhyolite was often used to make stone tools and weapons when more suitable materials were not available. It has been fashioned into scrapers, hoes, axe heads, spear points, and arrowheads.

Uses of Rhyolite Rhyolite is a rock that is rarely used in construction or manufacturing. It is often vuggy or highly fractured. Its composition is variable. When better materials are not locally available, rhyolite is sometimes used to produce crushed stone. People have also used rhyolite to manufacture stone tools, particularly scrapers, blades, and projectile points. It was probably not their material of choice, but a material used out of necessity.

Pumice

Pumice: This specimen shows the frothy vesicular texture of pumice. It has a specific gravity of less than one and will float on water. It is about five centimeters (two inches) across.

Pumice at Mount St. Helens: A pyroclastic flow will sometimes contain large pieces of pumice. This photograph shows a USGS scientist examining blocks of pumice at the toe of a pyroclastic flow at Mount St. Helens. Image by Terry Leighley, Sandia Labs.

What is Pumice? Pumice is a light-colored, extremely porous igneous rockthat forms during explosive volcanic eruptions. It is used as aggregate in lightweight concrete, as landscaping aggregate, and as an abrasive in a variety of industrial and consumer products. Many specimens have a high enough porosity that they can float on water until they slowly become waterlogged.

Pumice quarry: Photograph of stratified pumice deposit produced by pyroclastic flows at Mount St. Helens, Washington. USGS image by L. Topinka.

How Does Pumice Form? The pore spaces (known as vesicles) in pumice are a clue to how it forms. The vesicles are actually gas bubbles that were trapped in the rock during the rapid cooling of a gas-rich frothy magma. The material cools so quickly that atoms in the melt are not able to arrange themselves into a crystalline structure. Thus, pumice is an amorphous volcanic glass known as a "mineraloid." Some magmas contain several percent dissolved gas by weight while they are under pressure. Stop for a moment and think about that. Gas weighs very little at Earth's surface, but these magmas under pressure can contain several percent gas by weight held in solution. This is similar to the large amount of dissolved carbon dioxide in a sealed bottle of carbonated beverage such as beer or soda. If you shake the container, then immediately open the bottle, the sudden release of pressure allows the gas to come out of solution, and the beverage erupts from the container in a frothy mess. A rising body of magma, supercharged with dissolved gas under pressure, behaves in a similar way. As the magma breaks through Earth's surface, the sudden pressure drop causes the gas to come out of solution. This is what produces the enormous rush of high-pressure gas from the vent. This rush of gas from the vent shreds the magma and blows it out as a molten froth. The froth rapidly solidifies as it flies through the air and falls back to Earth as pieces of pumice. The largest volcanic eruptions can eject many cubic kilometers of material. This material can range in size from tiny dust particles to large blocks of pumice the size of a house. Large eruptions can blanket the landscape around the volcano with over 100 meters of pumice and launch dust and ash high into the atmosphere. The sections below give quotations from United States Geological Survey reports describing the production of pumice at two major eruptions.

Pinatubo eruption: The explosive eruption of Mount Pinatubo in the Philippines on June 12, 1991 ejected more than five cubic kilometers of material and was rated as a VEI 5 eruption on the volcanic explosivity index. Much of that material was pumice lapilli (see image below) that blanketed the landscape around the volcano. USGS image.

Pinatubo pumice: Dacitic pumice fragments erupted by Mount Pinatubo, Philippines, during an enormous eruption on 15 June 1991. Photo by W.E. Scott, USGS image.

Gas and Pumice at the Pinatubo Eruption The second most powerful volcanic eruption of the 20th century was at Mount Pinatubo in 1991. The description below explains how enormous volumes of dissolved gas powered the eruption and how a cubic mile of ash and pumice lapilli was blasted from the volcano. "From June 7 to 12, the first magma reached the surface of Mount Pinatubo. Because it had lost most of the gas contained in it on the way to the surface, the magma oozed out to form a lava dome but did not cause an explosive eruption. However, on June 12, millions of cubic yards of gas-charged magma reached the surface and exploded in the reawakening volcano's first spectacular eruption. When even more highly gas charged magma reached Pinatubo's surface on June 15, the volcano exploded in a cataclysmic eruption that ejected more than 1 cubic mile of material. [...] A blanket of volcanic ash and pumice lapilli blanketed the countryside. Huge avalanches of searing hot ash, gas, and pumice roared down the flanks of Mount Pinatubo, filling oncedeep valleys with fresh volcanic deposits as much as 660 feet thick. The eruption removed so much magma

and rock from below the volcano that the summit collapsed to form a large volcanic depression 1.6 miles across." [1]

Pumice raft: A "raft" of lightweight pumice floating on the surface of the South Pacific after an eruption in the Tonga Islands. NASA image.

Mount Mazama Eruption (Crater Lake) "The cataclysmic eruption of Mount Mazama 7,700 years ago started from a single vent on the northeast side of the volcano as a towering column of pumice and ash that reached some 30 miles high. Winds carried the ash across much of the Pacific Northwest and parts of southern Canada. So much magma erupted that the volcano began to collapse in on itself. As the summit collapsed, circular cracks opened up around the peak. More magma erupted through these cracks to race down the slopes as pyroclastic flows. Deposits from these flows partially filled the valleys around Mount Mazama with up to 300 feet of pumice and ash. As more magma was erupted, the collapse progressed until the dust settled to reveal a volcanic depression, called a caldera, 5 miles in diameter and one mile deep." [2]

Pumice raft: View of a pumice raft from a boat. Waves can be seen moving under the pumice. The rafts can float for years until all of the pumice becomes waterlogged and sinks or it is dissipated by waves and wind. USGS image.

Composition of Pumice Most pumice erupts from magmas that are highly charged with gas and have a rhyolitic composition. Rarely, pumice can erupt from gas-charged magmas of basaltic or andesitic composition.

Pantheon: Some of the concrete used to construct the Pantheon by the Romans in 126 AD was lightweight material made with pumice aggregate. Photography by Roberta Dragan, used under a Creative Commons license.

Pumice Has a Very Low Specific Gravity The abundant vesicles in pumice and the thin walls between them give the rock a very low specific gravity. It typically has a specific gravity of less than one, giving the rock an ability to float on water. Large amounts of pumice produced by some island and subsea eruptions will float on the surface and be pushed about by the winds. The pumice can float for long periods of time - sometimes years - before it finally becomes waterlogged and sinks. Large masses of floating pumice are known as "pumice rafts." They are large enough to be tracked by satellites and are a hazard to ships that sail through them (see images). [3] [4]

Pumice products: A variety of health and beauty products that contain pumice. They include the famous "Lava Soap" that cleans dirty hands with tiny pieces of pumice abrasive, a foot scrub cream that works as an exfoliant to smooth "sandal feet," two pumice stones, and a sponge with embedded pumice abrasive.

Uses of Pumice The largest use of pumice in the United States is the production of lightweight concrete blocks and other lightweight concrete products. When this concrete is mixed, the vesicles remain partially filled with air. That reduces the weight of the block. Lighter blocks can reduce the structural steel requirements of a building or reduce the foundation requirements. The trapped air also gives the blocks a greater insulating value.

The second most common use of pumice is in landscaping and horticulture. The pumice is used as a decorative ground cover in landscaping and planters. It is used as drainage rock and soil conditioner in plantings. Pumice and scoria are also popular rocks for use as substrates in hydroponic gardening. Pumice has many other uses. Together these account for less than a few percent of consumption in the United States, but these are the products that most people think of when they hear the word "pumice." Lots of people have found small pumice pebbles in the pockets of brand new "stone washed jeans," and almost everyone has seen the famous "Lava Soap" that uses pumice as an abrasive. Below we list these and some of the other minor uses of pumice (in no particular order). [5]          

an abrasive in conditioning "stone washed" denim an abrasive in bar and liquid soaps such as "Lava Soap" an abrasive in pencil erasers an abrasive in skin exfoliating products a fine abrasive used for polishing a traction material on snow-covered roads a traction enhancer in tire rubber an absorbent in cat litter a fine-grained filter media a lightweight filler for pottery clay Pumice Information

[1] The Cataclysmic 1991 Eruption of Mount Pinatubo, Philippines: Chris Newhall, James W. Hendley II, and Peter H. Stauffer; United States Geological Survey Fact Sheet 113-97, published 1997. [2] Mount Mazama and Crater Lake: Growth and Destruction of a Cascade Volcano: Ed Klimasauskas, Charles Bacon, and Jim Alexander; United States Geological Survey Fact Sheet 092-02, published 2002. [3] New Island and Pumice Raft in the Tongas: Earth Observatory image from NASA, November 16, 2006. [4] Maritime Impacts of Volcanic Eruptions: A Guide for the Prudent Mariner, National Weather Service, Ocean Prediction Center, NOAA, website last accessed June 2016. [5] Pumice and Pumicite: Robert D. Crangle, Jr., 2011 Minerals Yearbook, United States Geological Survey, August 2012.

Pumice and Pumicite Production Pumice is produced in two forms: rock pumice and pumicite. "Pumicite" is a name given to very fine-grained pumice (less than 4 millimeters in diameter down to submillimeter sizes). The word can be used synonymously with "volcanic ash." It is mined from volcanic ash deposits, or it can be produced by crushing rock pumice. About 500,000 metric tons of pumice and pumicite were mined in the United States in 2011. The total value of this pumice was about $11,200,000, or an average of about $23 per ton at the mine. The producing states were, in order of decreasing production:       

Oregon Nevada Idaho Arizona California New Mexico Kansas

Pumice Reticulite: Reticulite is a basaltic pumice in which all of the bubbles have burst, leaving a honeycomb structure. Photograph by J.D. Griggs, USGS image.

Imported Pumice and Substitutes All of the pumice production in the United States occurs west of the Mississippi River. In 2011, most of the pumice for consumption in the eastern United States was imported from Greece. In the eastern United States, expanded aggregate, produced by heating specific types of shale under controlled conditions, is used as a substitute for pumice in lightweight aggregate, horticultural, and landscaping applications. Contributor: Hobart King

Scoria

Scoria: A piece of scoria about 4 inches (10 centimeters) in diameter. A specimen with a rounded shape like this was most likely blown from a volcanic vent. This photograph was taken by Jonathan Zander and is used under a GNU Free Documentation License.

What is Scoria? Scoria is a dark-colored igneous rock with abundant round bubble-like cavities known as vesicles. It ranges in color from black or dark gray to deep reddish brown. Scoria usually has a composition similar to basalt, but it can also have a composition similar to andesite. Many people believe that small pieces of scoria look like the ash produced in a coal furnace. That has resulted in particles of scoria being called "cinders" and the small volcanoes that erupt scoria to be called "cinder cones."

Scoria: The specimen shown is about two inches (five centimeters) across.

How Does Scoria Form? Scoria forms when magma containing abundant dissolved gas flows from a volcano or is blown out during an eruption. As the molten rock emerges from the Earth, the pressure upon it is reduced and the dissolved gas starts to escape in the form of bubbles. If the molten rock solidifies before the gas has escaped, the bubbles become small rounded or elongated cavities in the rock. This dark-colored igneous rock with the trapped bubbles is known as scoria. When some volcanoes erupt, a rush of gas blows out of the vent. This gas was once dissolved in the magma below. The gas often blows out small bodies of magma that solidify as they fly through the air. This action can produce a ground cover of scoria all around the volcanic vent, with the heaviest deposits on the downwind side. Small particles of scoria that litter the landscape around the volcano are known as "lapilli" if they are between 2 millimeters and 64 millimeters in size. Larger particles are known as "blocks."

Scoria cinder cone: Artistic drawing illustrating the subsurface magma source and layer-by-layer build-up of scoria in a cinder cone eruption. Image by USGS.

Mauna Kea cinder cone: A red cinder cone and a cinder-covered landscape at Mauna Kea, Hawaii. Photo by Scot Izuka, USGS.

Cinder Cones Most of the scoria falls to the ground near the vent to build up a cone-shaped hill called a "cinder cone." Cinder cones are generally small volcanoes produced by brief eruptions with a total vertical relief of less than a few thousand feet. They are usually very steep because scoria has an angle of repose of 30 to 40 degrees. In some parts of the world, cinder cones occur in clusters of a few to hundreds of individual cones. These areas are called "volcano fields." (See the Google terrain image on this page for a view of the San Francisco Volcanic Field in Arizona, where hundreds of cinder cones can be seen.)

Stromboli ejecta: Magma being blown from the vent at Stromboli Volcano. This type of eruption would produce the small scoria cinders known as "lapilli." Photo by B. Chouet, USGS.

Lava Flows and Vesicular Basalts Some newly erupted lava flows contain abundant dissolved gas. The gas bubbles in the flow move upwards towards the surface in an attempt to escape while the lava is still molten. However, once the lava starts to solidify, the bubbles are trapped in the rock. These trapped gas bubbles are known as vesicles. If the upper portion of a lava flow contains a large concentration of vesicles, it is often called "scoria" or "vesicular basalt." This material often has fewer vesicles and a higher specific gravity than the scoria of lapilli.

Scoria on Mars: This image shows a field on Mars that is strewn with pieces of scoria, erupted from a Martian volcano. The piece of Martian scoria in the foreground is about 18 inches across and was found on the surface of Mars by the Spirit Rover. NASA image.

The Beverage Bottle Analogy Have you ever slowly opened a bottle that contains a carbonated beverage and watched the gas bubbles form on the walls of the bottle? Then as the seal on the bottle is broken, the bubbles grow larger and a hiss of gas escapes from the bottle, followed by a rush of foam. The depressurization and the escape of gas from a beverage is the same process that occurs when magma is depressurized as it emerges from a volcanic vent. The foam is equivalent to what will become scoria on solidification.

Sunset Crater cinder cone: Photograph of the Sunset Crater cinder cone that was formed by eruptions that occurred about 1000 years ago. It is located near Flagstaff, Arizona and is about 1000 feet tall. It is one of over 500 cinder cones in the San Francisco Volcanic Field. Image by USGS. (The Google terrain map on this page allows you to view this volcano on a topographic map or a satellite image.)

Not to be Confused with Pumice A vesicular igneous rock that is very similar to scoria is pumice. There are a few differences that can be used to distinguish them. First is their color. Scoria is almost always black or dark gray to reddish brown, while pumice is almost always white to light gray to light tan. This color difference is a result of their composition. Scoria forms from basaltic magmas, while pumice forms from rhyoliticmagmas - which usually contain more gas. Pumice has a much higher concentration of trapped bubbles - so many that the walls between them are very thin. The vesicles in pumice contain enough air that the rock will float on water. The thick walls of scoria make it heavy enough to sink. Finally, when observed closely with a hand lens, you can often see tiny mineral crystals in scoria. However, close observation of pumice reveals a "glassy" texture similar to obsidian. Pumice consists mainly of glass materials rather than mineral crystals. A "glass" is a noncrystalline substance. In the case of pumice, it cooled so quickly that the atoms were unable to arrange themselves into ordered crystal structures. Google terrain image showing a small cluster of cinder cones in the San Francisco Peaks Volcanic Field near Flagstaff, Arizona. The field contains over 500 cinder cones. View Larger Map

Expanded aggregate: Photograph of "light expanded clay aggregate," a scoria look-alike that is produced by heating certain types of clay in a rotating kiln. Organic material and moisture in the clay produce gas that causes vesicles similar to those found in scoria. Straight from the kiln, the material has a smooth exterior, but when broken the vesicular structure is exposed. Expanded aggregate is used as landscape stone, lightweight concrete, lightweight fill, and as a substrate for hydroculture. Public domain image by Leca67.

Uses of Scoria One of the main uses of scoria is in the production of lightweight aggregate. The scoria is crushed to desired sizes and sold for a variety of uses. Concrete made with scoria typically weighs about 100 pounds per cubic foot. This is a weight savings compared to concrete made with typical sand and gravel that weighs about 150 pounds per cubic foot. This savings in weight allows buildings to be constructed with less structural steel. The air trapped in the scoria makes the lightweight concrete a better insulator. Buildings constructed with this lightweight concrete can have lower heating and cooling costs. Crushed scoria is used as roofing granules, ground cover in landscape projects, and as a substrate in hydroponic gardening. Many dealers offer customers the option of choosing between black, brown, or red material. Scoria is also used as rip-rap, drainage stone, and low-quality road metal. Small amounts of scoria are used as sauna rock and as a heat sink in barbecue grills.

Scoria Substitutes Where scoria is not available, a lightweight aggregate can be produced by heating shale in a rotating kiln under controlled conditions. With the proper type of shale, the material will have the properties, appearance, and vesicles of scoria. It is sold under the name "expanded aggregate," "expanded clay," or "grow rocks" and used for the same purposes as crushed scoria. Contributor: Hobart King

Fire Opal A translucent to transparent opal with a wonderful fire-like background color of yellow, orange, or red.

Fire Opal: These three stones show the color range of "fire opal," a name given to specimens of translucent to transparent common opal with a wonderful fiery background hue that is present throughout the stone. The orange and yellow stones have a sleepy translucence, while the red stone is almost opaque.

What is Fire Opal? "Fire Opal" is a term used for colorful, transparent to translucent opal with a background color that is a firelike hue of yellow to orange to red. It might or might not exhibit "play-of-color" (the typical flashes of spectral colors that can be seen when a precious opal is turned under a source of light). Most fire opal does not have play-of-color. The defining characteristic of fire opal is the fiery hue of yellow, orange or red that serves as a uniform background color throughout the stone. These colors are thought to be caused by the presence of small amounts of iron in the opal. The value of a fire opal is based upon the desirability and uniformity of its color, with yellow being on the low end of value and red being on the high end. Transparent stones are preferred over translucent stones. The best fire opal typically sells for prices that are much lower than the best precious opal; however, fire opal specimens with exceptional color will sell for higher prices than some specimens of precious opal with less impressive play-of-color.

Oregon Fire Opal: The orange stone shown here is a faceted fire opal cut from material mined in Oregon. It is 9 x 7 millimeters in size and weighs about 1.2 carats.

"The defining characteristic of fire opal is the fiery hue of yellow, orange, or red that serves as a uniform background color throughout the stone."

How are Fire Opals Cut? Fire opals are cut in a variety of ways. Some are cut as faceted stones, others are cut as cabochons. The cutter decides how he/she thinks the stone will be most attractive. There is no rule for cutting fire opal. Transparent fire opals are often faceted so that they can be illuminated by incident light. If they have a spectacular play-of-color, they might be cut into a cabochon like most precious opal. If the play of color is minor, it might be cut into a faceted stone with a little surprise of flash. Translucent stones are often cut into cabochons, but it is not unusual to see a translucent to nearly opaque fire opal with an attractive color cut into a pretty faceted stone. The three stones in the photo at the top are wonderful examples of translucent stones that have been faceted.

Red fire opal: Red is the most desirable color of fire opal, with transparent specimens being more desirable than translucent. This is an 8 x 10 millimeter oval cut from material mined in Mexico. It weighs about 1.95 carats. Many people call a red fire opal like this one a "cherry opal" because of its color.

Durability of Fire Opal Fire opal has a Mohs hardness of 5.5 to 6, which is soft enough that it can be scratched by many objects that it might encounter if set into a ring without a setting especially designed to protect it. Fire opal also has a low tenacity, which means that it can easily be chipped or broken. So, fire opal is best used in jewelry such as earrings, pins, and pendants that usually are not subjected to rough wear.

Public Confusion with the Term "Fire Opal" There is some public confusion with the material known as "fire opal" and the gemstone phenomena known as "fire" and "play-of-color." What do the terms "fire" and "play-of-color" mean?

"Fire" "Fire" is a name used for flashes of spectral colors that are produced by the dispersion of light into its component colors as it passes through a transparent material. Most people are familiar with how a prism splits white light into a spectrum of its component colors. That phenomenon is known as "dispersion." Diamonds are famous for their colorful sparkle of light known as "fire" which is also caused by dispersion. Fire opal does not exhibit dispersion, thus it does not have "fire" like a diamond.

Precious fire opal: This is a faceted transparent opal with a light orange body color and a display of greenish-purple play-ofcolor. Because of its play-of-color and its orange body color, it could be called a "precious fire opal." It is a 12 x 8 millimeter oval that weighs about 2.2 carats, cut from material mined in Ethiopia.

"Play-of-Color" "Play-of-Color" is a name used for the flashes of spectral colors that are produced by "precious opal." Those colors are produced when light is scattered by an array of microscopic silica spheres that are stacked to make up the color-producing parts of a precious opal. The technical name of this light scattering is "diffraction." This same type of color is produced when droplets of water in Earth's atmosphere interact with sunlight to produce a rainbow. The rainbow colors of iris agate are also produced by diffraction. Fire opal will occasionally exhibit "play-of-color." When it does, some people call it "precious fire opal." A photo of "precious fire opal" can be seen on this page.

Nevada Fire Opal: The stone shown here is a faceted yellow fire opal cut from material mined in Nevada. It is a 9 millimeter round that weighs about 1.7 carats.

What is "Fire Opal"? Fire opal is the name used for a colorful material with a limited range of background colors. It is not a name given because of a phenomenon. It is an opal with an attractive background color that ranges from yellow to orange to red. The attractive background color is what defines the stone.

Fire Opal Localities Mexico has been the world's primary source of fire opal for decades. The Mexican deposits produce significant amounts of transparent to translucent, bright orange to orange-red material. Some of the transparent material is faceted, mounted in commercial jewelry, and described as "tangerine opal" because of its color. Smaller amounts of fire opal are produced in Australia, Brazil, Ethiopia, Honduras, Guatemala, Nevada, and Oregon.

Tuff An igneous rock that forms from the debris ejected by an explosive volcanic eruption.

Fish Canyon Tuff: Panoramic view of an outcrop of the Fish Canyon Tuff. The volcanic eruption(s) that produced this tuff occurred about 28 million years ago at the La Garita Caldera in southwestern Colorado. The original estimated volume of the Fish Canyon Tuff is about 1200 cubic miles (5000 cubic kilometers). It was one of the largest explosive volcanic eruptions known to have occurred. Enlarge. Image by USGS. [1]

Tuff: An igneous rock that contains the debris from an explosive volcanic eruption. It often contains fragments of bedrock, tephra, and volcanic ash. The specimen shown here is about two inches (five centimeters) across.

Beryllium tuff: A specimen of beryllium tuff from the Spor Mountain area of Utah. It is a porous tuff with abundant fragments of carbonate rock. Beryllium has been mined at Spor Mountain from stratified tuffs. Image by USGS. [3]

What is Tuff? Tuff is an igneous rock that forms from the products of an explosive volcanic eruption. In these eruptions, the volcano blasts rock, ash, magma and other materials from its vent. This ejecta travels through the air and falls back to Earth in the area surrounding the volcano. If the ejected material is compacted and cemented into a rock, that rock will be called "tuff." Tuff is usually thickest near the volcanic vent and decreases in thickness with distance from the volcano. Instead of being a "layer," a tuff is usually a "lens-shaped" deposit. Tuff can also be thickest on the downwind side of the vent or on the side of the vent where the blast was directed. Some tuff deposits are hundreds of meters thick and have a total eruptive volume of many cubic miles. That enormous thickness can be from a single eruptive blast or, more commonly, from successive surges of a single eruption - or eruptions that were separated by long periods of time.

Tuff ring: Drawing of a tuff ring surrounding a shallow, water-filled crater. The tuff ring is formed from materials that were ejected by the volcanic blast and fell back to Earth in the area surrounding the crater. Tuff rings generally have a gentle slope of between two and ten degrees.

Tuff Rings A "tuff ring" is a small volcanic cone of low relief that surrounds a shallow crater. These craters, known as maars, are formed by explosions caused by hot magma coming in contact with cold groundwater. The explosion blasts fragments of bedrock, tephra, and ash from the crater. The tuff ring forms as these ejected materials fall back to Earth. Tuff rings range in size from several hundred meters across to several thousand meters. They are typically less than a few hundred meters in height and have a very gentle slope of less than ten degrees.

Tuff: Close-up of a piece of tuff exposed at Hole-in-the-Wall, Mojave National Preserve, California. This specimen clearly displays the diversity of materials that compose a tuff. Public domain image by Mark A. Wilson, Department of Geology, The College of Wooster.

Welded Tuff Sometimes the ejecta is hot enough when it lands that the particles are soft and sticky. These materials "weld" together upon impact or upon compaction. The rock formed from this hot ejecta is known as a "welded tuff" - because the ejected particles are welded together. Some deposits might contain welded tuff near the vent and unwelded tuff at a distance where smaller, cooler particles fell to the ground.

Ettringer tuff: Close-up of a specimen of Ettringer Tuff showing a variety of rock fragments and tephra in a matrix of volcanic ash. Public domain image by Roll-Stone of Wikimedia.

Information about Tuff [1] Geologic Map of the Cochetopa Park and North Pass Calderas, Northeastern San Juan Mountains, Colorado: Peter W. Lipman, United States Geological Survey Scientific Investigations Map 3123, pamphlet 48 pages, 2 map sheets, scale 1:50,000, 2012. [2] Slides of the Fluorspar, Beryllium, and Uranium Deposits at Spor Mountain, Utah: David A. Lindsey, slide collection posted on the United States Geological Survey website, last accessed June 2016. [3] Pre-1980 Tephra-Fall Deposits Erupted from Mount St. Helens, Washington: Donal R. Mullineaux, United States Geological Survey Professional Paper 1563, last accessed June 2016.

Many Types of Tuff "Tuff" is a name that is used for a broad range of materials. The only requirement is that the materials are ejecta produced by a volcanic eruption. Tuff can contain fragments of dust-size particles to boulder-size particles and be composed of many different types of material.

Mount St. Helens tephra: Photograph of an outcrop of stratified tuff that formed from tephra produced by pre-1980 eruptions at Mount St. Helens, Washington. This photograph shows several layers of tephra with different textures and different compositions, each from a different eruptive event. [3]

Many tuff deposits contain fragments of bedrock that are unrelated to volcanic activity. These materials are involved when the volcanic explosion occurs below the ground. The subsurface explosion crushes the overlying bedrock and launches it into the air mixed with tephra and volcanic ash produced from the magma source below. Different volcanoes are supplied with magma of different compositions. Many tuff deposits form from magma with a rhyolitic composition, but andesitic, basaltic, and other types of magma might contribute to the tuff. Tuff also varies by particle size. Near the vent, a tuff might consist mainly of large blocks of material in a volcanic ash matrix. With distance from the vent, the clasts will be smaller in size. At the edges of the rock unit, the tuff might be mainly composed of very fine ash.

Pictures of Metamorphic Rocks Photos of Common Foliated and Non-Foliated Metamorphic Rock Types

Amphibolite is a non-foliated metamorphic rock that forms through recrystallization under conditions of high viscosity and directed pressure. It is composed primarily of hornblende(amphibole) and plagioclase, usually with very little quartz. The specimen shown above is about two inches (five centimeters) across.

What are Metamorphic Rocks? Metamorphic rocks have been modified by heat, pressure, and chemical processes, usually while buried deep below Earth's surface. Exposure to these extreme conditions has altered the mineralogy, texture, and chemical composition of the rocks. There are two basic types of metamorphic rocks. Foliated metamorphic rocks such as gneiss, phyllite, schist, and slate have a layered or banded appearance that is produced by exposure to heat and directed pressure. Non-foliated metamorphic rocks such as hornfels, marble, quartzite, and novaculite do not have a layered or banded appearance.

Hornfels is a fine-grained nonfoliated metamorphic rock with no specific composition. It is produced by contact metamorphism. Hornfels is a rock that was "baked" while near a heat source such as a magma chamber, sill, or dike. The specimen shown above is about two inches (five centimeters) across.

Gneiss is a foliated metamorphic rock that has a banded appearance and is made up of granular mineral grains. It typically contains abundant quartz or feldspar minerals. The specimen shown above is about two inches (five centimeters) across.

Marble is a non-foliated metamorphic rock that is produced from the metamorphism of limestone or dolostone. It is composed primarily of calcium carbonate. The specimen shown above is about two inches (five centimeters) across.

Phyllite is a foliated metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist. The specimen shown above is about two inches (five centimeters) across.

Novaculite is a dense, hard, fine-grained, siliceous rock that breaks with a conchoidal fracture. It forms from sediments deposited in marine environments where organisms such as diatoms (single-celled algae that secrete a hard shell composed of silicon dioxide) are abundant in the water. The specimen shown above is about three inches across.

Lapis Lazuli, the famous blue gem material, is actually a metamorphic rock. Most people are surprised to learn that, so we added it to this photo collection as a surprise. Blue rocks are rare, and we bet that it captured your eye. The round objects in the photo are lapis lazuli beads about 9/16 inch (14 millimeters) in diameter. Image © iStockPhoto / RobertKacpura.

Quartzite is a non-foliated metamorphic rock that is produced by the metamorphism of sandstone. It is composed primarily of quartz. The specimen above is about two inches (five centimeters) across.

Slate is a foliated metamorphic rock that is formed through the metamorphism of shale. It is a low-grade metamorphic rock that splits into thin pieces. The specimen shown above is about two inches (five centimeters) across.

Schist is a metamorphic rock with well-developed foliation. It often contains significant amounts of mica which allow the rock to split into thin pieces. It is a rock of intermediate metamorphic grade between phyllite and gneiss. The specimen shown above is a "chlorite schist" because it contains a significant amount of chlorite. It is about two inches (five centimeters) across.

Soapstone is a metamorphic rock that consists primarily of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, pyroxenes, and carbonates. It is a soft, dense, heat-resistant rock that has a high specific heat capacity. These properties make it useful for a wide variety of architectural, practical, and artistic uses.

Amphibolite A metamorphic rock composed primarily of amphibole minerals and plagioclase feldspar

Amphibolite: Amphibolite is a coarse-grained metamorphic rock that has amphibole minerals such as the hornblende group as its primary ingredient. The specimen shown is about two inches (five centimeters) across.

What is Amphibolite ? Amphibolite is a coarse-grained metamorphic rock that is composed mainly of green, brown, or black amphibole minerals and plagioclase feldspar. The amphiboles are usually members of the hornblende group. It can also contain minor amounts of other metamorphic minerals such as biotite, epidote, garnet, wollastonite, andalusite, staurolite, kyanite, and sillimanite. Quartz, magnetite, and calcite can also be present in small amounts.

How Does Amphibolite Form? Amphibolite is a rock of convergent plate boundaries where heat and pressure cause regional metamorphism. It can be produced through the metamorphism of mafic igneous rocks such as basalt and gabbro, or from the metamorphism of clay-rich sedimentary rocks such as marl or graywacke. The metamorphism sometimes flattens and elongates the mineral grains to produce a schistose texture.

Amphibolite: Some amphibolites are greenish, as determined by the color of the amphibole minerals. This specimen is actually an igneous rock. Some geologists call an igneous rock composed primarily of amphibole minerals an amphibolite or "hornblendite." USGS image.

Uses of Amphibolite Amphibolite has a variety of uses in the construction industry. It is harder than limestone and heavier than granite. These properties make it desirable for certain uses. Amphibolite is quarried and crushed for use as an aggregate in highway construction and as a ballast stone in railroad construction. It is also quarried and cut for use as a dimension stone. Higher quality stone is quarried, cut, and polished for architectural use. It is used as facing stone on the exterior of buildings, and used as floor tile and panels indoors. Some of the most attractive pieces are cut for use as countertops. In these architectural uses, amphibolite is one of the many types of stone sold as "black granite." Some amphibolite deposits, such as the one at Gore Mountain in the Adirondacks of New York, contain significant amounts of garnet. If enough garnet is present and of proper quality, the amphibolite can be mined and the garnet recovered for use as an abrasive.

Hornfels A metamorphic rock without foliation.

Hornfels: The specimen shown is about two inches (five centimeters) across.

What is Hornfels? Hornfels is a fine-grained nonfoliated metamorphic rock with no specific composition. It is produced by contact metamorphism. Hornfels is a rock that was "baked" while near a heat source such as a magma chamber, sill, or dike.

Gneiss

Gneiss: The specimen shown is about two inches (five centimeters) across. It is easy to see the "salt and pepper" banding of this rock.

What is Gneiss? Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. Other bands contain platy or elongate minerals with evidence of preferred orientation. It is this banded appearance and texture - rather than composition - that define a gneiss.

Gneissic Granodiorite: An outcrop of gneissic granodiorite in the Zarembo Island area of southeastern Alaska.

How Does Gneiss Form? Gneiss usually forms by regional metamorphism at convergent plate boundaries. It is a high-grade metamorphic rock in which mineral grains recrystallized under intense heat and pressure. This alteration increased the size of the mineral grains and segregated them into bands, a transformation which made the rock and its minerals more stable in their metamorphic environment. Gneiss can form in several different ways. The most common path begins with shale, which is a sedimentary rock. Regional metamorphism can transform shale into slate, then phyllite, then schist, and finally into gneiss. During this transformation, clay particles in shale transform into micas and increase in size. Finally, the platy micas begin to recrystallize into granular minerals. The appearance of granular minerals is what marks the transition into gneiss. Intense heat and pressure can also metamorphose granite into a banded rock known as "granite gneiss." This transformation is usually more of a structural change than a mineralogical transformation. Granite gneiss can also form through the metamorphism of sedimentary rocks. The end product of their metamorphism is a banded rock with a mineralogical composition like granite.

Folded Gneiss: A photograph of polished gneiss from the stock of a countertop vendor. The view shown in the photo is about 12 inches across. Click to enlarge.

Composition and Texture of Gneiss Although gneiss is not defined by its composition, most specimens have bands of feldspar and quartz grains in an interlocking texture. These bands are usually light in color and alternate with bands of darker-colored

minerals with platy or elongate habits. The dark minerals sometimes exhibit an orientation determined by the pressures of metamorphism. Some specimens of gneiss contain distinctive minerals characteristic of the metamorphic environment. These minerals might include biotite, cordierite, sillimanite, kyanite, staurolite, andalusite, and garnet. Gneiss is sometimes named for these minerals, examples of which include "garnet gneiss" and "biotite gneiss."

Garnet Gneiss: A coarse-grained gneiss composed mainly of hornblende (black), plagioclase (white), and garnet (red) from Norway. Public domain photo by Woudloper.

Garnet Gneiss: A cabochon cut and polished from garnet gneiss. A cabochon cut from this type of material is rarely seen, but it would be an interesting gem for a geologist. The stone is approximately 38 x 27 millimeters in size.

Garnet Gneiss: A photograph of polished garnet gneiss from the stock of a countertop vendor. The view shown in the photo is about 12 inches across. Click to enlarge.

Uses of Gneiss Gneiss usually does not split along planes of weakness like most other metamorphic rocks. This allows contractors to use gneiss as a crushed stone in road construction, building site preparation, and landscaping projects. Some gneiss is durable enough to perform well as a dimension stone. These rocks are sawn or sheared into blocks and slabs used in a variety of building, paving, and curbing projects. Some gneiss accepts a bright polish and is attractive enough for use as an architectural stone. Beautiful floor tiles, facing stone, stair treads, window sills, countertops, and cemetery monuments are often made from polished gneiss.

Corundum Gneiss: This is a specimen of corundum gneiss from Gallatin Valley, Montana. This specimen is about four inches across and has a round blue sapphire crystal on the left side.

Commercial Terminology Don't be surprised if you see gneiss labeled as "granite" at a cabinet shop or monument company. In the dimension stone trade, any rock with visible, interlocking grains of feldspar is considered to be "granite" in that industry. Seeing gneiss, gabbro, labradorite, diorite, and other types of rock marketed as "granite" disturbs many geologists. However, this long-time practice of the dimension stone trade simplifies discussions with customers since not everyone knows the technical names of unusual igneous and metamorphic rocks.

Gneiss in the Classroom Small rock and mineral specimens about one inch in size are usually adequate for student examination and identification. However, many rock units, identified as gneiss in the field, have bands that are thicker than one inch. If samples of these rock units are broken into one-inch pieces, many of them will be too small to exhibit the banding features of gneiss. This will confuse many students and cause others to incorrectly identify the rock. Teachers can avoid these problems by collecting specimens that clearly display a banded structure. Teachers who purchase specimens must examine them carefully before they are presented to students. After students have learned to identify gneiss and many other rock types, presenting specimens of gneiss that do not exhibit banding can be a challenging way to have students: A) consider possibilities that are not obvious, and, B) realize that a single rock specimen may not adequately represent a rock unit. Contributor: Hobart King

Marble A non-foliated metamorphic rock that forms when limestone is subjected to heat and pressure.

Pink Marble: A piece of pink marble about four inches (ten centimeters) across. The pink color is most likely derived from iron. Image by NASA.

What is Marble? Marble is a metamorphic rock that forms when limestoneis subjected to the heat and pressure of metamorphism. It is composed primarily of the mineral calcite (CaCO3) and usually contains other minerals, such as clay minerals, micas, quartz, pyrite, iron oxides, and graphite. Under the conditions of metamorphism, the calcite in the limestone recrystallizes to form a rock that is a mass of interlocking calcite crystals. A related rock, dolomitic marble, is produced when dolostone is subjected to heat and pressure.

Photo Gallery: The Many Uses of Marble

From monuments to crushed stone to cosmetics and pharmaceuticals, very few rocks have as many uses as marble.

Taj Mahal

The Taj Mahal is one of the most beautiful and famous buildings in the world. It was built between 1632 and 1653 as a mausoleum for Mumtaz Mahal, the third wife of Mughal emperor Shah Jahan. Marble was used extensively throughout the building, including the marble domes and towers. Photo © iStockphoto / standby.

The Properties of Marble and Its Uses Very few rocks have as many uses as marble. It is used for its beauty in architecture and sculpture. It is used for its chemical properties in pharmaceuticals and agriculture. It is used for its optical properties in cosmetics, paint, and paper. It is used because it is an abundant, low-cost commodity in crushed stone prepared for construction projects. Marble has many unique properties that make it a valuable rock in many different industries. The photographs and captions below illustrate just a few of its varied uses.

Many Colors of Marble

Marble occurs in a very wide range of colors. Marble formed from the purest limestones is white in color. Iron oxide impurities in the limestone will produce a yellow, orange, pink or red color. Clay minerals can produce gray colors that often occur in bands after the compositional stratification of the original limestone. Abundant bituminous materials can produce dark gray to black marble. Marble that contains serpentineoften has a green color. Photo © iStockphoto / Tina Lorien.

Supreme Court Building

The Supreme Court building was constructed between 1932 and 1935 using several different types of marble. Vermont marble was used extensively in the exterior. The inner courtyards were made using bright white marble from Georgia, and the interior corridors and entrance halls are made from creamy white marble from Alabama. Photo © iStockphoto / GBlakeley.

Washington Monument

The Washington Monument was built of marble between 1848 and 1884. Initial work on the structure was done using marble from a quarry located near Texas, Maryland. The project was then delayed for nearly 30 years due to a lack of funds. When construction resumed in 1876, similar stone from the Texas quarry was not available, so stone from the Sheffield quarry near Sheffield, Massachusetts was used. The Sheffield quarry had problems delivering stone in a timely manner, and in 1880 their contract was cancelled. A new contract then went to the Cockeysville Quarry near Baltimore, Maryland which supplied a slightly darker dolomitic marble. These different stone sources can be seen in the monument as labeled in the photo above. Photo and annotation by the United States Geological Survey.

Marble Stair Treads, Risers, Floor Tile

Marble is a material used in prestige architecture and interior design. This photo shows stair treads and risers made from brecciated marble and floor tiles made from marble in a variety of colors. Photo © iStockphoto / Nikada.

Bust of Artemis

Marble is a translucent stone that allows light to enter and produce a soft "glow." It also has the ability to take a very high polish. These properties make it a beautiful stone for producing sculptures. It is soft, making it easy to sculpt, and when it is fine-grained it has uniform properties in all directions. Some of the world's most famous sculptures have been produced from marble. This bust of the Greek goddess, Artemis, is a copy of an original Greek work. Photo © iStockphoto / Diane Diederich.

Lincoln Memorial

The Lincoln Memorial was built between 1914 and 1922. Many different stones were used in the memorial. The terrace walls and lower steps were made of granite from Massachusetts. The upper steps, columns, and outside facade were made using marble

from Colorado. The interior walls are Indiana limestone (called "Indiana Marble" by many architects). The floor was made using pink marble from Tennessee, and the statue of Lincoln is made from a very bright white marble from Georgia. Each type of stone was selected for its properties along with an effort to utilize stone from many parts of the United States. Photo © iStockphoto / ntn.

Cemetery Markers

Marble is often used as a cemetery marker. It is a very attractive stone. It is economical because it is relatively easy to cut and engrave. In comparison to rocks like granite, it is not as resistant to acid precipitation and tends to lose edges and detail over time. Photo © iStockphoto / JPecha.

Whiting

Marble of exceptionally white color is sometimes used to produce a product known as "whiting," a white powder that is used as a pigment, brightener, and filler in paint, paper and other products. Photo © iStockphoto / nsilcock.

Cutting Marble

A large-diameter diamond saw cuts a block of marble into dimension stone at a factory. Slabs and blocks of marble are used for stair treads, floor tiles, facing stone, cemetery stones, window sills, ashlars, sculptures, benches, paving stones and many other uses. Photo © iStockphoto / maskpro.

Agricultural Lime

Some marble is heated in a kiln to drive off the carbon dioxide that is contained within the calcite. What remains after kiln treatment is the calcium oxide - known as "lime." Lime is used as an agricultural soil treatment to reduce the acidity in soil. When applied in combination with fertilizer, it can increase the yield of a soil. This test plot shows a portion of a corn field where no lime and no fertilizer were applied. The plants in that plot are struggling to survive. Photo by the Agricultural Research Service, United States Department of Agriculture.

Marble Dimension Stone

Marble cut into blocks and slabs of specific size is known as "dimension stone." Photo © iStockphoto / Thomas Lehmann.

Marble Quarry

Equipment working in a marble quarry near Madrid, Spain. In this quarry the marble is being sawn into blocks for the production of dimension stone. Photo © iStockphoto / vallefrias.

Acid Neutralization

Marble is composed of calcium carbonate. That makes it very effective at neutralizing acids. Highest purity marble is often crushed to a powder, processed to remove impurities and then used to make products such as Tums and Alka-Seltzer that are used for the treatment of acid indigestion. Crushed marble is also used to reduce the acid content of soils, the acid levels of streams, and as an acid-neutralizing material in the chemical industry. Photo © iStockphoto / NoDerog.

Crushed Stone - Construction Aggregate

Some marble is mined, crushed, sized and sold as a construction aggregate. It can be used as fill, subbase, landscape stone and other uses where soundness and abrasion resistance are not critical. Because marble is composed of calcite, it cleaves more readily than limestone and does not have the strength, soundness, and abrasion resistance of granite and other more competent rocks. Photo © iStockphoto / AdShooter.

Soft Abrasive

Marble is composed of calcite, a mineral with a Mohs hardness of three. It is softer than most bathroom and kitchen surfaces and can be used on them as a scrubbing agent without producing scratches or other damage.

Calcium Feed Supplement

Dairy cows and chickens need a steady supply of calcium to produce milk and eggs. Farms that raise these animals often use animal feeds that have been supplemented with additional calcium. Powdered limestone and marble are used to produce these supplements because they are softer than the animal's teeth, soluble, and rich in calcium. Photo © iStockphoto / NiDerLander.

Ruby in Marble: Marble is often the host rock for corundum, spinel, and other gem minerals. This specimen is a piece of white marble with a large red ruby crystal from Afghanistan. Specimen is about 1 1/4 inches across (about 3 centimeters). Specimen and photo by Arkenstone / www.iRocks.com.

How Does Marble Form? Most marble forms at convergent plate boundaries where large areas of Earth's crust are exposed to regional metamorphism. Some marble also forms by contact metamorphism when a hot magma body heats adjacent limestone or dolostone. Before metamorphism, the calcite in the limestone is often in the form of lithified fossil material and biological debris. During metamorphism, this calcite recrystallizes and the texture of the rock changes. In the early stages of the limestone-to-marble transformation, the calcite crystals in the rock are very small. In a freshly-broken hand specimen, they might only be recognized as a sugary sparkle of light reflecting from their tiny cleavage faces when the rock is played in the light. As metamorphism progresses, the crystals grow larger and become easily recognizable as interlocking crystals of calcite. Recrystallization obscures the original fossils and sedimentary structures of the limestone. It also occurs without forming foliation, which normally is found in rocks that are altered by the directed pressure of a convergent plate boundary. Recrystallization is what marks the separation between limestone and marble. Marble that has been exposed to low levels of metamorphism will have very small calcite crystals. The crystals become larger as the level of metamorphism progresses. Clay minerals within the marble will alter to micas and more complex silicate structures as the level of metamorphism increases.

Marble Dimension Stone: Marble cut into blocks and slabs of specific size is known as "dimension stone." Photo © iStockphoto / Thomas Lehmann.

Physical Properties and Uses of Marble Marble occurs in large deposits that can be hundreds of feet thick and geographically extensive. This allows it to be economically mined on a large scale, with some mines and quarries producing millions of tons per year. Most marble is made into either crushed stone or dimension stone. Crushed stone is used as an aggregate in highways, railroad beds, building foundations, and other types of construction. Dimension stone is produced by sawing marble into pieces of specific dimensions. These are used in monuments, buildings, sculptures, paving and other projects. We have an article about "the uses of marble" that includes photos and descriptions of marble in many types of uses.

Gray Marble: This specimen has calcite cleavage faces up to several millimeters in size that are reflecting light. The specimen is about two inches (five centimeters) across.

Calcium carbonate medicines: Marble is composed of calcium carbonate. That makes it very effective at neutralizing acids. Highest purity marble is often crushed to a powder, processed to remove impurities, and then used to make products such as Tums and Alka-Seltzer that are used for the treatment of acid indigestion. Crushed marble is also used to reduce the acid content of soils, the acid levels of streams, and as an acid-neutralizing material in the chemical industry. Photo © iStockphoto / NoDerog.

Color: Marble is usually a light-colored rock. When it is formed from a limestone with very few impurities, it will be white in color. Marble that contains impurities such as clay minerals, iron oxides, or bituminous material can be bluish, gray, pink, yellow, or black in color. Marble of extremely high purity with a bright white color is very useful. It is often mined, crushed to a powder, and then processed to remove as many impurities as possible. The resulting product is called "whiting." This powder is used as a coloring agent and filler in paint, whitewash, putty, plastic, grout, cosmetics, paper, and other manufactured products. Acid Reaction: Being composed of calcium carbonate, marble will react in contact with many acids, neutralizing the acid. It is one of the most effective acid neutralization materials. Marble is often crushed and used for acid neutralization in streams, lakes, and soils. It is used for acid neutralization in the chemical industry as well. Pharmaceutical antacid medicines such as "Tums" contain calcium carbonate, which is sometimes made from powdered marble. These medicines are helpful to people who suffer from acid reflux or acid indigestion. Powdered marble is used as an inert filler in other pills. Hardness: Being composed of calcite, marble has a hardness of three on the Mohs hardness scale. As a result, marble is easy to carve, and that makes it useful for producing sculptures and ornamental objects. The translucence of marble makes it especially attractive for many types of sculptures. The low hardness and solubility of marble allows it to be used as a calcium additive in animal feeds. Calcium additives are especially important for dairy cows and egg-producing chickens. It is also used as a low-hardness abrasive for scrubbing bathroom and kitchen fixtures. Ability to Accept a Polish: After being sanded with progressively finer abrasives, marble can be polished to a high luster. This allows attractive pieces of marble to be cut, polished, and used as floor tiles, architectural panels, facing stone, window sills, stair treads, columns, and many other pieces of decorative stone.

Another Definition of Marble The name "marble" is used in a different way in the dimension stone trade. Any crystalline carbonate rock that has an ability to accept a polish is called "marble." The name is sometimes used for other soft rocks such as travertine, verd antique, serpentine, and some limestones. Contributor: Hobart King

Phyllite What is Phyllite?

Phyllite: The specimen shown is about two inches (five centimeters) across.

Phyllite is a foliate metamorphic rock that is made up mainly of very fine-grained mica. The surface of phyllite is typically lustrous and sometimes wrinkled. It is intermediate in grade between slate and schist.

Novaculite Native Americans and European settlers valued this rock for different reasons

Novaculite: Specimen of novaculite showing its fine-grained texture and conchoidal fracture. Specimen is approximately 3 inches across.

What is Novaculite? Novaculite is a dense, hard, fine-grained siliceous rock that breaks with a conchoidal fracture. It forms from sediments deposited in marine environments where organisms such as diatoms (single-celled algae that secrete a hard shell composed of silicon dioxide) are abundant in the water. When the diatoms die, their silicon dioxide shells fall to the seafloor. In some areas these diatom shells are the primary ingredient of the seafloor sediments. During diagenesis (the transformation from sediment to rock) the silicon dioxide from the diatom shells is transformed into chalcedony (a microcrystalline silicon dioxide). At this point the rock is chert. The chert is transformed into novaculite as further diagenesis and low-grade metamorphism recrystallize the chalcedony into microcrystalline quartz grains. The two primary differences between chert and novaculite are: 1) chert is composed mainly of chalcedony while novaculite is composed mainly of microcrystalline quartz grains; and, 2) chert is a sedimentary rock, while novaculite is a chert that has experienced a higher level of diagenetic alteration and low-grade metamorphism.

Arkansas novaculite sharpening stones: Sharpening stones made of Arkansas novaculite. The white stone has a coarse texture for initial sharpening, the mottled stone has an intermediate texture for resharpening, and the black stone has a very fine texture for honing an ultrasharp edge. The stones are used with a drop of oil that lubricates the sharpening strokes and keeps metal from loading the pore spaces in the stone. Stones are about two inches wide, six inches in length and 1/2 inch thick.

Novaculite Localities The most famous locality for novaculite is where the Arkansas Novaculite Formation outcrops in the Ouachita Mountains of central Arkansas and southeastern Oklahoma. It is a Devonian to Mississippian-age rock unit that ranges from about 60 feet thick in the northern Ouachitas to about 900 feet thick in the southern Ouachitas. Outcrops of the Arkansas Novaculite Formation are prominent landscape features of the Ouachita Mountains. Compared to most other types of rock, novaculite is very resistant to chemical and physical weathering. This makes it a ridge-former and a cliff-former in the areas where it outcrops. Peaks, cliffs, and ridges formed by novaculite are prominent landscape features of the Ouachitas.

Novaculite ridges: Ridges of the Caballos Novaculite in the Lightning Hills of Brewster County, Texas. The Lightning overthrust crops out in the valley between the ridges. In the subsurface, the Caballos novaculite yields oil and gas from a tripolitic zone near the top of the rock unit and from fracture porosity in the lower portion of the rock unit. USGS Photo taken in November, 1930 and included in U.S. Geological Survey Professional Paper 187.

First Use of Arkansas Novaculite Native Americans were the first people to mine the Arkansas Novaculite Formation. They noticed its conchoidal fracture and discovered that it could be knapped - just like flint - into projectile points, scrapers, and cutting tools. They mined novaculite, used it to manufacture cutting tools and weapons, and traded the material and products over a broad area. The Quapaw, Osage, Caddo, Tunica, Chickasaw, and Natchez tribes were especially involved in the mining. Prehistoric people in other parts of the world have worked novaculite deposits to manufacture weapons and cutting tools. Novaculite and manufactured products from these areas were transported and traded across great distances.

Water wells in novaculite: Novaculite is often a highly fractured rock unit that can serve as an adequate aquifer for private water supplies. United States Geological Survey image.

A World-Famous Sharpening Stone European settlers in the Ouachita region were the second people to mine the Arkansas Novaculite Formation. They valued it for a different reason. They found that novaculite could be used to sharpen metal tools and weapons. They soon began producing sharpening tools and trading them with distant partners. Arkansas “whetstones,” “oil stones,” and “sharpening stones” became world-famous for their ability to produce a sharp edge on a metal blade. This created a demand for novaculite that was strong in the 1800s but declined as people used fewer blades that required resharpening. In the early 1900s, demand declined further as artificial abrasives and sharpening machines began to replace the sharpening stone. Although sharpening stones made with synthetic abrasives are cost-competitive with novaculite and perform well, a steady demand for novaculite still supports several producers of novaculite sharpening tools. The Arkansas Novaculite Formation yields sharpening-grade stones in a range of textures. “Washita Stone” has the appearance of unglazed porcelain, a porosity of several percent and serves as a good stone for coarse sharpening. An extremely fine-grained material known as “Arkansas Stone” has almost no porosity and is an excellent tool for honing a razor-sharp blade. These stones are broken from the quarry with black-powder blasting, sawn to shape with a diamond saw, and then lapped to form a surface that is perfectly flat and smooth.

Physical Properties of Novaculite Color

Usually ranges from white, through gray, to black. Other colors occur when the stone has impurities - brown, red, pink, bluish.

Streak

Colorless (harder than the streak plate)

Luster

Usually dull to earthy when porous. Specimens with very little porosity can have a waxy luster.

Diaphaneity

Translucent on thin edges to opaque.

Cleavage

None - typically breaks with a conchoidal fracture.

Mohs Hardness

7

Specific Gravity

2.5 - 2.7

Distinguishing

Conchoidal fracture, glassy luster, hardness.

Characteristics

Chemical Composition

Uses

Silicon dioxide (SiO2). Some specimens contain up to 99% silicon dioxide. Some specimens have a high carbonate content of up to 30%. Used to make cutting tools, sharpening stones, streak plates, and tripoli. Reservoir rock.

Novaculite Information [1] Novaculite: H. Pennington, Encyclopedia of Arkansas, website article last accessed June 2016. [2] Novaculite (Silica Stone): Arkansas Geological Survey, Industrial Minerals of Arkansas, website article last accessed June 2016. [3] Exploration for the Arkansas Novaculite Reservoir, in the Southern Ouachita Mountains, Arkansas: T.J. Godo, P. Li and M.E. Ratchford, Search and Discovery Article #10337, July 2011.

Other Uses for Novaculite AGGREGATE Novaculite is a very durable rock that resists abrasion and is well suited for use as road base, railroad ballast, and rip-rap. Although it works especially well, its use in these applications is avoided. The reason: novaculite is so abrasive on metal that it causes excessive wear on excavating equipment used to mine it, on crushers and classifiers used to process it, and it wears out the beds of trucks that haul it. Novaculite is also not used as a concrete aggregate for the same reasons and because it reacts with cement to produce pop-outs (aggregate grains that separate from the concrete to produce a pit in the pavement surface). REFRACTORY Novaculite’s heat-resistant properties make it a good material for making refractory products. It has also been used in glass manufacturing, with some of it being used in Pyrex products. The abrasive properties of novaculite make it useful for manufacturing deburring media, files, and grinding media.

TRIPOLI The upper portion of the Arkansas Novaculite Formation in some areas has a significant carbonate content. In these areas the novaculite weathers to yield a granular quartz residue with a very high silica content and a very fine grain size. This material is mined and processed into a product known as tripoli. Most of the tripoli used in the United States today is a filler or extender in plastics, rubber, paint, caulking compounds, and other products. Tripoli is added to soaps and scouring powders to serve as an abrasive. It is also used as an abrasive in metal finishing, woodworking, lapidary, and auto painting shops. RESERVOIR ROCK Novaculite sometimes serves as a reservoir for oil and natural gas. Several oil and gas fields in the Ouachita overthrust belt of Oklahoma and Texas produce from the Caballos Novaculite. Tripolitic chert zones near the top of the rock unit can have significant porosity, and the highly fractured novaculite is another form of porosity. Areas underlain by fractured novaculite are also preferred drilling sites when drilling for groundwater. GOLD TESTING Small blocks of black novaculite are also used in the "acid test" for determining the gold content of jewelry. In this, the jeweler rubs the suspected gold item across a fine-grained block of black novaculite to produce a tiny streak of metal. A drop of aqua regia (a mixture of hydrochloric acid and nitric acid) of known concentration is placed on the streak. If the streak disappears, it was dissolved by the aqua regia. Aqua regia solutions of different concentration will dissolve different karat weights of gold. Standard aqua regia solutions have been developed to identify gold of 10k, 12k, 14k, 18k, 20k, and 22k purity. Contributor: Hobart King

Lapis Lazuli A metamorphic rock, gem material, and mineral pigment that obtains its blue color from the mineral lazurite.

Lapis Lazuli Gemstones: As a general rule, solid blue lapis or solid blue with a few grains of gold pyrite are the most desirable colors. In the photo above the bottom two cabochons approach that ideal. The large cabochon on the top right has a few thin veins of calcite and some calcite mottling. This stone is attractive and some people might prefer it, but the calcite reduces its desirability for most people. The top left cabochon has large patches of calcite that are intergrown with blue lazurite to yield a faded denim color. It also contains many visible grains of pyrite. For most people, it would be the least desirable stone in the photo; however, some people will enjoy it. Desirability in lapis varies from stone to stone and from person to person.

What is Lapis Lazuli? Lapis lazuli, also known simply as "lapis," is a blue metamorphic rock that has been used by people as a gemstone, sculpting material, and ornamental material for thousands of years. Unlike most other gem materials, lapis lazuli is not a mineral. Instead, it is a rock composed of multiple minerals. The blue color of lapis lazuli is mainly derived from the presence of lazurite, a blue silicate mineral of the sodalite group with a chemical composition of (Na,Ca)8(AlSiO4)6(S,Cl,SO4,OH)2.

Lapis Lazuli - The Rock: This photo shows a specimen of marble in which small patches of lazurite and abundant crystals of pyrite have formed. This is a beautiful rock specimen, but its usefulness as a rough for cutting high-quality lapis lazuli cabochons or beads is limited because the amount of lazurite present at any location within the rock is lower than optimal. However, this type of rock can be dyed to look like reasonable quality lapis. This image © iStockphoto / Epitavi.

Geologic Occurrence of Lapis Lazuli Lapis lazuli forms near igneous intrusions where limestoneor marble has been altered by contact metamorphism or hydrothermal metamorphism. In these rocks, lazurite replaces portions of the host rock and often preferentially develops within certain bands or layers. Afghanistan is the world's leading source of lapis lazuli. Some parts of the country have been actively mined for thousands of years. Other countries that produce notable amounts of lapis lazuli include Chile, Russia, Canada, Argentina, and Pakistan. In the United States small amounts of lapis lazuli have been produced in California, Colorado, and Arizona. Physical Properties of Lapis Lazuli Classification

A metamorphic rock that contains enough of the mineral lazurite to impart a distinct blue color. It may also contain significant amounts of calcite, pyrite, and minor amounts of other minerals.

Color

Blue. Often with white calcite veining or mottling, and gold grains of pyrite.

Streak

Blue.

Luster

Dull, but polishes to a bright luster.

Diaphaneity Cleavage

Semi-translucent to opaque. None, though it may split easily along foliation or calcite veins and layers.

Mohs Hardness

Varies between the 3 of calcite and the 5 to 5.5 of lazurite. Not well suited for use as a ring stone or in bracelets.

Specific Gravity

2.7 to 2.9 or more depending upon the amount of pyrite

Diagnostic Properties Uses

Blue color, association with pyrite, and hardness.

Cabochons, beads, carvings, spheres, inlay, and pigments.

Banded Lapis: A piece of rough lapis lazuli showing distinct calcite banding and pyrite on a fracture face. Image © iStockphoto / J-Palys.

Composition and Properties of Lapis In addition to lazurite, specimens of lapis lazuli usually contain calcite and pyrite. Sodalite, hauyne, wollastonite, afghanite, mica, dolomite, diopside, and a diversity of other minerals might also be present. To be called "lapis lazuli," a rock must have a distinctly blue color and contain at least 25% blue lazurite. Calcite is often the second most abundant mineral present in lapis lazuli. Its presence can be very obvious, appearing as white layers, fractures, or mottling. It can also be finely intermixed with lazurite to produce a rock with a faded denim color. Pyrite usually occurs in lapis lazuli as tiny, randomly spaced grains with a contrasting gold color. When abundant, the grains can be concentrated or intergrown into distinct layers or patches. It can occasionally occur as a fracture-filling mineral. As a rock, lapis lazuli is composed of several minerals, each with its own hardness, cleavage/fracture characteristics, specific gravity, and color. Hardness ranges from a Mohs 3 for calcite to the 6.5 of pyrite. The hardness of the material depends upon where you test it.

Ancient lapis pendant: A Mesopotamian pendant made of lapis lazuli, c. 2900 BC. Public domain image by Randy Benzie.

Lapis Lazuli History Lapis lazuli has been popular through most of recorded human history. Mining for lapis occurred in the Badakhshan Province of northeastern Afghanistan as early as 7000 BC. The lapis was used to make beads, small jewelry items and small sculptures. These have been found at Neolithic archaeological sites dating back to about 3000 BC in Iraq, Pakistan, and Afghanistan. Lapis lazuli appears in many Egyptian archaeological sites that date back to about 3000 BC. It was used in many ornamental objects and jewelry. Powdered lapis was used as a cosmetic and a pigment. In Biblical times the word "sapphire" was often used as a name for lapis lazuli. For that reason, many scholars believe that at least some of the references to sapphire in the Bible are actually references to lapis lazuli. Some modern translations of the Bible use the word "lapis" instead of "sapphire." Lapis lazuli started to be seen in Europe during the Middle Ages. It arrived in the form of jewelry, cutting rough, and finely ground pigment. Today lapis lazuli is still used in jewelry and ornamental objects. As a pigment it has been replaced with modern materials except by artists who strive to use historical methods.

Lazurite Crystal: A crystal of lazurite on marble from Badakhshan Province, Afghanistan. The specimen is about 3.1 x 3.1 x 1.5 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Lapis Lazuli as a "Conflict Mineral"? Afghanistan has been one of the world's primary sources of lapis lazuli through most of recorded history. Most of the country's production comes from thousands of small mines in the Badakhshan Province. This is an area with a destitute economy, where opium poppy growing and gemstone mining are the only important sources of outside revenue. Much of the area where the lapis lazuli mining occurs is occupied by the Taliban and local members of the Islamic State. They operate illegal mines, attack other mines to capture their production, and demand protection payments from intimidated mine operators. Revenue from these activities is used to fund war and terrorism.

Numerous advocacy groups and some members of the Afghanistan government would like to see Afghanistan's lapis lazuli classified as an international "conflict mineral." This would require the country's government to track the production and sale of lapis lazuli from mine to market. It would also involve an international effort to keep illicit lapis lazuli from being traded. The Kimberly Process, used for tracking the flow of diamonds, would serve as a model for the tracking of illicit lapis lazuli.

Lapis and Turquoise Necklace: Lapis lazuli and turquoise beads in a necklace with sterling silver. Lapis lazuli and turquoise are a common pairing in beaded jewelry. Image © iStockphoto / Alexander Kuzovlev.

Use as a Gem and Ornamental Material Lapis lazuli is most widely known for its use as a gemstone. It is a popular material for cutting into cabochons and beads. It is also used in inlay or mosaic projects and often as a material for small sculptures. These uses made lapis the most popular opaque blue gemstone. Although personal preferences vary, the most popular lapis has a uniform, deep blue to violet blue color. Many people enjoy a few randomly placed grains of gold pyrite or a few fractures or mottles of white calcite. However, when pyrite or calcite is present in more than minor amounts, the desirability of the material and the value are significantly lowered. Gray inclusions or mottling also quickly lowers desirability. Lapis lazuli has some durability problems that limit its suitability for certain uses. Lapis has a Mohs hardness of about 5, which makes it very soft for use in a ring, cuff links or bracelet - especially if the top of the stone is raised above the top of the setting or bezel. In these uses, lapis will show signs of abrasion with continued use. Lapis is best used in earrings, pins, and pendants, where abrasion is less likely to occur. When stored as unmounted stones or in jewelry, lapis can be damaged if the pieces are not isolated from one another. Jewelry is best stored in separate boxes or bags, or in trays with separate compartments for each item. Loose cut stones should be stored in separate papers, in bags, or in gem containers where the stones will not rub or abrade one another.

Lapis Lazuli Spheres and Rough: Small blue spheres of lapis lazuli shown together with two pieces of high-quality, solid blue untreated lapis rough from Afghanistan. The spheres are approximately 14 to 15 millimeters in diameter. Image © iStockphoto / RobertKacpura.

Treatment of Lapis Lazuli Lapis lazuli is frequently treated after it is cut and before it is sold as finished gemstones, sculptures, or ornaments. Lapis lazuli is slightly porous and that allows it to accept and hold dye. Much of the material that enters the market has been treated with a blue dye to remove the visibility of white calcite. It is then frequently treated with wax or oil that improve the luster of polished surfaces and seal the dyed calcite.

Ultramarine Pigment: Photo looking down into a small jar of ultramarine pigment made from finely ground and beneficiated lapis lazuli.

Lapis Lazuli Used as a Pigment High-quality lapis lazuli has been used as a mineral pigment for over 1,000 years. Bright blue pieces of lapis are trimmed of impurities and ground to a fine powder; the powder can then be mixed with oil or another vehicle for use as a paint.

Higher-grade pigments can be produced by washing the powder with mild acid to remove calcite and dolomite that dilute the blue color. The material is then processed to remove grains of pyrite and other foreign minerals. This lapis-derived pigment was named "ultramarine blue," a name that has been subsequently used for hundreds of years. During the Renaissance and into the 1800s, paintings done with ultramarine blue were considered to be a luxury because of their high cost. High-quality lapis lazuli was mined in Afghanistan and transported to Europe to manufacture ultramarine blue. This costly pigment was normally used by only the most accomplished artists and those who had wealthy clients to support the additional expense. Ultramarine blue made from lapis lazuli is one of the few natural pigments with a permanent and vivid blue color, good opacity, and high stability. It has always been very expensive and today can sell for over $1,000 per pound. Starting in the mid-1800s, artists and chemists began developing synthetic blue pigments for use as alternatives to ultramarine blue made from lapis lazuli. Some of these pigments also bear the name "ultramarine." An artist who wants an ultramarine pigment made from lapis lazuli today must be sure that the pigment is not synthetic and is actually made from lapis lazuli. Synthetic ultramarine pigments have their advantages. Their blue color is usually deeper and more consistent than traditional ultramarine, and they also cost far less. Today, because of cost, very little ultramarine made from lapis lazuli is used, mainly by artists who are striving to learn historical techniques or achieve results similar to master painters of the past. It is prepared by a few pigment manufacturers who continue to use lapis lazuli from the historical sources in Afghanistan.

Paintings Done With Ultramarine Blue: Four well-known paintings done using ultramarine pigment. Clockwise from top right: The Starry Night by Vincent Van Gogh; Girl With a Pearl Earring by Johannes Vermeer; Bacchus and Ariadne by Titian; and, The Virgin in Prayer by Sassoferrato. All images are in the public domain and were obtained from Wikimedia.org.

Examples of Ultramarine in Paintings A few master painters (examples of which are provided below) considered the use of ultramarine and other costly pigments an essential part of producing paintings with optimum color. Vincent Van Gogh (1853-1890) used ultramarine to paint The Starry Night in 1889. The oil on canvas painting is considered to be one of his best works and is today in the collection of the Museum of Modern Art in New York City. It is a widely recognized painting. Johannes Vermeer (1632-1675) used ultramarine to paint the headscarf of the Girl with a Pearl Earring in about 1665. The oil on canvas painting has been exhibited at museums throughout the world, and also served as the inspiration for a novel and a film. It is currently in the collection of the Mauritshuis in The Hague. Titian (1488-1576) used ultramarine blue to paint the dramatic sky and draperies in his oil on canvas painting of Bacchus and Ariadne. The painting is now on display at the National Gallery in London. Many painters have used ultramarine blue to paint the robe of Mary, mother of Jesus. Giovanni Sassoferrato (1609-1685) produced one of the most vivid examples when he painted The Virgin in Prayer between 1640 and 1650. The oil on canvas painting is on exhibit at the National Gallery in London.

Quartzite The metamorphic rock composed almost entirely of quartz.

Quartzite: A specimen of quartzite showing its conchoidal fracture and granular texture. The specimen shown is about two inches (five centimeters) across.

What is Quartzite? Quartzite is a nonfoliated metamorphic rock composed almost entirely of quartz. It forms when a quartzrich sandstone is altered by the heat, pressure, and chemical activity of metamorphism. These conditions recrystallize the sand grains and the silica cement that binds them together. The result is a network of interlocking quartz grains of incredible strength. The interlocking crystalline structure of quartzite makes it a hard, tough, durable rock. It is so tough that it breaks through the quartz grains rather than breaking along the boundaries between them. This is a characteristic that separates true quartzite from sandstone.

Quartzite Under a Microscope: A specimen of the Bo Quartzite collected near South Troms, Norway, observed through a microscope in thin-section under cross-polarized light. The quartz grains in this view range in color from white to gray to black, and they form a tight interlocking network. Photograph by Jackdann88, used here under a Creative Commons license.

Physical Properties of Quartzite Quartzite is usually white to gray in color. Some rock units that are stained by iron can be pink, red, or purple. Other impurities can cause quartzite to be yellow, orange, brown, green, or blue. The quartz content of quartzite gives it a hardness of about seven on the Mohs Hardness Scale. Its extreme toughness made it a favorite rock for use as an impact tool by early people. Its conchoidal fracture allowed it to be shaped into large cutting tools such as ax heads and scrapers. Its coarse texture made it less suitable for producing tools with fine edges such as knife blades and projectile points.

Quartzite scree: A steep slope covered with an unstable blanket of quartzite scree. Scree is a name used for resistant pieces of broken rock that cover a talus slope. This photo was taken near Begunje na Gorenjskem, Slovenia. A Creative Commons image by Pinky sl.

Where Does Quartzite Form? Most quartzite forms during mountain-building events at convergent plate boundaries. There, sandstone is metamorphosed into quartzite while deeply buried. Compressional forces at the plate boundary fold and fault the rocks and thicken the crust into a mountain range. Quartzite is an important rock type in folded mountain ranges throughout the world.

Ridge-Forming Quartzite: An outcrop of the Chimney Rock Formation in Catoctin Mountain Park near Thurmont, Maryland. Catoctin Mountain is part of the Blue Ridge Mountains. The Chimney Rock Formation in this area caps many of the ridges, drapes the flanks of the mountains as scree, and is made up mostly of quartzite. Photo by Alex Demas, United States Geological Survey.

Quartzite as a Ridge-Former Quartzite is one of the most physically durable and chemically resistant rocks found at Earth's surface. When the mountain ranges are worn down by weathering and erosion, less-resistant and less-durable rocks are destroyed, but the quartzite remains. This is why quartzite is so often the rock found at the crests of mountain ranges and covering their flanks as a litter of scree. Quartzite is also a poor soil-former. Unlike feldspars which break down to form clay minerals, the weathering debris of quartzite is quartz. It is therefore not a rock type that contributes well to soil formation. For that reason it is often found as exposed bedrock with little or no soil cover.

Fuchsitic Quartzite: A specimen of quartzite that contains significant amounts of green fuchsite, a chromium-rich muscovite mica. This specimen measures about 7 centimeters across and was collected from a small abandoned quarry where the flaggy rocks were produced and cut for use as decorative stones. The quarry is in the Elmers Rock Greenstone Belt, Wyoming. Photograph by James St. John, used here under a Creative Commons license.

How the Name "Quartzite" Is Used Geologists have used the name "quartzite" in a few different ways, each with a slightly different meaning. Today most geologists who use the word "quartzite" are referring to rocks that they believe are metamorphic and composed almost entirely of quartz. A few geologists use the word "quartzite" for sedimentary rocks that have an exceptionally high quartz content. This usage is falling out of favor but remains in older textbooks and other older publications. The name "quartz arenite" is a more appropriate and less confusing name for these rocks. It is often difficult or impossible to differentiate quartz arenite from quartzite. The transition of sandstone into quartzite is a gradual process. A single rock unit such as the Tuscarora Sandstone might fully fit the definition of quartzite in some parts of its extent and be better called "sandstone" in other areas. Between these areas, the names "quartzite" and "sandstone" are used inconsistently and often guided by habit. It is often called "quartzite" when rock units above and below it are clearly sedimentary. This contributes to the inconsistency in the ways that geologists use the word "quartzite."

"Aventurine": Pieces of green, yellow, and reddish orange "aventurine" from India. These pieces of rough average about 1 inch across and were sold for making tumbled stones in a rock tumbler. Much of the "aventurine" sold for lapidary use is actually quartzite. Often it exhibits no aventurescence.

Hammer With Caution ! Wise geologists, who have memorable experiences with quartzites, hit them with a rock hammer only when necessary. If a freshly broken piece is needed for examination, they break off a small protrusion with a light tap. That small piece is usually more than enough. Don't hit quartzite hard with a rock hammer. It's not a good idea. If you must, be sure that you are wearing impact-resistant goggles, gloves, long sleeves, long pants, and sturdy shoes. A sharp hammer blow usually bounces off. That bounce can cause injury. When the rock does break, the impact often yields sparks and sharp pieces of rock traveling at high velocity. Be certain that nearby field partners are warned and safely away. Hold the base of your goggles with your free hand before striking the rock. That will protect the lower half of your face from sparks and sharp flakes of high velocity rock. You have been warned.

Quartzite Countertop: A kitchen island countertop made of quartzite. In the dimension stone industry, some quartzite is sold as "granite" because in that industry, any hard silicate rock is often called "granite." Image © iStockphoto and Theanthrope.

Quartzite arrowhead: Quartzite was often used as a tool by early people. It is durable enough for use as impact tools such as hammerstones. It breaks with a conchoidal fracture, which made it useful for tools with sharp edges, such as hoes, axes, and scrapers. Although it is very difficult to knap, some ancient people were able to knap it into knife blades and projectile points. The photo shows a quartzite arrowhead found in Alabama. If the arrowhead is turned under a bright light, the grains in the quartzite produce a sparkling luster.

Uses of Quartzite Quartzite has a diversity of uses in construction, manufacturing, architecture, and decorative arts. Although its properties are superior to many currently used materials, its consumption has always been low for various reasons. The uses of quartzite and some reasons that it is avoided are summarized below. Architectural Use

In architecture, marble and granite have been the favorite materials for thousands of years. Quartzite, with a Mohs hardness of seven along with greater toughness, is superior to both in many uses. It stands up better to abrasion in stair treads, floor tiles, and countertops. It is more resistant to most chemicals and environmental

conditions. It is available in a range of neutral colors that many people prefer. The use of quartzite in these uses is growing slowly as more people learn about it. Construction Use

Quartzite is an extremely durable crushed stone that is suitable for use in the most demanding applications. Its soundness and abrasion resistance are superior to most other materials. Unfortunately, the same durability that makes quartzite a superior construction material also limits its use. Its hardness and toughness cause heavy wear on crushers, screens, truck beds, cutting tools, loaders, tires, tracks, drill bits, and other equipment. As a result, the use of quartzite is mainly limited to geographic areas where other aggregates are not available. Manufacturing Use

Quartzite is valued as a raw material because of its high silica content. A few unusual deposits have a silica content of over 98%. These are mined and used to manufacture glass, ferrosilicon, manganese ferrosilicon, silicon metal, silicon carbide, and other materials. Decorative Use

Quartzite can be a very attractive stone when it is colored by inclusions. Inclusions of fuchsite (a green chromium-rich variety of muscovite mica) can give quartzite a pleasing green color. If the quartzite is semitransparent to translucent, the flat flakes of mica can reflect light to produce a glittering luster known as aventurescence. Material that displays this property is known as "aventurine," a popular material used to produce beads, cabochons, tumbled stones, and small ornaments. Aventurine can be pink or red when stained with iron. Included dumortierite produces a blue color. Other inclusions produce white, gray, orange, or yellow aventurine. Stone Tools

Quartzite has been used by humans to make stone tools for over one million years. It was mainly used for impact tools, but its conchoidal fracture allowed it to be broken to form sharp edges. Broken pieces of quartzite were used for crude cutting and chopping tools. Quartzite was not the preferred material for producing cutting tools. Flint, chert, jasper, agate, and obsidian all can be knapped to produce fine cutting edges, which are difficult to produce when working quartzite. Quartzite served as an inferior substitute for these preferred materials.

Slate What is Slate? What Minerals are in Slate? What is Slate Used For?

Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. The specimen shown above is about two inches (five centimeters) across.

What is Slate? Slate is a fine-grained, foliated metamorphic rock that is created by the alteration of shale or mudstone by low-grade regional metamorphism. It is popular for a wide variety of uses such as roofing, flooring, and flagging because of its durability and attractive appearance.

Composition of Slate Slate is composed mainly of clay minerals or micas, depending upon the degree of metamorphism to which it has been subjected. The original clay minerals in shale alter to micas with increasing levels of heat and pressure. Slate can also contain abundant quartz and small amounts of feldspar, calcite, pyrite, hematite, and other minerals.

Slate roof: Most of the slate mined throughout the world is used to produce roofing slates. Slate performs well in this application because it can be cut into thin sheets, absorbs minimal moisture, and stands up well in contact with freezing water. A disadvantage is the cost of the slate and its installation in comparison with other roofing materials. As a result, in new construction slate is mainly confined to high-end projects and prestige architecture. Image © iStockphoto / Iain Sarjeant.

Color of Slate Most slates are gray in color and range in a continuum of shades from light to dark gray. Slate also occurs in shades of green, red, black, purple, and brown. The color of slate is often determined by the amount and type of iron and organic material that are present in the rock.

How Does Slate Form? The tectonic environment for producing slate is usually a former sedimentary basin that becomes involved in a convergent plate boundary. Shales and mudstones in that basin are compressed by horizontal forces with minor heating. These forces and heat modify the clay minerals in the shale and mudstone. Foliation develops at right angles to the compressive forces of the convergent plate boundary to yield a vertical foliation that usually crosses the bedding planes that existed in the shale.

School slate: School slate used for writing practice and arithmetic. Students wrote on the slate with a "pencil" made from slate, soapstone, or clay. These slates were widely used until the late 1800s, when wood-case pencils were easily produced and the price of paper became affordable. Image © iStockphoto / Bruce Lonngren.

Uses of the Word "Slate" The word "slate" has not been used consistently over time and in some industries. Today most geologists are careful not to use the word "slate" when talking about "shale." However, in the past the word slate was often used freely in reference for shale. This confusion of terms partially arises from the fact that shale is progressively converted into slate. Imagine driving your car eastwards in Pennsylvania through areas of increasing metamorphism, starting where the rock is definitely "shale" and stopping to examine rock at each outcrop. You will have a difficult time deciding where on that route "shale" has been converted into "slate." It can be difficult to pick up a rock and apply the proper name where the rocks have been lightly metamorphosed. In the coal mining industry of the Appalachian Basin, the word "slate" is still used by many miners in reference to the shale that forms the roof and floor of a mine, and for fragments of shale that are separated from the coal in preparation plants. Experienced miners train newer miners, and archaic language is passed along.

In the 1800s, elementary school students used a small piece of slate mounted in a wooden frame for writing practice and arithmetic problems. Writing was done with a small pencil made of slate, soapstone, or clay. The slate could be wiped clean with a soft cloth. Small slates were also used in schools and businesses to list daily events, schedules, menus, prices, and other notices. Today, over 150 years after writing slates started to disappear from schools, the word "slate" is still used in phrases such as "clean slate," "wipe the slate clean," "slated for today," "put it on the slate" and more.

Slate siding: Slate is sometimes used as facing stone on building exteriors. Image © iStockphoto / John Bloor.

Slaty Cleavage Foliation in slate is caused by the parallel orientation of platy minerals in the rock, such as microscopic grains of clay minerals and mica. These parallel mineral grain alignments give the rock an ability to break smoothly along planes of foliation. People exploit this property of slate to produce thin sheets of slate that are used in construction projects and manufacturing.

Slate tile flooring: Slate is a durable rock that is suitable for use as flooring, stair treads, sidewalk slabs, and patio stone. It is also produced in a variety of colors that allow it to be incorporated into a variety of design projects. Shown above are multi-color flooring tiles. Image © iStockphoto / Chad Truemper.

Uses of Slate Most of the slate mined throughout the world is used to produce roofing slates. Slate performs well in this application because it can be cut into thin sheets, absorbs minimal moisture, and stands up well in contact with freezing water. A disadvantage is the cost of the slate and its installation in comparison with other roofing materials. As a result, in new construction slate is mainly confined to high-end projects and prestige architecture. Slate is also used for interior flooring, exterior paving, dimension stone, and decorative aggregate. Small pieces of slate are also used to make turkey calls. The photos on this page document several uses of slate. Historically slate has been used for chalkboards, student writing slates, billiard tables, cemetery markers, whetstones, and table tops. Because it is a good electrical insulator, it was also used for early electric panels and switch boxes. Contributor: Hobart King

Schist A foliated metamorphic rock that contains abundant platy mineral grains.

Muscovite schist: The dominant visible mineral in this schist is muscovite. Its platy grains are aligned in a common orientation, and that allows the rock to be split easily in the direction of the grain orientation. The specimen shown is about two inches (five centimeters) across.

What is Schist? Schist is a foliated metamorphic rock made up of plate-shaped mineral grains that are large enough to see with an unaided eye. It usually forms on a continental side of a convergent plate boundary where sedimentary rocks, such as shales and mudstones, have been subjected to compressive forces, heat, and chemical activity. This metamorphic environment is intense enough to convert the clay minerals of the sedimentary rocks into platy metamorphic minerals such as muscovite, biotite, and chlorite. To become schist, a shale must be metamorphosed in steps through slate and then through phyllite. If the schist is metamorphosed further, it might become a granular rock known as gneiss. A rock does not need a specific mineral composition to be called “schist.” It only needs to contain enough platy metamorphic minerals in alignment to exhibit distinct foliation. This texture allows the rock to be broken into thin slabs along the alignment direction of the platy mineral grains. This type of breakage is known as schistosity. In rare cases the platy metamorphic minerals are not derived from the clay minerals of a shale. The platy minerals can be graphite, talc, or hornblende from carbonaceous, basaltic, or other sources.

Chlorite schist: A schist with chlorite as the dominant visible mineral is known as a "chlorite schist." The specimen shown is about two inches (five centimeters) across.

How Does Schist Form? Schist is a rock that has been exposed to a moderate level of heat and a moderate level of pressure. Let’s trace its formation from its protoliths - the sedimentary rocks from which it forms. These are usually shales or mudstones. In the convergent plate boundary environment, heat and chemical activity transform the clay minerals of shales and mudstones into platy mica minerals such as muscovite, biotite, and chlorite. The directed pressure pushes the transforming clay minerals from their random orientations into a common parallel alignment where the long axes of the platy minerals are oriented perpendicular to the direction of the compressive force. This transformation of minerals marks the point in the rock’s history when it is no longer sedimentary but becomes the low-grade metamorphic rock known as “slate.” Slate is has a dull luster, it can be split into thin sheets along the parallel mineral alignments, and the thin sheets will ring when they are dropped onto a hard surface. If the slate is exposed to additional metamorphism, the mica grains in the rock will begin to grow. The grains will elongate in a direction that is perpendicular to the direction of compressive force. This alignment and increase in mica grain size gives the rock a silky luster. At that point the rock can be called a “phyllite.” When the platy mineral grains have grown large enough to be seen with the unaided eye, the rock can be called “schist.” Additional heat, pressure, and chemical activity might convert the schist into a granular metamorphic rock known as “gneiss.”

Garnet mica schist: This rock is composed of fine-grained muscovite mica with numerous visible grains of red garnet. The specimen shown is about two inches (five centimeters) across.

Emeralds in mica schist: Photograph of emerald crystals in mica schist from the Malyshevskoye Mine, Sverdlovsk Region, Southern Ural, Russia. The large crystal is about 21 millimeters in length. Photograph © iStockphoto and Epitavi.

Types of Schist and Their Composition As explained above, mica minerals such as chlorite, muscovite, and biotite are the characteristic minerals of schist. These were formed through metamorphism of the clay minerals present in the protolith. Other common minerals in schist include quartz and feldspars that are inherited from the protolith. Micas, feldspars, and quartz usually account for most of the minerals present in a schist. Schists are often named according to the eye-visible minerals of metamorphic origin that are obvious and abundant when the rock is examined. Muscovite schist, biotite schist, and chlorite schist (often called “greenstone”) are commonly used names. Other names based upon obvious metamorphic minerals are garnet schist, kyaniteschist, staurolite schist, hornblende schist, and graphiteschist. Some names used for schist often consist of three words, such as garnet graphite schist. In these cases the dominant metamorphic mineral’s name is used second, and the less abundant mineral name is used first. Garnet graphite schist is a schist that contains graphite as its dominant mineral, but abundant garnet is visible and present.

Garnet mica schist in thin section: This is a microscopic view of a garnet grain that has grown in schist. The large black grain is the garnet, the red elongate grains are mica flakes. The black, gray, and white grains are mostly silt or smaller size grains of quartz and feldspar. The garnet has grown by replacing, displacing, and including the mineral grains of the surrounding rock. You can see many of these grains as inclusions within the garnet. From this photo it is easy to understand why clean, gem-quality garnets with no inclusions are very hard to find. It is also hard to understand how garnet can grow into nice euhedral crystals under these conditions. Photo by Jackdann88, used here under a Creative Commons license.

Schist as a Construction Material Schist is not a rock with numerous industrial uses. Its abundant mica grains and its schistosity make it a rock of low physical strength, usually unsuitable for use as a construction aggregate, building stone, or decorative stone. The only exception is for its use as a fill when the physical properties of the material are not critical.

Schist as a Gem Material Host Rock Schist is often the host rock for a variety of gemstonesthat form in metamorphic rocks. Gemquality garnet, kyanite, tanzanite, emerald, andalusite, sphene, sapphire, ruby, scapolite, iolite, chryso beryl and many other gem materials are found in schist. Gem materials found in schist are often highly included. This is because their mineral crystals grow within the rock matrix, often including mineral grains of the host rock instead of replacing them or pushing them aside. The best metamorphic host rock for gem materials is usually limestone, which is easily dissolved or replaced when the gem materials are formed.

Soapstone What is Soapstone? How does it Form? How is it Used ?

Soapstone: A metamorphic rock that consists primarily of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, pyroxenes, and carbonates. It is a soft, dense, heat-resistant rock that has a high specific heat capacity. These properties make it useful for a wide variety of architectural, practical, and artistic uses.

Some Soapstone History People have quarried soapstone for thousands of years. Native Americans in eastern North America used the soft rock to make bowls, cooking slabs, smoking pipes, and ornaments as early as the Late Archaic Period (3000 to 5000 years ago). [1] Native Americans on the west coast traveled in canoes from the mainland to San Clemente Island (60 miles offshore!) to obtain soapstone for cooking bowls and effigy carving as early as 8000 years ago. [2] The people of Scandinavia began using soapstone during the Stone Age, and it helped them enter the Bronze Age when they discovered that it could be easily carved into molds for casting metal objects such as knife blades and spearheads. They were among the first to discover the ability of soapstone to absorb heat and radiate it slowly. That discovery inspired them to make soapstone cooking pots, bowls, cooking slabs, and hearth liners. Throughout the world, in locations where the soapstone is exposed at the surface, it was one of the first rocks to be quarried. Soapstone's special properties continue to make it the "material of choice" for a wide variety of uses.

Soapstone statue: The famous "Christ the Redeemer" statue that overlooks the city of Rio de Janeiro, Brazil is made of reinforced concrete and faced with soapstone. The statue is 120 feet tall and was built on Corcovado Mountain. CIA image.

Steatite: A traditional Inuit carving of a female's head done in black steatite, a very fine-grained variety of soapstone. Photo © iStockphoto / Pierre Chouinard.

What is Soapstone? Soapstone is a metamorphic rock that is composed primarily of talc, with varying amounts of chlorite, micas, amphiboles, carbonates, and other minerals. [4] Because it is composed primarily of talc it is usually very soft. Soapstone is typically gray, bluish, green, or brown in color, often variegated. Its name is derived from its "soapy" feel and softness. The name "soapstone" is often used in other ways. Miners and drillers use the name for any soft rock that is soapy or slippery to the touch. In the craft marketplace, sculptures and ornamental objects made from soft rocks such as alabaster or serpentine are often said to be made from "soapstone." Be careful when purchasing if the type of rock used in making the object is important to you. Many people use the name "steatite" interchangeably with "soapstone." However, some people reserve the name "steatite" for a fine-grained unfoliated soapstone that is nearly 100% talc and highly suited for carving.

Soapstone pencils: Talc is very soft and has a white streak. Since soapstone is made primarily of talc, it will deposit a white powder when it is rubbed against almost any object. This white mark is similar to talcum powder and is easily brushed off without leaving a permanent mark. Soapstone pencils are used by tailors to mark fabric. Soapstone markers are also used by welders. The heat-resistant powder does not burn away and continues to be visible when the workpiece is heated during the welding process.

How Does Soapstone Form? Soapstone most often forms at convergent plate boundaries where broad areas of Earth’s crust are subjected to heat and directed pressure. Peridotites, dunites, and serpentinites in this environment can be metamorphosed into soapstone. On a smaller scale, soapstone can form where siliceous dolostones are altered by hot, chemically active fluids in a process known as metasomatism.

Physical Properties of Soapstone Soapstone is composed primarily of talc and shares many physical properties with that mineral. These physical properties make soapstone valuable for many different uses. These useful physical properties include:       

soft and very easy to carve nonporous nonabsorbent low electrical conductivity heat resistant high specific heat capacity resistant to acids and alkalis

Soapstone is a rock, and its mineral composition can vary. Its composition depends upon the parent rock material and the temperature/pressure conditions of its metamorphic environment. As a result, the physical properties of the soapstone can vary from quarry to quarry and even within a single rock unit. The level of metamorphism sometimes determines its grain size. Soapstone with a fine grain size works best for highly detailed carvings. The presence of minerals other than talc and the level of metamorphism can influence its hardness. Some of the harder varieties of soapstone are preferred for countertops because they are more durable than a pure talc soapstone.

Soapstone bullet mold from the Revolutionary War era. The two halves of this mold would be placed together and secured with wooden sticks through the four holes. Then molten lead would be poured into the five bullet molds. The mold would be opened after cooling, the lead sprue would be cut from the bullet, and the bullet surface would be filed smooth. Soapstone was used to make bullet molds because it was easily carved, heat resistant, and durable enough to be used hundreds of times. Image from the Guilford Courthouse National Military Park, National Park Service.

How is Soapstone Used? The special properties of soapstone make it suitable, or the material of choice, for a wide variety of uses. A number of examples of soapstone use are explained below and in the photograph captions on this page.               

Countertops in kitchens and laboratories Sinks Cooking pots, cooking slabs, boiling stones Bowls and plates Cemetery markers Electrical panels Ornamental carvings and sculptures Fireplace liners and hearths Woodstoves Wall tiles and floor tiles Facing stone Bed warmers Marking pencils Molds for metal casting Cold stones

Soapstone countertops: The dark countertops and sink in this photo are made from soapstone. Soapstone is heat resistant, stain resistant, nonporous, and resistant to attack from acids and bases. It is often used as a natural stone countertop in kitchens and laboratories. Image © iStockphoto / Virginia Hamrick.

Soapstone Kitchen and Laboratory Countertops Soapstone is often used as an alternative natural stone countertop instead of granite or marble. In laboratories it is unaffected by acids and alkalis. In kitchens it is not stained or altered by tomatoes, wine, vinegar, grape juice, and other common food items. Soapstone is unaffected by heat. Hot pots can be placed directly on it without fear of melting, burning, or other damage. Soapstone is a soft rock, and it is easily scratched in countertop use. However, a gentle sanding and treatment with mineral oil will easily remove shallow scratches. Soapstone is not suitable for use as a workbench top where it will receive rough treatment and where sharp or abrasive objects will be placed upon it.

Soapstone electrical panels: Remains of the original 1907 soapstone control panel of the Cos Cob Power Plant near Greenwich, Connecticut. Thick slabs of soapstone were often used to hold high-voltage equipment and wiring because soapstone is heat resistant and does not conduct electricity. Image by Jet Lowe, Historic American Buildings Survey, National Park Service.

Soapstone Tiles and Wall Panels Soapstone tiles and panels are an excellent choice where heat and moisture are present. Soapstone is dense, without pores, does not stain, and repels water. Those properties make soapstone tiles and wall panels a good choice for showers, tub surrounds, and backsplashes. Soapstone is heat resistant and does not burn. That makes it an excellent wall covering behind wood-burning stoves and ovens. Fireplaces are also lined with soapstone to create a hearth that quickly absorbs heat and radiates it long after the fire is out. This property of soapstone was recognized in Europe over 1000 years ago, and many early hearths there were lined with soapstone.

Whiskystones are small soapstone cubes that are refrigerated and then used to chill a glass of whisky. They do not melt and dilute the drink. Since soapstone has a very high specific heat capacity and changes temperature very slowly, a few stones can keep a drink cold for 30 minutes or more.

Soapstone Woodstoves Soapstone does not burn or melt at wood-burning temperatures, and it has the ability to absorb heat, hold heat, and radiate heat. These properties make it an excellent material for making wood-burning stoves. The stove becomes hot and radiates that heat into the room. It also holds heat, keeping the coals hot and often allowing the owner to add more wood without the need for kindling.

Soapstone pipe: Native Americans have used soapstone to make smoking pipes and pipe bowls. They used soapstone because it is easy to carve and drill. Its high specific heat capacity enabled the outside of the bowl to have a lower temperature than the burning tobacco inside. Image © iStockphoto / Gill André.

Boiling stones: Native Americans made "boiling stones" from soapstone. Cooking was done in a small pit lined with a thick animal skin. A boiling stone would be placed in a nearby fire until it was very hot. A stick was then poked through the hole in the stone, and the stone was lifted from the fire, carried to the cooking pit, and dropped into the stew. National Park Service photo, Ocmulgee National Monument.

Soapstone bowls: Native Americans made cooking bowls from soapstone. These bowls would be placed in a fire and used to cook stews and meat. The mouth of the unbroken bowl is about four inches across. Soapstone worked well for this type of cooking because it is heat resistant and can withstand the heat of a wood fire. National Park Service photo, Grand Teton National Park.

Soapstone Cooking Pots Soapstone cooking pots absorb heat readily from the stove and radiate it into the soup or stew. Because their walls are thick, they take a little longer to heat than a thin metal pot. However, they heat their contents evenly and retain their heat when removed from the stove - the contents of the pot keep cooking until the pot itself begins to cool. Soapstone pots are highly prized by people who learn how to use them. Stone Age people made the first cooking pots from soapstone without the aid of metal tools. The soft rock could be worked with sharp stones, antlers, or bone. Skilled craftsmen carved the pots directly from the outcrop. Small soapstone pots were highly prized and traded widely. Large soapstone pots were very heavy and difficult to move. Archaeologists believe that large soapstone pots were used at sites where the residents had intentions of living there for a long time. Contributor: Hobart King

Soapstone Information [1] Origin of Soapstone within the Wissahickon Formation: Analyses of Native American Quarries along the Lower Patuxent River, Maryland; Rachel Burks, Steven Lev and Wayne Clark, Geological Society of America Abstracts with Programs, Vol. 38, No. 7, p. 234, October 2006. [2] California's Ancient Maritime Heritage, John W. Foster, California Department of Parks and Recreation, website article last accessed June 2016. [3] Soapstone Production through Norwegian History: Geology, Properties, Quarrying and Use; Per Storemyr and Tom Heldal; in: Asmosia 5: Interdisciplinary Studies on Ancient Stone; p. 359-369; J.J. Herrmann, N. Herz and R. Newman, editors; Archetype Publications Ltd., 2002. [4] Talc: The Softest Mineral: Website article by Geology.com staff, April 2012.

Soapstone ink well: Soapstone inkwell from the 1700s with the initials "AL" carved on one side. Image from Guilford Courthouse National Military Park, National Park Service.

Pictures of Sedimentary Rocks Photos of Common Clastic, Chemical, and Organic Sedimentary Rock Types.

Breccia is a clastic sedimentary rock that is composed of large (over two-millimeter diameter) angular fragments. The spaces between the large fragments can be filled with a matrix of smaller particles or a mineral cement which binds the rock together. The specimen shown above is about two inches (five centimeters) across.

What Are Sedimentary Rocks? Sedimentary rocks are formed by the accumulation of sediments. There are three basic types of sedimentary rocks. Clastic sedimentary rocks such as breccia, conglomerate, sandstone, siltstone, and shale are formed from mechanical weathering debris. Chemical sedimentary rocks, such as rock salt, iron ore, chert, flint, some dolomites, and some limestones, form when dissolved materials precipitate from solution. Organic sedimentary rocks such as coal, some dolomites, and some limestones, form from the accumulation of plant or animal debris.

Coal is an organic sedimentary rock that forms mainly from plant debris. The plant debris usually accumulates in a swamp environment. Coal is combustible and is often mined for use as a fuel. The specimen shown above is about two inches (five centimeters) across.

Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO 2). It occurs as nodules and concretionary masses, and less frequently as a layered deposit. It breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The specimen shown above is about two inches (five centimeters) across.

Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded particles. The space between the pebbles is generally filled with smaller particles and/or a chemical cement that binds the rock together. The specimen shown above is about two inches (five centimeters) across.

Flint is a hard, tough, chemical or biochemical sedimentary rock that breaks with a conchoidal fracture. It is a form of microcrystalline quartz that is typically called “chert” by geologists. It often forms as nodules in sedimentary rocks such as chalk and marine limestones.

Dolomite (also known as "dolostone" and "dolomite rock") is a chemical sedimentary rock that is very similar to limestone. It is thought to form when limestone or lime mud is modified by magnesium-rich ground water. The specimen shown above is about four inches (ten centimeters) across.

Limestone is a rock that is composed primarily of calcium carbonate. It can form organically from the accumulation of shell, coral, algal, and fecal debris. It can also form chemically from the precipitation of calcium carbonate from lake or ocean water. Limestone is used in many ways. Some of the most common are: production of cement, crushed stone, and acid neutralization. The specimen shown above is about two inches (five centimeters) across.

Iron Ore is a chemical sedimentary rock that forms when iron and oxygen (and sometimes other substances) combine in solution and deposit as a sediment. Hematite (shown above) is the most common sedimentary iron ore mineral. The specimen shown above is about two inches (five centimeters) across.

Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name "halite." It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food. The specimen shown above is about two inches (five centimeters) across.

Oil Shale is a rock that contains significant amounts of organic material in the form of kerogen. Up to 1/3 of the rock can be solid organic material. Liquid and gaseous hydrocarbons can be extracted from the oil shale, but the rock must be heated and/or treated with solvents. This is usually much less efficient than drilling rocks that will yield oil or gasdirectly into a well. The processes used for hydrocarbon extraction also produce emissions and waste products that cause significant environmental concerns.

Shale is a clastic sedimentary rock that is made up of clay-size (less than 1/256 millimeter in diameter) weathering debris. It typically breaks into thin flat pieces. The specimen shown above is about two inches (five centimeters) across.

Sandstone is a clastic sedimentary rock made up mainly of sand-size (1/16 to 2 millimeter diameter) weathering debris. Environments where large amounts of sand can accumulate include beaches, deserts, flood plains, and deltas. The specimen shown above is about two inches (five centimeters) across.

Siltstone is a clastic sedimentary rock that forms from silt-size (between 1/256 and 1/16 millimeter diameter) weathering debris. Specimens in the photo are about two inches (five centimeters) across.

Breccia What Is Breccia, How Does It Form, and What Is Its Composition?

Chert Breccia: The angular clasts in this breccia are chert fragments. The matrix is an iron-stained mix of clay- through sand-size particles. The specimen is about two inches (five centimeters) across.

What is Breccia? Breccia is a term most often used for clastic sedimentary rocks that are composed of large angular fragments (over two millimeters in diameter). The spaces between the large angular fragments can be filled with a matrix of smaller particles or a mineral cement that binds the rock together.

How Does Breccia Form? Breccia forms where broken, angular fragments of rock or mineral debris accumulate. One possible location for breccia formation is at the base of an outcrop where mechanical weathering debris accumulates. Another would be in stream deposits near the outcrop such as an alluvial fan. Some breccias form as debris flow deposits. The angular particle shape reveals that they have not been transported very far (transport wears the sharp points and edges of angular particles into rounded shapes). After deposition, the fragments are bound together by a mineral cement or by a matrix of smaller particles that fills the spaces between the fragments.

Limestone Breccia: A breccia that contains clasts of multiple types of limestone. Specimen is about four inches (ten centimeters) across.

How Does Breccia Differ From Conglomerate? Breccia and conglomerate are very similar rocks. They are both clastic sedimentary rocks composed of particles larger than two millimeters in diameter. The difference is in the shape of the large particles. In breccia the large particles are angular in shape, but in conglomerate the particles are rounded. This reveals a difference in how far the particles were transported. Near the outcrop where the fragments were produced by mechanical weathering, the shape is angular. However, during transport by water away from the outcrop, the sharp points and edges of those angular fragments are rounded. The rounded particles would form a conglomerate.

Debris Flow Breccia: Outcrop of a breccia thought to have formed from debris flow deposits in Death Valley National Park. The largest clasts are about three feet (one meter) across and are thought to be from the Noonday Dolomite. United States Geological Survey image.

What is Breccia's Composition? Breccia has many compositions. Its composition is mainly determined by the rock and mineral material that the angular fragments were produced from. The climate of the source area can also influence composition. Most breccias are a mix of rock fragments and mineral grains. The type of rock that the fragments were

produced from is often used as an adjective when referring to the rock. Some examples: sandstone breccia, limestone breccia, granite breccia, chert breccia, basalt breccia, and others. Often a breccia will contain many types of angular rock fragments. These are known as polymict breccias or polymictic breccias.

Talus Slopes: Scene of a mountain environment where talus, the angular mechanical weathering debris that might form breccia, is produced in abundance. Panorama from Kearsarge Pass looking east over Big Pothole Lake into the Owens Valley. Image © iStockphoto / Tom Grundy.

What Color is Breccia? Breccia can be any color. The color of the matrix or cement along with the color of the angular rock fragments determine its color. Breccia can be a colorful rock, as shown in the photos on this page.

Alluvial Fan: An alluvial fan in Death Valley National Park. Material on the fan was weathered from the mountains in the background and transported a very short distance. United States Geological Survey image.

Impact Breccia: A 457.7-gram breccia specimen from the Popigai impact crater in northern Siberia. Note the variety of colors, sizes, shapes, and textures within a single mass - the result of a major meteorite impact which threw millions of tons of rock into the air. As fragments fell back to Earth, rocks from different strata were mixed together. Photograph by Geoffrey Notkin © Aerolite Meteorites.

Is the Word "Breccia" Used in Other Ways? Geologists have been very generous in their use of the word "breccia." It is common to hear the term used when referring to a rock or rock debris made up of angular fragments. Although it is mainly used for rocks of sedimentary origin, it can be used for other types of rocks. A few more uses of the word are given below. Collapse

Breccia:

Broken

rock

that

originates

from

a

cavern

or

magma

chamber

collapse.

Fault Breccia: Broken rock found in the contact area between two fault blocks and produced by movement of the fault. Flow Breccia: A lava texture produced when the crust of a lava flow is broken and jumbled during movement. Igneous Breccia: A term used for a rock composed of angular fragments of igneous rocks. "Flow breccia" and "pyroclastic breccia" could be called "igneous breccia." Impact Breccia: A deposit of angular rock debris produced by the impact of an asteroid or other cosmic body. See an article about "impactites." Pyroclastic Breccia: A term used for a deposit of igneous rock debris that was ejected by a volcanic blast or pyroclastic flow.

When you hear the word "breccia" used in reference to a rock or rock material, it is fairly safe to assume that it means angular-shaped pieces.

What are the Uses of Breccia? The rock, breccia, has very few uses. However, the word "breccia" is used as a trade name for a group of dimension stone products with a broken, angular pattern. Names such as "Breccia Oniciata," "Breccia Pernice," and "Breccia Damascata" are cut and polished limestones and marbles that reveal a broken, angular

pattern. These breccias are used as architectural stones for interior building veneers, tiles, window sills, and other decorative applications. These are proprietary names applied to the rock from specific quarries. Contributor: Hobart King

Coal What Is Coal and How Does It Form?

Bituminous Coal: Bituminous coal is typically a banded sedimentary rock. In this photo you can see bright and dull bands of coal material oriented horizontally across the specimen. The bright bands are well-preserved woody material, such as branches or stems. The dull bands can contain mineral material washed into the swamp by streams, charcoal produced by fires in the swamp, or degraded plant materials. This specimen is approximately three inches across (7.5 centimeters). Photo by the West Virginia Geological and Economic Survey.

What is Coal? Coal is an organic sedimentary rock that forms from the accumulation and preservation of plant materials, usually in a swamp environment. Coal is a combustible rock and, along with oil and natural gas, it is one of the three most important fossil fuels. Coal has a wide range of uses; the most important use is for the generation of electricity.

Coal-Forming Environments: A generalized diagram of a swamp, showing how water depth, preservation conditions, plant types, and plant productivity can vary in different parts of the swamp. These variations will yield different types of coal. Illustration by the West Virginia Geological and Economic Survey.

Peat: A mass of recently accumulated to partially carbonized plant debris. This material is on its way to becoming coal, but its plant debris source is still easily recognizable.

How Does Coal Form? Coal forms from the accumulation of plant debris, usually in a swamp environment. When a plant dies and falls into the swamp, the standing water of the swamp protects it from decay. Swamp waters are usually deficient in oxygen, which would react with the plant debris and cause it to decay. This lack of oxygen allows the plant debris to persist. In addition, insects and other organisms that might consume the plant debris on land do not survive well under water in an oxygen-deficient environment. To form the thick layer of plant debris required to produce a coal seam, the rate of plant debris accumulation must be greater than the rate of decay. Once a thick layer of plant debris is formed, it must be buried by sediments such as mud or sand. These are typically washed into the swamp by a flooding river. The weight of these materials compacts the plant debris and aids in its transformation into coal. About ten feet of plant debris will compact into just one foot of coal. Plant debris accumulates very slowly. So, accumulating ten feet of plant debris will take a long time. The fifty feet of plant debris needed to make a five-foot thick coal seam would require thousands of years to accumulate. During that long time, the water level of the swamp must remain stable. If the water becomes too deep, the plants of the swamp will drown, and if the water cover is not maintained the plant debris will decay. To form a coal seam, the ideal conditions of perfect water depth must be maintained for a very long time. If you are an astute reader you are probably wondering: "How can fifty feet of plant debris accumulate in water that is only a few feet deep?" The answer to that question is the primary reason that the formation of a coal seam is a highly unusual occurrence. It can only occur under one of two conditions: 1) a rising water level that perfectly keeps pace with the rate of plant debris accumulation; or, 2) a subsiding landscape that perfectly keeps pace with the rate of plant debris accumulation. Most coal seams are thought to have formed under condition #2 in a delta environment. On a delta, large amounts of river sediments are being deposited on a small area of Earth's crust, and the weight of those sediments causes the subsidence. For a coal seam to form, perfect conditions of plant debris accumulation and perfect conditions of subsidence must occur on a landscape that maintains this perfect balance for a very long time. It is easy to understand

why the conditions for forming coal have occurred only a small number of times throughout Earth's history. The formation of a coal requires the coincidence of highly improbable events. Rank (From Lowest to Highest)

Properties

Peat

A mass of recently accumulated to partially carbonized plant debris. Peat is an organic sediment. Burial, compaction, and coalification will transform it into coal, a rock. It has a carbon content of less than 60% on a dry ash-free basis.

Lignite

Lignite is the lowest rank of coal. It is a peat that has been transformed into a rock, and that rock is a brown-black coal. Lignite sometimes contains recognizable plant structures. By definition it has a heating value of less than 8300 British Thermal Units per pound on a mineral-matter-free basis. It has a carbon content of between 60 and 70% on a dry ashfree basis. In Europe, Australia, and the UK, some low-level lignites are called "brown coal."

Sub Bituminous

Sub bituminous coal is a lignite that has been subjected to an increased level of organic metamorphism. This metamorphism has driven off some of the oxygen and hydrogen in the coal. That loss produces coal with a higher carbon content (71 to 77% on a dry ashfree basis). Sub bituminous coal has a heating value between 8300 and 13000 British Thermal Units per pound on a mineral-matter-free basis. On the basis of heating value, it is subdivided into sub bituminous A, sub bituminous B, and sub bituminous C ranks.

Bituminous

Bituminous is the most abundant rank of coal. It accounts for about 50% of the coal produced in the United States. Bituminous coal is formed when a sub bituminous coal is subjected to increased levels of organic metamorphism. It has a carbon content of between 77 and 87% on a dry ash-free basis and a heating value that is much higher than lignite or sub bituminous coal. On the basis of volatile content, bituminous coals are subdivided into low-volatile bituminous, medium-volatile bituminous, and high-volatile bituminous. Bituminous coal is often referred to as "soft coal"; however, this designation is a layman's term and has little to do with the hardness of the rock.

Anthracite

Anthracite is the highest rank of coal. It has a carbon content of over 87% on a dry ashfree basis. Anthracite coal generally has the highest heating value per ton on a mineralmatter-free basis. It is often subdivided into semi-anthracite, anthracite, and metaanthracite on the basis of carbon content. Anthracite is often referred to as "hard coal"; however, this is a layman's term and has little to do with the hardness of the rock.

Anthracite coal: Anthracite is the highest rank of coal. It has a bright luster and breaks with a semi-conchoidal fracture.

What is Coal "Rank"? Plant debris is a fragile material compared to the mineral materials that make up other rocks. As plant debris is exposed to the heat and pressure of burial, it changes in composition and properties. The "rank" of a coal is a measure of how much change has occurred. Sometimes the term "organic metamorphism" is used for this change. Based upon composition and properties, coals are assigned to a rank progression that corresponds to their level of organic metamorphism. The basic rank progression is summarized in the table here.

Lignite: The lowest rank of coal is "lignite." It is peat that has been compressed, dewatered, and lithified into a rock. It often contains recognizable plant structures.

What are the Uses of Coal? Electricity production is the primary use of coal in the United States. Most of the coal mined in the United States is transported to a power plant, crushed to a very small particle size, and burned. Heat from the burning

coal is used to produce steam, which turns a generator to produce electricity. Most of the electricity consumed in the United States is made by burning coal.

Coal-Fired Power Plant: Photo of a power plant where coal is burned to produce electricity. The three large stacks are cooling towers where water used in the electricity generation process is cooled before reuse or release to the environment. The emission streaming from the right-most stack is water vapor. The combustion products from burning the coal are released into the tall, thin stack on the right. Within that stack are a variety of chemical sorbents to absorb polluting gases produced during the combustion process. Image © iStockphoto / Michael Utech.

Coal has many other uses. It is used as a source of heat for manufacturing processes. For example, bricks and cement are produced in kilns heated by the combustion of a jet of powdered coal. Coal is also used as a power source for factories. There it is used to heat steam, and the steam is used to drive mechanical devices. A few decades ago most coal was used for space heating. Some coal is still used that way, but other fuels and coalproduced electricity are now used instead. Coke production remains an important use of coal. Coke is produced by heating coal under controlled conditions in the absence of air. This drives off some of the volatile materials and concentrates the carbon content. Coke is then used as a high-carbon fuel for metal processing and other uses where an especially hotburning flame is needed. Coal is also used in manufacturing. If coal is heated the gases, tars, and residues produced can be used in a number of manufacturing processes. Plastics, roofing, linoleum, synthetic rubber, insecticides, paint products, medicines, solvents, and synthetic fibers all include some coal-derived compounds. Coal can also be converted into liquid and gaseous fuels; however, these uses of coal are mainly experimental and done on a small scale. Contributor: Hobart King

Chert What Is Chert, How Does It Form, and What Is It Used For?

Chert: This specimen of chert is about two inches (five centimeters) across. It displays conchoidal fracture and has broken to produce sharp edges.

What is Chert? Chert is a microcrystalline or cryptocrystalline sedimentary rock material composed of silicon dioxide (SiO2). It occurs as nodules, concretionary masses, and as layered deposits. Chert breaks with a conchoidal fracture, often producing very sharp edges. Early people took advantage of how chert breaks and used it to fashion cutting tools and weapons. The name "flint" is also used for this material.

How Does Chert Form? Chert can form when microcrystals of silicon dioxide grow within soft sediments that will become limestone or chalk. In these sediments, enormous numbers of silicon dioxide microcrystals grow into irregularly-shaped nodules or concretions when dissolved silica is transported to the formation site by the movement of groundwater. If the nodules or concretions are numerous, they can enlarge and merge with one another to form a nearly continuous layer of chert within the sediment mass. Chert formed in this manner is a chemical sedimentary rock.

Diatoms are microscopic, single-celled algae that live in marine or fresh water. They produce hard parts made of silicon dioxide. NASA Image.

Some of the silicon dioxide in chert is thought to have a biological origin. In some oceans and shallow seas, large numbers of diatoms and radiolarians live in the water. These organisms have a glassy silica skeleton. Some sponges also produce "spicules" that are composed of silica. When these organisms die, their silica skeletons fall to the bottom, dissolve, recrystallize, and might become part of a chert nodule or chert layer. Chert formed in this way could be considered a biological sedimentary rock.

Marble Bar Chert: Outcrop of the 3.4 Ga Marble Bar Chert, Pilbara Craton, Australia. The hematite-rich chert has been used as evidence of high levels of atmospheric oxygen in the early Archean. Image by NASA Astrobiological Institute.

What is Chert's Composition? Chert is a microcrystalline silicon dioxide (SiO2). As chert nodules or concretions grow within a sediment mass, their growth can incorporate significant amounts of the surrounding sediment as inclusions. These inclusions can impart a distinctive color to the chert.

What Color is Chert? Chert occurs in a wide variety of colors. Continuous color gradients exist between white and black or between cream and brown. Green, yellow, and red cherts are also common. The darker colors can result from inclusions of sediment or organic matter. The name "flint" is often used in reference to the darker colors of chert. Red to reddish brown cherts receive their color from included iron oxide. The name "jasper" is frequently used for these reddish cherts.

Chert Arrowhead: A chert (flint) arrowhead bound to a wooden arrow shaft with sinew. Image © iStockphoto / Brian Brockman.

Flintlock: A close-up of the lock of a flintlock rifle, a weapon of the 18th century used in the Revolutionary War. Note the piece of chert (flint) in the hammer. Image © iStockphoto / Kakupacal.

Chert cabochons: Occasionally, specimens of chert with attractive colors or interesting patterns are cut as gemstones. These chert cabochons are examples.

What are the Uses of Chert? Chert has very few uses today; however, it was a very important tool-making material in the past. Chert has two properties that made it especially useful: 1) it breaks with a conchoidal fracture to form very sharp edges, and, 2) it is very hard (7 on the Mohs Scale). The edges of broken chert are sharp and tend to retain their sharpness because chert is a very hard and very durable rock. Thousands of years ago people discovered these properties of chert and learned how to intentionally break it to produce cutting tools such as knife blades, arrowheads, scrapers, and ax heads. Tons of chert fragments have been found at locations where these objects were produced in what was one of the earliest manufacturing activities of people. Chert is not found everywhere. It was a precious commodity that early people traded and transported long distances. As early as 8000 BC, the people of what are now England and France dug shafts up to 300 feet deep into layers of soft chalk to mine chert nodules. These are some of the oldest mining operations ever discovered. Chert is a very hard material that produces a spark when it is struck against steel. The heat from this spark can be used to start fires. A "flintlock" is an early firearm in which a charge of gunpowder is ignited by a flint hammer striking a metal plate (see photo). A variety of metamorphosed chert known as "novaculite" has a porous, even texture that makes it useful as a sharpening stone. The Arkansas Novaculite Formation has become world famous as a source of high-quality sharpening stones and novaculite abrasive products.

Conglomerate What Is Conglomerate? How Does It Form? What Is It Used For?

Conglomerate: The specimen shown is about two inches (five centimeters) across. It is made up of chert and limestone clasts bound in a matrix of sand and clay.

What is Conglomerate? Conglomerate is a clastic sedimentary rock that contains large (greater than two millimeters in diameter) rounded clasts. The space between the clasts is generally filled with smaller particles and/or a chemical cement that binds the rock together.

What is the Composition of Conglomerate? Conglomerate can have a variety of compositions. As a clastic sedimentary rock, it can contain clasts of any rock material or weathering product that is washed downstream or down current. The rounded clasts of conglomerate can be mineral particles such as quartz, or they can be sedimentary, metamorphic, or igneous rock fragments. The matrix that binds the large clasts together can be a mixture of sand, mud, and chemical cement.

Conglomerate-Forming Environment: A beach where strong waves have deposited rounded, cobble-size rocks. If buried and lithified, these materials might be transformed into a conglomerate. Image © iStockphoto / Jason van der Valk.

Conglomerate-Size Sediment Clasts: Pebble-size clasts of many compositions deposited together on a beach. Quartz, sandstone, and limestone clasts are all easily recognizable. Largest clast is about two inches (five centimeters) across. Image © iStockphoto / Ivan Ivanov.

How Does Conglomerate Form? Conglomerate forms where sediments of rounded clasts at least two millimeters in diameter accumulate. It takes a strong water current to transport and shape particles this large. So the environment of deposition might be along a swiftly flowing stream or a beach with strong waves. There must also be a source of large-size sediment particles somewhere up current. The rounded shape of the clasts reveals that they were tumbled by running water or moving waves. In September 2012, NASA's Mars rover Curiosity discovered an outcrop of conglomerate exposed on the surface of Mars. The rounded clasts within the conglomerate provide evidence that a stream or a beach had moved the rocks and tumbled them into rounded pebbles. This conglomerate was the most convincing evidence that water once flowed on the surface of Mars. See photo below.

Martian Conglomerate: This image was acquired by NASA's Curiosity rover on the surface of Mars. It shows an outcrop of conglomerate and some pebble-size weathering debris. The round pebbles are too large to have been moved and shaped by wind, thus they had to have been transported a significant distance by water. This photo from September 2012 was the strongest evidence of the existence of water on Mars that had been obtained at that time.

Conglomerates often begin by being deposited as a sediment consisting mainly of pebble and cobble-size clasts. The finer-size sand and clay, which fill the spaces between the larger clasts, is often deposited later on top of the large clasts and then sifts down between them to fill the interstitial spaces. The deposition of a chemical cement then binds the sediment into a rock.

Conglomerate Close-Up: A detailed view of conglomerate showing the pebble-size clasts with sand and smaller size particles filling the spaces between them. The largest pebbles in this view are about ten millimeters across. Image by the United States Geological Survey.

What is Conglomerate Used For? Conglomerate has very few commercial uses. Its inability to break cleanly makes it a poor candidate for dimension stone, and its variable composition makes it a rock of unreliable physical strength and durability. Conglomerate can be crushed to make a fine aggregate that can be used where a low-performance material is suitable. Many conglomerates are colorful and attractive rocks, but they are only rarely used as an ornamental stone for interior use.

Analysis of conglomerate can sometimes be used as a prospecting tool. For example, most diamond deposits are hosted in kimberlite. If a conglomerate contains clasts of kimberlite, then the source of that kimberlite must be somewhere upstream. Contributor: Hobart King

Flint A hard, tough material that humans have used to make tools for millions of years

Flint arrowheads: One of the most common uses of flint by prehistoric people was in the making of arrowheads. They were hard, tough and very sharp. Image by Derek McLean.

What is Flint? Flint is a hard, tough chemical or biochemical sedimentary rock that breaks with a conchoidal fracture. It is a form of microcrystalline quartz that is typically called “chert” by geologists. It often forms as nodules in sedimentary rocks such as chalk and marine limestones. The nodules can be dispersed randomly throughout the rock unit but are often concentrated in distinct layers. Some rock units form through the accumulation of siliceous skeletal material. These can recrystallize to form a layer of bedded flint. Flint is highly resistant to weathering and is often found as pebbles or cobbles along streams and beaches.

Flintknapping: Prehistoric people became highly skilled at flintknapping, a method of shaping flint into useful objects such as drills, arrowheads, knife blades, and spearheads. National Park Service image.

A Preferred Material for Making Tools Flint has been used by humans to make stone tools for at least two million years. [1] The conchoidal fracture of flint causes it to break into sharp-edged pieces. Early people recognized this property of flint and learned how to fashion it into knife blades, spear points, arrowheads, scrapers, axes, drills, and other sharp tools using a method known as flintknapping. If these tools were broken or damaged in use, they were often reshaped into smaller tools of similar function. The value of flint for making sharp tools was discovered and utilized by Stone Age people in almost every early culture located where flint could easily be found. Their survival depended upon having a durable material that could be used to produce sharp tools.

Flint knife: A lithic knife made from flint. Photo © iStockphoto / Martin Vallière.

Ohio Flint: The Vanport Flint has been quarried by people for at least 12,000 years. It outcrops in a layer between one and twelve feet thick along Flint Ridge in eastern Ohio. Native Americans produced the flint from hundreds of quarries along the ridge. Some of these people travelled hundreds of miles to collect the flint, used it to make a variety of tools and weapons, and traded it widely throughout what is now the eastern United States.

Flint Ridge Quarries, Ohio One of the most important localities for flint in eastern North America is Flint Ridge in eastern Ohio. Native Americans discovered this deposit and produced flint from hundreds of small quarries along the ridge. [2] This “Ohio flint” occurred in distinctive colors and was treasured by Native Americans. They travelled hundreds of miles to collect it and spread the distinctive material in trade across eastern North America. It has been found as artifacts as far south as the Gulf of Mexico and as far west as the Rocky Mountains. [3]

Alibates flint quarries: Heavily quarried landscape at the Alibates Flint Quarry National Monument. Over 700 quarries can still be seen today. These were all dug by hand without metal tools. National Park Service image.

Alibates flint: The Alibates Flint has been used by people of southwestern North America for about 13,000 years. The quarries used by these people have been preserved as part of the Alibates Flint Quarry National Monument. National Park Service image.

Alibates Flint Quarries In the area that is now the Texas panhandle, Native Americans discovered an area where weathered flint littered the ground. This flint was weathering out of a dolomite beneath the thin soil cover. These people discovered that fresh, unweathered flint of high quality could be obtained by digging down a few feet.

From about 13,000 years ago into the 1800's, this area was continuously mined for the high-quality flint. The flint was used to produce projectile points, scrapers, knives, and other stone tools. In the 1800's the flint was also mined for use as gunflints. Over 700 small quarries are still visible today and have been preserved as part of the Alibates Flint National Monument.

Grime's Graves flint mines: Shown in this satellite view are the remains of mining pits at the Grime's Graves flint mining complex, near Brandon, England. Neolithic people constructed vertical shafts down through the Cretaceous chalk to a layer of flint about 40 feet below the surface. Each shaft required the removal of about 2000 tonnes of chalk and required a team of workers several months to construct. About 60 tons of flint could be removed from each of these pits and the short horizontal excavations that followed the high-quality flint layer at the base. Starting about 3000 BC until about 1900 BC, these miners built over 400 shafts over an area of about 100 acres and removed thousands of tons of flint. [4] View Larger Map

Neolithic Flint Miners Perhaps the most impressive story about flint is that of the ancient mining complexes that were built in what is now England during Neolithic times. These excavations began about 4000 BC and continued until the widespread use of metals about 2,000 years later. [5] One flint mining complex of particular note was Grime's Graves located near Brandon, England. Here ancient miners dug shafts down through 40 feet of Cretaceous chalk to a layer of high-quality flint below. Each shaft was several feet in diameter and required the removal of about 2,000 tonnes of chalk. Most of the digging was done without metal tools, using red deer antlers as picks. Over 400 of these shafts were sunk over an area of about 100 acres. [6] Although these mining operations were amazing feats of engineering, just as impressive was the geological understanding of the workers. They knew that the flint was below the ground even though it did not outcrop anywhere in the immediate area. They also knew that the highest quality flint layer was below lower quality zones that were encountered during the early digging.

Flintlock: Close-up of a French flintlock rifle showing a flint ready to strike the steel frizzen, which will produce the spark needed to ignite the powder. Photo © iStockphoto / Michael Westhoff.

Flint as a Source of Fire Another important property of flint is its ability to generate sparks of hot material when it is struck against steel. This property allows flint to be used as a fire-starter. Skilled people can use a piece of flint, a piece of steel, and a little tinder to quickly start a fire. Early firearms, such as a flintlock, had a piece of flint attached to a spring-loaded hammer that was released when the trigger was pulled. The hammer struck a piece of steel known as a "frizzen" to create a shower of sparks that ignited a small pan of powder. That touched off the primary charge which exploded to propel the ball down the barrel.

Flint gemstone: Flint is often cut into dome-shaped stones known as cabochons. These can be set into pins, belt buckles, pendants, bolos, and other jewelry items.

Flint as a Gemstone Flint is a very durable material that accepts a bright polish and often occurs in attractive colors. It is occasionally cut into cabochons, beads, and baroque shapes for use as a gemstone. It is also used to produce tumbled stones in a rock tumbler.

Flint nodule: Flint is a variety of microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses and less frequently as a layered deposit. It breaks consistently with a conchoidal fracture and was one of the first materials used to make tools by early people. They used it to make cutting tools. After thousands of years, people continue to use it. It is presently used as the cutting edge in some of the finest surgical tools. This specimen is about four inches (ten centimeters) across and is from Dover Cliffs, England.

Chalk cliffs: Chalk cliffs can be an excellent place to find flint. As the soft chalk weathers away, flint nodules fall to the beach below. Image of chalk cliffs along the Baltic Sea, photo © iStockphoto / hsvrs.

Flint as a Construction Material Where flint is abundant it is sometimes used as a construction material. It is very durable and resists weathering better than almost any other natural stone. It is common to see walls, homes, and larger buildings that are built partially or entirely with flint as a facing stone in southern England and many parts of Europe.

Flint wall: A portion of a wall of a medieval building in Suffolk, UK, built with split flints. Photo © iStockphoto / John Woodcock. Flint Information [1] The World’s Oldest Stone Artefacts from Gona, Ethiopia: Their Implications for Understanding Stone Technology and Patterns of Human Evolution Between 2.6 and 1.5 Million Years Ago, Sileshi Semaw, Journal of Archaeological Science, Volume 27, pages 1197-1214, February

2000. [2] Flint: Ohio’s Official Gemstone: Garry L. Getz, Educational Leaflet Number 6, Ohio Geological Survey, 2012. [3] Ohio’s State Gemstone - Flint: Website article, Ohio History Central: An Online Encyclopedia of Ohio History, Ohio Historical Society, 1999. [4] The Neolithic Flint Mines of Sussex: Britain's Earliest Monuments, website article, Bournemouth University Archaeology Group, 2011. [5] Emmer Green (Hanover) South Chalk Mine Site Records, Subterranea Britannica, 2003. (This reference is provided for the excellent photographs of the underground workings showing the main flint seam.) [6] Grime's Graves Flint Mining Complex: Article from Wikipedia, the free encyclopedia, April 2012.

A Confusion of Names Flint is a microcrystalline variety of quartz. Materials of this description have been given a wide variety of names, including chert, jasper, agate, and chalcedony. Most geologists use the word "chert" for this material. Some people believe that the name "flint" should be reserved for dark-colored chert that formed as nodules in limestone or chalk. Some archaeologists believe that the name "flint" should only be used when the material has been fashioned into an artifact. The name "flint" has been so closely associated with starting fires that man-made materials used to produce sparks in cigarette lighters and survival kits have been given the name "flints." "Novaculite" is another similar material. It has a sedimentary origin that is similar to flint, but diagenesis and metamorphism have increased the size of the quartz microcrystals. It has been used for thousands of years for making sharp tools and weapons. Some specimens have a texture that make them useful as a sharpening stone. Contributor: Hobart King

Dolomite A sedimentary rock similar to limestone. Also known as "dolostone" and "dolomite rock."

"The Dolomites" are a mountain range in northeastern Italy and part of the Italian Alps. They are one of the largest exposures of dolomite rock on Earth - from which the name is obtained. The Dolomites are a UNESCO World Heritage Site. Image © iStockphoto / Dan Breckwoldt.

Dolomite: A Mineral and a Rock "Dolomite" is a word that is used by geologists in two different ways: 1) as the name of the mineral dolomite; and, 2) as the name of a rock known as dolomite, dolostone, or dolomite rock. This page is about dolomite rock. If you are looking for an article about the mineral, please go here.

Dolomite rock: A specimen of fine-grained dolomite rock from Lee, Massachusetts. It is about four inches (ten centimeters) across.

What is Dolomite? Dolomite, also known as "dolostone" and "dolomite rock," is a sedimentary rock composed primarily of the mineral dolomite, CaMg(CO3)2. Dolomite is found in sedimentary basins worldwide. It is thought to form by the postdepositional alteration of lime mud and limestone by magnesium-rich groundwater. Dolomite and limestone are very similar rocks. They share the same color ranges of white-to-gray and whiteto-light brown (although other colors such as red, green, and black are possible). They are approximately the same hardness, and they are both soluble in dilute hydrochloric acid. They are both crushed and cut for use as construction materials and used for their ability to neutralize acids.

Dolomitization Dolomite is very common in the rock record, but the mineral dolomite is rarely observed forming in sedimentary environments. For this reason it is believed that most dolomites form when lime muds or limestones are modified by postdepositional chemical change. Dolomite originates in the same sedimentary environments as limestone - warm, shallow, marine environments where calcium carbonate mud accumulates in the form of shell debris, fecal material, coral fragments, and carbonate precipitates. Dolomite is thought to form when the calcite(CaCO3) in carbonate mud or limestone is modified by magnesium-rich groundwater. The available magnesium facilitates the conversion of calcite into dolomite (CaMg(CO3)2). This chemical change is known as "dolomitization." Dolomitization can completely alter a limestone into a dolomite, or it can partially alter the rock to form a "dolomitic limestone."

Dolomite aggregate: Dolomite aggregate used for asphalt paving, from Penfield, New York. These specimens are approximately 1/2 inch to 1 inch (1.3 centimeters to 2.5 centimeters) across.

Identification in the Field and Classroom Dolomite is slightly harder than limestone. Dolomite has a Mohs hardness of 3.5 to 4, and limestone (composed of the mineral calcite) has a hardness of 3. Dolomite is slightly less soluble in dilute hydrochloric acid. Calcite will effervesce vigorously in contact with cold, dilute (5%) hydrochloric acid, while dolomite produces a very weak effervescence. These differences are often not significant enough to make a positive identification in the field. Distinguishing the rocks in the field is further complicated by a compositional continuum that ranges from limestone to dolomitic limestone to dolomite. A chemical analysis that determines the relative abundances of calcium and magnesium is needed to accurately name the rocks.

Dolostone: Photograph of a specimen of the Little Falls Dolostone from Herkimer County, New York. This dolostone is the host rock for the doubly-terminated quartz crystals known as "Herkimer Diamonds." It is vuggy, has a high silica content, and is much harder and tougher than the typical dolomite. The Herkimer Diamonds are found in petroleum-lined vugs in the rock unit. Part of a Herkimer Diamond is visible in the large vug on the left side of this specimen.

"Dolomite Rock" and "Dolostone" Some geologists are uncomfortable using the word "dolomite" for both a mineral and a rock of the same composition. They instead prefer using "dolomite rock" or "dolostone" when speaking of the sedimentary rock and "dolomite" when speaking of the mineral. Although these terms simplify communication and improve accuracy, many geologists continue to use the word "dolomite" for both the mineral and the rock.

Granular dolomite: A specimen of coarsely crystalline dolomitic marble from Thornwood, New York. This specimen is approximately 3 inches (6.7 centimeters) across.

Metamorphism of Dolomite Dolomite behaves like limestone when it is subjected to heat and pressure. It begins to recrystallize as the temperature rises. As this occurs, the size of the dolomite crystals in the rock increases, and the rock develops a distinctly crystalline appearance. If you examine the photo of granular dolomite, you will see that the rock is composed of easily recognizable dolomite crystals. The coarse crystalline texture is a sign of recrystallization, most often caused by metamorphism. Dolomite that has been transformed into a metamorphic rock is called "dolomitic marble."

Lime kiln: Dolomite and limestone have been heated in kilns to produce lime for thousands of years. This stone structure is the Olema Lime Kiln, located in Marin County, California. It was built in 1850 for the production of lime. National Park Service photo.

Uses of Dolomite Dolomite and limestone are used in similar ways. They are crushed and used as an aggregate in construction projects. They are kiln-fired in the manufacture of cement. They are cut into blocks and slabs for use as a dimension stone. They are calcined to produce lime. In some of these uses, dolomite is preferred. Its greater hardness makes it a superior construction material. Its lower solubility makes it more resistant to the acid content of rain and soil. The dolomitization process results in a slight volume reduction when limestone is converted into dolomite. This can produce a porosity zone in the strata where dolomitization has occurred. These pore spaces can be traps for subsurface fluids like oil and natural gas. This is why dolomite is often a reservoir rock that is sought in the exploration for oil and natural gas. Dolomite can also serve as a host rock for lead, zinc, and copper deposits. In the chemical industry, dolomite is used as a source of magnesia (MgO). The steel industry uses dolomite as a sintering agent in processing iron ore and as a flux in the production of steel. In agriculture, dolomite is used as a soil conditioner and as a feed additive for livestock. Dolomite is used in the production of glass and ceramics. Dolomite has been used as a minor source of magnesium, but today most magnesium is produced from other sources. Contributor: Hobart King

Limestone What Is Limestone and How Is It Used?

Limestone: The specimen shown is about two inches (five centimeters) across.

What is Limestone? Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3) in the form of the mineral calcite. It most commonly forms in clear, warm, shallow marine waters. It is usually an organic sedimentary rock that forms from the accumulation of shell, coral, algal, and fecal debris. It can also be a chemical sedimentary rock formed by the precipitation of calcium carbonate from lake or ocean water.

A Limestone-Forming Environment: An underwater view of a coral reef system from the Kerama Islands in the East China Sea southwest of Okinawa. Here the entire seafloor is covered by a wide variety of corals which produce calcium carbonate skeletons. A United States Geological Survey image by Curt Storlazzi.

Limestone-Forming Environment: Marine Most limestones form in shallow, calm, warm marine waters. That type of environment is where organisms capable of forming calcium carbonate shells and skeletons can easily extract the needed ingredients from ocean water. When these animals die, their shell and skeletal debris accumulate as a sediment that might be lithified into limestone. Their waste products can also contribute to the sediment mass. Limestones formed from this type of sediment are biological sedimentary rocks. Their biological origin is often revealed in the rock by the presence of fossils. Some limestones can form by direct precipitation of calcium carbonate from marine or fresh water. Limestones formed this way are chemical sedimentary rocks. They are thought to be less abundant than biological limestones. Today Earth has many limestone-forming environments. Most of them are found in shallow water areas between 30 degrees north latitude and 30 degrees south latitude. Limestone is forming in the Caribbean Sea, Indian Ocean, Persian Gulf, Gulf of Mexico, around Pacific Ocean islands, and within the Indonesian archipelago. One of these areas is the Bahamas Platform, located in the Atlantic Ocean about 100 miles southeast of southern Florida (see satellite image). There, abundant corals, shellfish, algae, and other organisms produce vast amounts of calcium carbonate skeletal debris that completely blankets the platform. This is producing an extensive limestone deposit.

The Bahamas Platform: A NASA satellite image of the Bahamas Platform where active limestone formation occurs today. The main platform is over 100 miles wide, and a great thickness of calcium carbonate sediments have accumulated there. In this image the dark blue areas are deep ocean waters. The shallow Bahamas Platform appears as light blue. Enlarge image.

Limestone-Forming Environment: Evaporative

Limestone stalactite: A drop of water hangs and evaporates on the tip of a stalactite. National Park Service image.

Limestone can also form through evaporation. Stalactites, stalagmites, and other cave formations (often called "speleothems") are examples of limestone that formed through evaporation. In a cave, droplets of water seeping down from above enter the cave through fractures or other pore spaces in the cave ceiling. There they might evaporate before falling to the cave floor. When the water evaporates, any calcium carbonate that was dissolved in the water will be deposited on the cave ceiling. Over time, this evaporative process can result in an accumulation of icicle-shaped calcium carbonate on the cave ceiling. These deposits are known as stalactites. If the droplet falls to the floor and evaporates there, a stalagmite could grow upwards from the cave floor. The limestone that makes up these cave formations is known as "travertine" and is a chemical sedimentary rock. A rock known as "tufa" is a limestone formed by evaporation at a hot spring, lake shore, or other area.

Related: The "Acid Test" for Carbonate Minerals

What is the Acid Test? To most geologists, the term "acid test" means placing a drop of dilute (5% to 10%) hydrochloric acid on a rock or mineral and watching for bubbles of carbon dioxide gas to be released. The bubbles signal the presence of carbonate minerals such as calcite, dolomite, or one of the minerals listed in Table 1. The bubbling release of carbon dioxide gas can be so weak that you need a hand lens to observe single bubbles slowly growing in the drop of hydrochloric acid - or so vigorous that a flash of effervescence is produced. These variations in effervescence vigor are a result of the type of carbonate minerals present, the amount of carbonate present, the particle size of the carbonate, and the temperature of the acid.

Magnesite: The mineral magnesite, which has a chemical composition of MgCO3, will effervesce weakly with warm hydrochloric acid and very weakly with cold acid. Specimen is about 6.4 cm across.

What Causes the Fizz? Carbonate minerals are unstable in contact with hydrochloric acid. When acid begins to effervesce (fizz) on a specimen, a reaction similar to the one shown below is taking place.

On the left side of this reaction, the mineral calcite (CaCO3) is in contact with hydrochloric acid (HCl). These react to form carbon dioxide gas (CO2), water (H2O), dissolved calcium (Ca++), and dissolved chlorine (Cl--). The carbon dioxide bubbles that you observe are evidence that the reaction is taking place. When that occurs, calcite or another carbonate mineral is present. Many other carbonate minerals react with hydrochloric acid. Each of these minerals consists of one or more metal ions combined with a carbonate ion (CO3--). The chemistry of these reactions is similar to the calcite reaction above. The mineral reacts with hydrochloric acid to produce carbon dioxide gas, water, a dissolved metal ion, and dissolved chlorine. The reactions for magnesite (MgCO3) and siderite (FeCO3) are shown below.

Calcite: This transparent specimen of calcite shows cleavage that is characteristic of the mineral. Calcite, with a composition of CaCO3, will react strongly with either cold or warm hydrochloric acid. Specimen measures about 10 cm across.

Acid Reactions of Carbonate Minerals Mineral

Chemical Composition

Cold Acid Reaction

Warm Acid Reaction

Aragonite

CaCO3

strong

strong

Azurite

Cu3(CO3)2(OH)2

yes

strong

Calcite

CaCO3

strong

strong

Dolomite

CaMg(CO3)2

weak

yes

Magnesite

MgCO3

very weak

weak

Malachite

Cu2CO3(OH)2

yes

yes

Rhodochrosite

MnCO3

weak

yes

Siderite

FeCO3

very weak

weak

Smithsonite

ZnCO3

weak

yes

Strontianite

SrCO3

yes

yes

Witherite

BaCO3

weak

weak

Table 1: A list of commonly and occasionally encountered carbonate minerals with their chemical formula and reactions to cold and warm hydrochloric acid. Test results can vary because of weathering, previous testing, contamination, and specimen purity.

The Vigor of Carbonate Reactions Careful observation is important because some carbonate minerals react vigorously and others barely react with cold acid.

The carbonate mineral that is most commonly encountered by geologists is calcite (CaCO3). Calcite is a "ubiquitous" mineral. Ubiquitous means "found everywhere." Calcite occurs in igneous, metamorphic, and sedimentary rocks and is the most commonly encountered carbonate mineral. If you place one drop of cold hydrochloric acid on calcite, the entire drop of acid will erupt with bubbles and a vigorous fizz will last for a few seconds. Dolomite CaMg(CO3)2 is another commonly encountered carbonate mineral. If you place one drop of cold hydrochloric acid on a piece of dolomite, the reaction is weak or not observed. Instead of seeing an obvious fizz, you will see a drop of acid on the surface of the mineral that might have a few bubbles of carbon dioxide gas slowly growing on the dolomite surface. However, if warm acid is placed on dolomite an obvious fizz will occur. This occurs because the acid and rock react more vigorously at higher temperatures. If you place a drop of hydrochloric acid on powdered dolomite, a visible reaction will occur. This is because the surface area has been increased, making more dolomite available to the acid. (You can easily make dolomite powder by scratching a specimen of dolomite across a streak plate. Then test the powder by placing a drop of hydrochloric acid on the powder. Another easy way to produce a small amount of mineral powder is to scratch the specimen with a nail.) Different carbonate minerals have different responses to hydrochloric acid. A list of common and occasionally encountered carbonate minerals is given in Table 1 with their chemical composition and their relative reaction with cold and warm hydrochloric acid. When a mineral has a weak response to acid, you must be observant and patient to see it. For example, magnesite has a very weak reaction with cold HCl. If you powder a small amount of magnesite on a streak plate and place a drop of acid on it, you might not see any action for several seconds. Then, as tiny bubbles begin to form on particles of magnesite, the drop of acid will appear to grow larger in size. That occurs as carbon dioxide is liberated from the mineral and displaces the water. Observing the formation of bubbles with a hand lens can be helpful.

Dolostone: Dolostone is a sedimentary rock composed primarily of the mineral dolomite, which has a chemical composition of CaMg(CO3)2. Dolomite will effervesce weakly with cold hydrochloric acid, producing a few bubbles. The reaction is more noticeable when the acid is warm and/or the stone is powdered. The specimen in the photo is about 10 cm across.

The Acid Test on Rocks LIMESTONE, DOLOSTONE, AND MARBLE

Some rocks contain carbonate minerals, and the acid test can be used to help identify them. Limestone is composed almost entirely of calcite and will produce a vigorous fizz with a drop of hydrochloric acid. Dolostone is a rock composed of almost entirely of dolomite. It will produce a very weak fizz when a drop of cold hydrochloric acid is placed upon it, a more obvious fizz when powdered dolostone is tested, and a stronger fizz when hot hydrochloric acid is used. Limestone and dolostone can be a little more complex. They are sometimes composed of a mixture of calcite and dolomite and have acid reactions that are deceptive. A dolostone can contain enough calcite to fool you into calling it a limestone. For these rocks the acid test might not be enough for a confident identification - but at least you will know that the rock has a significant carbonate mineral content. Marble is a limestone or a dolostone that has been metamorphosed. It will have an acid reaction that is similar to the limestone or dolostone from which it was formed.

Other Applications of the "Acid Test": Geologists can use dilute hydrochloric acid to help identify the cementing agent of sandstones. They place a drop of dilute HCl on the sandstone and closely observe. If calcite is the cementing agent, an effervescence will occur and some of the sand grains might be liberated. A hand lens or small microscope is used to make the observations. The photo above is a magnified view of a piece of Oriskany Sandstone, an Ordovician-age rock unit from the Appalachian Basin that serves as a natural gas reservoir and a natural gas storage unit. Oriskany sandstone is often cemented by calcite.

OTHER ROCKS THAT FIZZ Always remember that "calcite is ubiquitous." (Ubiquitous means that it is found almost everywhere.) Many rocks contain small amounts of calcite or other carbonate minerals. All of these can produce a fizz even though the carbonate is only a minor part of a rock's composition. These rocks might contain small veins or crystals of carbonate minerals that produce a fizz in contact with acid. These veins and crystals can be so tiny that they are not visible to the unaided eye. This small amount of carbonate might fizz the first time a drop of acid is applied but be depleted and not fizz if acid is applied a second time to the same location on the rock. Some sedimentary rocks are bound together with calcite or dolomite cement. Sandstone, siltstone, and conglomerate sometimes have calcite cement that will produce a vigorous fizz with cold hydrochloric acid. Some conglomerates and breccias contain clasts of carbonate rocks or minerals that react with acid. Many shales were deposited in marine environments and contain enough calcium carbonate to produce a vigorous acid fizz. These shales were formed when mud was deposited in an environment similar to or adjacent

to where limestone was formed. They are composed of sedimentary clay minerals intermixed with a small amount of calcite. They are known as "calcareous shales." Don't allow an acid fizz to guide the identification process. In many cases it will instead add detail to your observation such as: "calcareous shale" or "sandstone with carbonate cement." This is valuable information.

Vinegar can be used for the acid test: Vinegar can be a safe, economical and easy-to-obtain "acid" for identifying calcite and dolomite. Vinegar is dilute acetic acid that produces a very weak reaction with calcite and dolomite - best observed with a hand lens.

The "Vinegar Test" Vinegar is a dilute acetic acid solution (about 5% to 10%) that produces a weak effervescent reaction with calcite and dolomite. It can be used instead of hydrochloric acid for introducing students to the acid test. Vinegar is easy to obtain, inexpensive, and safer to use than hydrochloric acid. The effervescence using vinegar usually requires a hand lens for clear observation and is only observable with carbonate minerals that have a strong reaction with hydrochloric acid. Vinegar is often used when the acid test is part of a precollege course. Protective gloves, glasses, paper towels, and immediate access to an eyewash station are recommended.

EXTREME ACID REACTIONS A few rocks can produce an extreme reaction with hydrochloric acid. These are usually rocks composed of calcite or aragonite with abundant pore space or extremely high surface areas. Some specimens of chalk, coquina, oolite, and tufa are examples. When a drop of dilute hydrochloric acid is placed on these specimens, an eruption of acid foam can rise up off of the rock and spread to an unexpected diameter. The reaction is very brief (and may not be repeatable), but it is so sudden and vigorous that it can surprise an inexperienced person. This description is for one drop of acid. If more is used an even more vigorous reaction will occur. (These extreme reactions will not occur with every specimen of these rocks. Be aware when testing them or presenting them to students for testing.) The extremely vigorous reaction of cold hydrochloric acid with these specimens occurs because the rocks are so porous or because they have a very high surface area under a single drop of acid.

TEST UNWEATHERED MATERIAL Calcite and other carbonate minerals have a low resistance to weathering and can be attacked by acids in natural waters and soils. When testing material that has been exposed at Earth's surface, it is very important to test unweathered material. A fresh surface can usually be obtained by breaking the rock. DECEIVED BY POROSITY! Some rocks are porous and contain a reservoir of air. Small amounts of air escaping into a drop of acid from below can give the appearance of a gentle acid reaction. Don't be fooled. If you place a drop of acid on some sandstones, a few bubbles will emerge out of pore spaces. It's not a carbonate cement. To avoid this problem scratch the rock across a streak plate and test the powder or the grains that are produced.

Contamination in Mineral Identification Labs When students are given minerals to identify, two situations can cause problems with their work. 1) In mineral identification labs, some students are ready to call any mineral that produces an acid reaction "calcite" or another carbonate. However, calcite is a ubiquitous mineral and it is often present as an intimate part of other mineral specimens and rocks. These can produce a false acid reaction. To avoid being misled, students should always be cautioned to confirm a specimen's identity with multiple properties. If a specimen fizzes with acid but has a Mohs hardness of seven and breaks with a conchoidal fracture, then it certainly isn't calcite! Depending upon the experience of the students, specimens that are very true to their properties can be presented to the class, or specimens with some challenges can be used. Lots of minerals found in the field will not be absolutely true to properties. It's better to learn that lesson in the lab and go into the field with wisdom. 2) Since calcite is one of the index minerals of the Mohs Hardness Scale, it is often used to test the hardness of mineral specimens. This can place small amounts of calcite potentially on every unknown specimen in the lab! Don't assume that a single acid reaction is correct. Test the specimen in a second location if you suspect that contamination has occurred. In a mineral identification lab, barite is commonly confused with calcite because of contamination. The barite might naturally contain small amounts of calcite, or the hardness testing of a previous student might have left small amounts of calcite on a barite specimen. Students are often drawn to an identification as "calcite" simply because of the acid test. If that mineral exhibits a bit of cleavage and is not very hard, then many students will arrive at an incorrect identification. Acid Test Safety Hydrochloric acid, properly diluted to a 10% concentration, can cause irritation if it contacts the skin or eyes. It can also fade clothing. Hydrochloric acid should kept in clearly-labeled dispensing bottles and used with quick and easy access to paper towels, water, and an eyewash station. Safety glasses and protective gloves are recommended. If skin contact occurs the area should be flushed with plenty of water. If eye contact occurs the eye should be flushed for 15 minutes with plenty of water. If a contact lens is worn, the eye should be flushed, contact lens removed, and flushing continued. Seek prompt medical attention for eye contact. Specimens that are tested with acid should be rinsed after testing to remove or dilute unreacted acid.

Limiting Frivolous Acid Use in Labs Most students are intrigued with the acid test and want to try it. To limit frivolous acid use, students should be instructed to use a single drop of acid for the test and to only test specimens when carbonate minerals are suspected. If that is not done, some students will use the acid frivolously. This behavior is encouraged if the classroom is equipped with large acid bottles that are filled to the top. However, if the acid bottles are small and nearly empty at the beginning of class, students usually ration their use of the acid to appropriate amounts. Small, nearly empty bottles makes less acid available to spill.

Acid dispensing bottles: Small acid dispensing bottles work well for the acid test. They dispense the acid one-drop-at-a-time and will not spill if they are knocked over. If you are a teacher supervising the acid test in a classroom, give students small bottles that are nearly empty. That will reduce the amount of frivolous acid use that might otherwise occur. Label the bottles clearly and instruct students in acid use before making them available.

Acid Bottle Selection The type of bottle selected for dispensing the acid is important. Laboratory supply stores sell bottles that are designed for dispensing acid one-drop-at-a-time. The lid is always on these bottles (except when they are being cleaned or refilled), and they do not produce a spill when they are knocked over. Bottles with a removable lid that has a squeeze bulb dispenser will be occasionally knocked over when the lid is off if they are being used by normal humans. Acid dispensing bottles should be made of rigid plastic with a small opening which allows acid to be easily dispensed one-drop-at-a-time. Soft dispensing bottles or bottles with a larger opening can dispense a large amount of acid with an accidental squeeze.

Sources of Hydrochloric Acid Hydrochloric acid diluted to a 10% solution cannot be purchased in most communities. The best place to purchase commercially prepared solutions is from a laboratory supply company. Purchasing it ready-for-use is the recommended way to obtain it. Don't try to prepare your own solution if you don't know exactly what

you are doing and have an equipped laboratory. Your chemistry department might be able to assist you with ordering acid. Some generous chemists will prepare a 10% solution for you.

Mineral Specimens as "Consumables" Mineral specimens that are used properly in the science classroom or laboratory will need to be replaced frequently. Students will be investigating them with hardness tests, streak tests, acid tests and other experiments. All of these tests damage the specimen and make it less fit for the next group of students. To keep the acid test from fouling your entire collection, ask students to rinse specimens after testing with acid and limit testing to only when it is needed. Contributor: Hobart King

Composition of Limestone Limestone is by definition a rock that contains at least 50% calcium carbonate in the form of calcite by weight. All limestones contain at least a few percent other materials. These can be small particles of quartz, feldspar, clay minerals, pyrite, siderite, and other minerals. It can also contain large nodules of chert, pyrite, or siderite. The calcium carbonate content of limestone gives it a property that is often used in rock identification - it effervesces in contact with a cold solution of 5% hydrochloric acid.

Chalk: A fine-grained, light-colored limestone formed from the calcium carbonate skeletal remains of tiny marine organisms.

Coquina: This photo shows the shell hash known as coquina. The rock shown here is about two inches (five centimeters) across.

Tufa: A porous limestone that forms from the precipitation of calcium carbonate, often at a hot spring or along the shoreline of a lake where waters are saturated with calcium carbonate.

Varieties of Limestone There are many different names used for limestone. These names are based upon how the rock formed, its appearance or its composition, and other factors. Here are some of the more commonly used varieties. Chalk: A soft limestone with a very fine texture that is usually white or light gray in color. It is formed mainly from the calcareous shell remains of microscopic marine organisms such as foraminifers, or the calcareous remains from numerous types of marine algae. Coquina: A poorly-cemented limestone that is composed mainly of broken shell debris. It often forms on beaches where wave action segregates shell fragments of similar size. Fossiliferous Limestone: A limestone that contains obvious and abundant fossils. These are normally shell and skeletal fossils of the organisms that produced the limestone. Lithographic Limestone: A dense limestone with a very fine and very uniform grain size that occurs in thin beds which separate easily to form a very smooth surface. In the late 1700s, a printing process (lithography) was developed to reproduce images by drawing them on the stone with an oil-based ink and then using that stone to press multiple copies of the image.

Oolitic Limestone: A limestone composed mainly of calcium carbonate "oolites," small spheres formed by the concentric precipitation of calcium carbonate on a sand grain or shell fragment. Travertine: A limestone that forms by evaporative precipitation, often in a cave, to produce formations such as stalactites, stalagmites, and flowstone. Tufa: A limestone produced by precipitation of calcium-laden waters at a hot spring, lake shore, or other location.

Crinoidal Limestone: A limestone that contains a significant amount of crinoid fossils. Crinoids are organisms that have the morphology of a stemmed plant but are actually animals. Rarely, crinoidal and other types of limestone, have the ability to accept a bright polish and have interesting colors. These specimens can be made into unusual organic gems. This cabochon is about 39 millimeters square and was cut from material found in China.

Arenaceous Limestone: This image is a microscopic view of a polished surface of the Loyalhanna Limestone from Fayette County, Pennsylvania. The Loyalhanna is a Late Mississippian calcareous sandstone to arenaceous limestone, composed of siliceous sand grains embedded in a calcium carbonate matrix or bound by a calcium carbonate cement. It is cross-bedded with features that have caused geologists to argue if it is of marine bar or eolian dune origin. This view shows about one centimeter of rock between opposing corners of the photo with sand grains measuring about 1/2 millimeter in diameter. The Loyalhanna is

valued as an antiskid aggregate. When it is used to make concrete paving, sand grains in aggregate particles exposed on a wet pavement surface provide traction for tires, giving the pavement an antiskid quality.

Uses of Limestone Limestone is a rock with an enormous diversity of uses. It could be the one rock that is used in more ways than any other. Most limestone is made into crushed stone and used as a construction material. It is used as a crushed stone for road base and railroad ballast. It is used as an aggregate in concrete. It is fired in a kiln with crushed shale to make cement. Some varieties of limestone perform well in these uses because they are strong, dense rocks with few pore spaces. These properties enable them to stand up well to abrasion and freeze-thaw. Although limestone does not perform as well in these uses as some of the harder silicate rocks, it is much easier to mine and does not exert the same level of wear on mining equipment, crushers, screens, and the beds of the vehicles that transport it. Some additional but also important uses of limestone include: Dimension Stone: Limestone is often cut into blocks and slabs of specific dimensions for use in construction and in architecture. It is used for facing stone, floor tiles, stair treads, window sills, and many other purposes. Roofing Granules: Crushed to a fine particle size, crushed limestone is used as a weather and heat-resistant coating on asphalt-impregnated shingles and roofing. It is also used as a top coat on built-up roofs. Flux Stone: Crushed limestone is used in smelting and other metal refining processes. In the heat of smelting, limestone combines with impurities and can be removed from the process as a slag. Portland Cement: Limestone is heated in a kiln with shale, sand, and other materials and ground to a powder that will harden after being mixed with water. AgLime: Calcium carbonate is one of the most cost-effective acid-neutralizing agents. When crushed to sand-size or smaller particles, limestone becomes an effective material for treating acidic soils. It is widely used on farms throughout the world. Lime: If calcium carbonate (CaC03) is heated to high temperature in a kiln, the products will be a release of carbon dioxide gas (CO2) and calcium oxide (CaO). The calcium oxide is a powerful acid-neutralization agent. It is widely used as a soil treatment agent (faster acting than aglime) in agriculture and as an acid-neutralization agent by the chemical industry. Animal Feed Filler: Chickens need calcium carbonate to produce strong egg shells, so calcium carbonate is often offered to them as a dietary supplement in the form of "chicken grits." It is also added to the feed of some dairy cattle who must replace large amounts of calcium lost when the animal is milked. Mine Safety Dust: Also known as "rock dust." Pulverized limestone is a white powder that can be sprayed onto exposed coal surfaces in an underground mine. This coating improves illumination and reduces the amount of coal dust that activity stirs up and releases into the air. This improves the air for breathing, and it also reduces the explosion hazard produced by suspended particles of flammable coal dust in the air.

Limestone has many other uses. Powdered limestone is used as a filler in paper, paint, rubber, and plastics. Crushed limestone is used as a filter stone in on-site sewage disposal systems. Powdered limestone is also used as a sorbent (a substance that absorbs pollutants) at many coal-burning facilities. Limestone is not found everywhere. It only occurs in areas underlain by sedimentary rocks. Limestone is needed in other areas and is so important that buyers will pay five times the value of the stone in delivery charges so that limestone can be used in their project or process. Contributor: Hobart King

Iron Ore What Is Iron Ore, How Does It Form, and What Is It Used For?

Iron Ore: A specimen of oolitic hematite iron ore. The specimen shown is about two inches (five centimeters) across.

What is Iron Ore? Earth's most important iron ore deposits are found in sedimentary rocks. They formed from chemical reactions that combined iron and oxygen in marine and fresh waters. The two most important minerals in these deposits are iron oxides: hematite (Fe2O3) and magnetite (Fe3O4). These iron ores have been mined to produce almost every iron and steel object that we use today - from paper clips to automobiles to the steel beams in skyscrapers.

How Does Iron Ore Form? Nearly all of Earth's major iron ore deposits are in rocks that formed over 1.8 billion years ago. At that time Earth's oceans contained abundant dissolved iron and almost no dissolved oxygen. The iron ore deposits began forming when the first organisms capable of photosynthesis began releasing oxygen into the waters. This oxygen immediately combined with the abundant dissolved iron to produce hematite or magnetite. These minerals deposited on the sea floor in great abundance, forming what are now known as the "banded iron formations." The rocks are "banded" because the iron minerals deposited in alternating bands with silica and sometimes shale. The banding might have resulted from seasonal changes in organism activity.

Steel Mill: Most iron ore is used to make steel. Here a steel slab is being cut to length in a steel mill. Image © iStockphoto / Alfredo Tisi.

What is Iron Ore Used For? The primary use of iron ore is in the production of iron. Most of the iron produced is then used to make steel. Steel is used to make automobiles, locomotives, ships, beams used in buildings, furniture, paper clips, tools, reinforcing rods for concrete, bicycles, and thousands of other items. It is the most-used metal by both tonnage and purpose. Contributor: Hobart King

Banded Iron Formation: Close-up view of a banded iron formation. In this specimen bands of hematite (silver) alternate with bands of jasper (red). This photo spans an area of rock about one foot wide. Photo taken by André Karwath, GNU Free Documentation License.

Rock Salt What is Rock Salt?

Rock Salt: The specimen shown is about two inches (five centimeters) across.

Rock Salt is a chemical sedimentary rock that forms from the evaporation of ocean or saline lake waters. It is also known by the mineral name "halite". It is rarely found at Earth's surface, except in areas of very arid climate. It is often mined for use in the chemical industry or for use as a winter highway treatment. Some halite is processed for use as a seasoning for food.

Geology and Resources of Some World Oil-Shale Deposits Reprint of: USGS Scientific Investigations Report 2005-5294 By John R. Dyni

Introduction

What is Oil Shale?

Oil shale is commonly defined as a fine-grained sedimentary rock containing organic matter that yields substantial amounts of oil and combustible gas upon destructive distillation. Most of the organic matter is insoluble in ordinary organic solvents; therefore, it must be decomposed by heating to release such materials. Underlying most definitions of oil shale is its potential for the economic recovery of energy, including shale oil and combustible gas, as well as a number of byproducts. A deposit of oil shale having economic potential is generally one that is at or near enough to the surface to be developed by open-pit or conventional underground mining or by in-situ methods. Oil shales range widely in organic content and oil yield. Commercial grades of oil shale, as determined by their yield of shale oil, ranges from about 100 to 200 liters per metric ton (l/t) of rock. The U.S. Geological Survey has used a lower limit of about 40 l/t for classification of Federal oil-shale lands. Others have suggested a limit as low as 25 l/t. Deposits of oil shale are in many parts of the world. These deposits, which range from Cambrian to Tertiary age, may occur as minor accumulations of little or no economic value or giant deposits that occupy thousands of square kilometers and reach thicknesses of 700 m or more. Oil shales were deposited in a variety of depositional environments, including fresh-water to highly saline lakes, epicontinental marine basins and subtidal shelves, and in limnic and coastal swamps, commonly in association with deposits of coal. In terms of mineral and elemental content, oil shale differs from coal in several distinct ways. Oil shales typically contain much larger amounts of inert mineral matter (60-90 percent) than coals, which have been defined as containing less than 40 percent mineral matter. The organic matter of oil shale, which is the source of liquid and gaseous hydrocarbons, typically has a higher hydrogen and

Oil shale is a rock that contains significant amounts of organic material in the form of kerogen. Up to 1/3 of the rock can be solid organic material. Liquid and gaseous hydrocarbons can be extracted from the oil shale but the rock must be heated and/or treated with solvents. This is usually much less efficient than drilling rocks that will yield oil or gas directly into a well. The processes used for hydrocarbon extraction also produce emissions and waste products that cause significant environmental concerns. Oil shale usually meets the definition of "shale" in that it is "a laminated rock consisting of at least 67% clay minerals," however, it sometimes contains enough organic material and carbonate minerals that clay minerals account for less than 67% of the rock.

lower oxygen content than that of lignite and bituminous coal.

United States of America Oil Shale

In general, the precursors of the organic matter in oil shale and coal also differ. Much of the organic matter in oil shale is of algal origin, but may also include remains of vascular land plants that more commonly compose much of the organic matter in coal. The origin of some of the organic matter in oil shale is obscure because of the lack of recognizable biologic structures that would help identify the precursor organisms. Such materials may be of bacterial origin or the product of bacterial degradation of algae or other organic matter. The mineral component of some oil shales is composed of carbonates including calcite, dolomite, and siderite, with lesser amounts of aluminosilicates. For other oil shales, the reverse is true-silicates including quartz, feldspar, and clay minerals are dominant and carbonates are a minor component. Many oil-shale deposits contain small, but ubiquitous, amounts of sulfides including pyrite and marcasite, indicating that the sediments probably accumulated in dysaerobic to anoxic waters that prevented the destruction of the organic matter by burrowing organisms and oxidation. Although shale oil in today's (2004) world market is not competitive with petroleum, natural gas, or coal, it is used in several countries that possess easily exploitable deposits of oil shale but lack other fossil fuel resources. Some oil-shale deposits contain minerals and metals that add byproduct value such as alum [KAl(SO4)2.12H2O], nahcolite (NaHCO3), dawsonite [NaAl(OH)2CO3], sulfur, ammonium sulfate, vanadium, zinc, copper, and uranium.

Areas underlain by the Green River Formation in Colorado, Utah, and Wyoming, United States (after Dyni, 2005) and major areas of surface minable Devonian oil shale in the eastern United States (after Matthews and others 1980). More information on United States oil shale.

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Australia Oil Shale

The gross heating value of oil shales on a dryweight basis ranges from about 500 to 4,000 kilocalories per kilogram (kcal/kg) of rock. The high-grade kukersite oil shale of Estonia, which fuels several electric power plants, has a heating value of about 2,000 to 2,200 kcal/kg. By comparison, the heating value of lignitic coal ranges from 3,500 to 4,600 kcal/kg on a dry, mineral-free basis (American Society for Testing Materials, 1966). Tectonic events and volcanism have altered some deposits. Structural deformation may impair the mining of an oil-shale deposit, whereas igneous intrusions may have thermally degraded the organic matter. Thermal alteration of this type may be restricted to a small part of the deposit, or it may be widespread making most of the deposit unfit for recovery of shale oil.

Deposits of oil shale in Australia (locations after Crisp and others, 1987; and, Cook and Sherwood 1989). More information on Australia oil shale.

The purpose of this report is to (1) discuss the geology and summarize the resources of selected deposits of oil shale in varied geologic settings from different parts of the world and (2) present new information on selected deposits developed since 1990 (Russell, 1990).

Brazil Oil Shale

Recoverable Resources The commercial development of an oil-shale deposit depends upon many factors. The geologic setting and the physical and chemical characteristics of the resource are of primary importance. Roads, railroads, power lines, water, and available labor are among the factors to be considered in determining the viability of an oilshale operation. Oil-shale lands that could be mined may be preempted by present land usage such as population centers, parks, and wildlife refuges. Development of new in-situ mining and processing technologies may allow an oil-shale operation in previously restricted areas without causing damage to the surface or posing problems of air and water pollution. The availability and price of petroleum ultimately effect the viability of a large-scale oil-shale industry. Today, few, if any deposits can be economically mined and processed for shale oil in competition with petroleum. Nevertheless, some countries with oil-shale resources, but lack petroleum reserves, find it expedient to operate an oil-shale industry. As supplies of petroleum diminish in future years and costs for petroleum increase, greater use of oil shale for the production of electric power, transportation fuels, petrochemicals, and other industrial products seems likely.

Deposits of oil shale in Brazil (locations after Padula, 1969). More information on Brazil oil shale .

Canada Oil Shale

Determining Grade of Oil Shale

The grade of oil shale has been determined by many different methods with the results expressed in a variety of units. The heating value of the oil shale may be determined using a calorimeter. Values obtained by this method are reported in English or metric units, such as British thermal units (Btu) per pound of oil shale, calories per gram (cal/gm) of rock, kilocalories per kilogram (kcal/kg) of rock, megajoules per kilogram (MJ/kg) of rock, and other units. The heating value is useful for determining the quality of an oil shale that is burned directly in a power plant to produce electricity. Although the heating value of a given oil shale is a useful and fundamental property of

Oil-shale deposits in Canada (locations after Macauley, 1981). More information on Canada oil shale.

Estonia and Sweden Oil Shale

the rock, it does not provide information on the amounts of shale oil or combustible gas that would be yielded by retorting (destructive distillation). The grade of oil shale can be determined by measuring the yield of oil of a shale sample in a laboratory retort. This is perhaps the most common type of analysis that is currently used to evaluate an oil-shale resource. The method commonly used in the United States is called the "modified Fischer assay," first developed in Germany, then adapted by the U.S. Bureau of Mines for analyzing oil shale of the Green River Formation in the western United States (Stanfield and Frost, 1949). The technique was subsequently standardized as the American Society for Testing and Materials Method D-390480 (1984). Some laboratories have further modified the Fischer assay method to better evaluate different types of oil shale and different methods of oil-shale processing. The standardized Fischer assay method consists of heating a 100-gram sample crushed to -8 mesh (2.38-mm mesh) screen in a small aluminum retort to 500ºC at a rate of 12ºC per minute and held at that temperature for 40 minutes. The distilled vapors of oil, gas, and water are passed through a condenser cooled with ice water into a graduated centrifuge tube. The oil and water are then separated by centrifuging. The quantities reported are the weight percentages of shale oil (and its specific gravity), water, shale residue, and "gas plus loss" by difference. The Fischer assay method does not determine the total available energy in an oil shale. When oil shale is retorted, the organic matter decomposes into oil, gas, and a residuum of carbon char remaining in the retorted shale. The amounts of individual gases-chiefly hydrocarbons, hydrogen, and carbon dioxide-are not normally determined but are reported collectively as "gas plus loss," which is the difference of 100 weight percent minus the sum of the weights of oil, water, and spent shale. Some oil shales may have a greater energy potential than that reported by the Fischer assay method depending on the components of the "gas plus loss." The Fischer assay method also does not necessarily indicate the maximum amount of oil that can be produced by a given oil shale. Other retorting methods, such as the Tosco II process, are known to yield in excess of 100 percent of the yield reported by Fischer assay. In fact, special methods of retorting, such as the Hytort process, can increase oil yields of some oil shales by as much as three to four times the yield obtained by the Fischer assay method (Schora and others, 1983; Dyni and others, 1990). At best, the Fischer

Location of the kukersite deposits in northern Estonia and Russia (locations after Kattai and Lokk, 1998; and Bauert, 1994). Also, areas of Alum Shale in Sweden (locations after Andersson and others, 1985). More information on Estonia and Sweden oil shale

assay method only approximates the energy potential of an oil-shale deposit.

Israel and Jordan Oil Shale

Newer techniques for evaluating oil-shale resources include the Rock-Eval and the "material-balance" Fischer assay methods. Both give more complete information about the grade of oil shale, but are not widely used. The modified Fischer assay, or close variations thereof, is still the major source of information for most deposits. It would be useful to develop a simple and reliable assay method for determining the energy potential of an oil shale that would include the total heat energy and the amounts of oil, water, combustible gases including hydrogen, and char in sample residue.

Origin of Organic Matter

Organic matter in oil shale includes the remains of algae, spores, pollen, plant cuticle and corky fragments of herbaceous and woody plants, and other cellular remains of lacustrine, marine, and land plants. These materials are composed chiefly of carbon, hydrogen, oxygen, nitrogen, and sulfur. Some organic matter retains enough biological structures so that specific types can be identified as to genus and even species. In some oil shales, the organic matter is unstructured and is best described as amorphous (bituminite). The origin of this amorphous material is not well known, but it is likely a mixture of degraded algal or bacterial remains. Small amounts of plant resins and waxes also contribute to the organic matter. Fossil shell and bone fragments composed of phosphatic and carbonate minerals, although of organic origin, are excluded from the definition of organic matter used herein and are considered to be part of the mineral matrix of the oil shale.

Deposits of oil shale in Israel (locations after Minster, 1994). Also, oil-shale deposits in Jordan (locations after Jaber and others, 1997; and, Hamarneh, 1998). More information on Israel and Jordan oil shale.

Morocco Oil Shale

Most of the organic matter in oil shales is derived from various types of marine and lacustrine algae. It may also include varied admixtures of biologically higher forms of plant debris that depend on the depositional environment and geographic position. Bacterial remains can be volumetrically important in many oil shales, but they are difficult to identify. Most of the organic matter in oil shale is insoluble in ordinary organic solvents, whereas some is bitumen that is soluble in certain organic solvents. Solid hydrocarbons, including gilsonite, wurtzilite, grahamite, ozokerite, and albertite, are present as veins or pods in some oil shales. These hydrocarbons have somewhat varied chemical

Oil-shale deposits in Morocco (locations after Bouchta, 1984). More information on Morocco oil shale.

China, Russia, Syria, Thailand and Turkey

and physical characteristics, and several have been mined commercially.

Thermal Maturity of Organic Matter The thermal maturity of an oil shale refers to the degree to which the organic matter has been altered by geothermal heating. If the oil shale is heated to a high enough temperature, as may be the case if the oil shale were deeply buried, the organic matter may thermally decompose to form oil and gas. Under such circumstances, oil shales can be source rocks for petroleum and natural gas. The Green River oil shale, for example, is presumed to be the source of the oil in the Red Wash field in northeastern Utah. On the other hand, oil-shale deposits that have economic potential for their shale-oil and gas yields are geothermally immature and have not been subjected to excessive heating. Such deposits are generally close enough to the surface to be mined by open-pit, underground mining, or by in-situ methods. The degree of thermal maturity of an oil shale can be determined in the laboratory by several methods. One technique is to observe the changes in color of the organic matter in samples collected from varied depths in a borehole. Assuming that the organic matter is subjected to geothermal heating as a function of depth, the colors of certain types of organic matter change from lighter to darker colors. These color differences can be noted by a petrographer and measured using photometric techniques. Geothermal maturity of organic matter in oil shale is also determined by the reflectance of vitrinite (a common constituent of coal derived from vascular land plants), if present in the rock. Vitrinite reflectance is commonly used by petroleum explorationists to determine the degree of geothermal alteration of petroleum source rocks in a sedimentary basin. A scale of vitrinite reflectances has been developed that indicates when the organic matter in a sedimentary rock has reached temperatures high enough to generate oil and gas. However, this method can pose a problem with respect to oil shale, because the reflectance of vitrinite may be depressed by the presence of lipid-rich organic matter. Vitrinite may be difficult to recognize in oil shale because it resembles other organic material of algal origin and may not have the same reflectance response as vitrinite, thereby leading to erroneous conclusions. For this reason, it may be necessary to measure vitrinite reflectance from

Other countries with oil shale. More information on China, Russia, Syria, Thailand and Turkey oil shale

laterally equivalent vitrinite-bearing rocks that lack the algal material. In areas where the rocks have been subjected to complex folding and faulting or have been intruded by igneous rocks, the geothermal maturity of the oil shale should be evaluated for proper determination of the economic potential of the deposit.

Classification of Oil Shale Oil shale has received many different names over the years, such as cannel coal, boghead coal, alum shale, stellarite, albertite, kerosene shale, bituminite, gas coal, algal coal, wollongite, schistes bitumineux, torbanite, and kukersite. Some of these names are still used for certain types of oil shale. Recently, however, attempts have been made to systematically classify the many different types of oil shale on the basis of the depositional environment of the deposit, the petrographic character of the organic matter, and the precursor organisms from which the organic matter was derived. A useful classification of oil shales was developed by A.C. Hutton (1987, 1988, 1991), who pioneered the use of blue/ultraviolet fluorescent microscopy in the study of oil-shale deposits of Australia. Adapting petrographic terms from coal terminology, Hutton developed a classification of oil shale based primarily on the origin of the organic matter. His classification has proved to be useful for correlating different kinds of organic matter in oil shale with the chemistry of the hydrocarbons derived from oil shale. Hutton (1991) visualized oil shale as one of three broad groups of organic-rich sedimentary rocks: (1) humic coal and carbonaceous shale, (2) bitumen-impregnated rock, and (3) oil shale. He then divided oil shale into three groups based upon their environments of deposition - terrestrial, lacustrine, and marine. Terrestrial oil shales include those composed of lipid-rich organic matter such as resin spores, waxy cuticles, and corky tissue of roots, and stems of vascular terrestrial plants commonly found in coal-forming swamps and bogs. Lacustrine oil shales include lipid-rich organic matter derived from algae that lived in freshwater, brackish, or saline lakes. Marine oil shales are composed of lipid-rich organic matter derived from marine algae, acritarchs (unicellular organisms of questionable origin), and marine dinoflagellates. Several

quantitatively

important

petrographic

components of the organic matter in oil shaletelalginite, lamalginite, and bituminite-are adapted from coal petrography. Telalginite is organic matter derived from large colonial or thick-walled unicellular algae, typified by genera such as Botryococcus. Lamalginite includes thin-walled colonial or unicellular algae that occurs as laminae with little or no recognizable biologic structures. Telalginite and lamalginite fluoresce brightly in shades of yellow under blue/ultraviolet light. Bituminite, on the other hand, is largely amorphous, lacks recognizable biologic structures, and weakly fluoresces under blue light. It commonly occurs as an organic groundmass with fine-grained mineral matter. The material has not been fully characterized with respect to its composition or origin, but it is commonly an important component of marine oil shales. Coaly materials including vitrinite and inertinite are rare to abundant components of oil shale; both are derived from humic matter of land plants and have moderate and high reflectance, respectively, under the microscope. Within his three-fold grouping of oil shales (terrestrial, lacustrine, and marine), Hutton (1991) recognized six specific oil-shale types: cannel coal, lamosite, marinite, torbanite, tasmanite, and kukersite. The most abundant and largest deposits are marinites and lamosites. Cannel coal is brown to black oil shale composed of resins, spores, waxes, and cutinaceous and corky materials derived from terrestrial vascular plants together with varied amounts of vitrinite and inertinite. Cannel coals originate in oxygendeficient ponds or shallow lakes in peat-forming swamps and bogs (Stach and others, 1975, p. 236-237). Lamosite is pale- and grayish-brown and dark gray to black oil shale in which the chief organic constituent is lamalginite derived from lacustrine planktonic algae. Other minor components in lamosite include vitrinite, inertinite, telalginite, and bitumen. The Green River oil-shale deposits in western United States and a number of the Tertiary lacustrine deposits in eastern Queensland, Australia, are lamosites. Marinite is a gray to dark gray to black oil shale of marine origin in which the chief organic components are lamalginite and bituminite derived chiefly from marine phytoplankton. Marinite may also contain small amounts of bitumen, telalginite, and vitrinite. Marinites are deposited typically in epeiric seas such as on broad shallow marine shelves or inland seas where wave action is restricted and currents are minimal. The Devonian-Mississippian oil shales of eastern United States are typical marinites. Such deposits

are generally widespread covering hundreds to thousands of square kilometers, but they are relatively thin, often less than about 100 m. Torbanite, tasmanite, and kukersite are related to specific kinds of algae from which the organic matter was derived; the names are based on local geographic features. Torbanite, named after Torbane Hill in Scotland, is a black oil shale whose organic matter is composed mainly of telalginite derived largely from lipid-rich Botryococcus and related algal forms found in fresh- to brackishwater lakes. It also contains small amounts of vitrinite and inertinite. The deposits are commonly small, but can be extremely high grade. Tasmanite, named from oil-shale deposits in Tasmania, is a brown to black oil shale. The organic matter consists of telalginite derived chiefly from unicellular tasmanitid algae of marine origin and lesser amounts of vitrinite, lamalginite, and inertinite. Kukersite, which takes its name from Kukruse Manor near the town of KohtlaJärve, Estonia, is a light brown marine oil shale. Its principal organic component is telalginite derived from the green alga, Gloeocapsomorpha prisca. The Estonian oil-shale deposit in northern Estonia along the southern coast of the Gulf of Finland and its eastern extension into Russia, the Leningrad deposit, are kukersites.

Evaluation of Oil-Shale Resources Relatively little is known about many of the world's deposits of oil shale and much exploratory drilling and analytical work need to be done. Early attempts to determine the total size of world oilshale resources were based on few facts, and estimating the grade and quantity of many of these resources were speculative, at best. The situation today has not greatly improved, although much information has been published in the past decade or so, notably for deposits in Australia, Canada, Estonia, Israel, and the United States. Evaluation of world oil-shale resources is especially difficult because of the wide variety of analytical units that are reported. The grade of a deposit is variously expressed in U.S. or Imperial gallons of shale oil per short ton (gpt) of rock, liters of shale oil per metric ton (l/t) of rock, barrels, short or metric tons of shale oil, kilocalories per kilogram (kcal/kg) of oil shale, or gigajoules (GJ) per unit weight of oil shale. To bring some uniformity into this assessment, oil-shale resources in this report are given in both metric tons of shale oil and in equivalent U.S. barrels of shale oil, and the grade of oil shale, where known, is expressed in liters of

shale oil per metric ton (l/t) of rock. If the size of the resource is expressed only in volumetric units (barrels, liters, cubic meters, and so on), the density of the shale oil must be known or estimated to convert these values to metric tons. Most oil shales produce shale oil that ranges in density from about 0.85 to 0.97 by the modified Fischer assay method. In cases where the density of the shale oil is unknown, a value of 0.910 is assumed for estimating resources. Byproducts may add considerable value to some oil-shale deposits. Uranium, vanadium, zinc, alumina, phosphate, sodium carbonate minerals, ammonium sulfate, and sulfur are some of the potential byproducts. The spent shale after retorting is used to manufacture cement, notably in Germany and China. The heat energy obtained by the combustion of the organic matter in oil shale can be used in the cement-making process. Other products that can be made from oil shale include specialty carbon fibers, adsorbent carbons, carbon black, bricks, construction and decorative blocks, soil additives, fertilizers, rock wool insulating material, and glass. Most of these uses are still small or in experimental stages, but the economic potential is large. This appraisal of world oil-shale resources is far from complete. Many deposits are not reviewed because data or publications are unavailable. Resource data for deeply buried deposits, such as a large part of the Devonian oil-shale deposits in eastern United States, are omitted, because they are not likely to be developed in the foreseeable future. Thus, the total resource numbers reported herein should be regarded as conservative estimates. This review focuses on the larger deposits of oil shale that are being mined or have the best potential for development because of their size and grade.

Shale Shale is the most abundant sedimentary rock and is in sedimentary basins worldwide.

Shale: Shale breaks into thin pieces with sharp edges. It occurs in a wide range of colors that include red, brown, green, gray, and black. It is the most common sedimentary rock and is found in sedimentary basins worldwide.

What is Shale? Shale is a fine-grained sedimentary rock that forms from the compaction of silt and clay-size mineral particles that we commonly call "mud." This composition places shale in a category of sedimentary rocks known as "mudstones." Shale is distinguished from other mudstones because it is fissile and laminated. "Laminated" means that the rock is made up of many thin layers. "Fissile" means that the rock readily splits into thin pieces along the laminations.

Uses of Shale Some shales have special properties that make them important resources. Black shales contain organic material that sometimes breaks down to form natural gas or oil. Other shales can be crushed and mixed with water to produce clays that can be made into a variety of useful objects.

Conventional Oil and Natural Gas Reservoir: This drawing illustrates an "anticlinal trap" that contains oil and natural gas. The gray rock units are impermeable shale. Oil and natural gas forms within these shale units and then migrates upwards. Some of the oil and gas becomes trapped in the yellow sandstone to form an oil and gas reservoir. This is a "conventional" reservoir - meaning that the oil and gas can flow through the pore space of the sandstone and be produced from the well.

Conventional Oil and Natural Gas Black organic shales are the source rock for many of the world's most important oil and natural gas deposits. These shales obtain their black color from tiny particles of organic matter that were deposited with the mud from which the shale formed. As the mud was buried and warmed within the earth, some of the organic material was transformed into oil and natural gas. The oil and natural gas migrated out of the shale and upwards through the sediment mass because of their low density. The oil and gas were often trapped within the pore spaces of an overlying rock unit such as a sandstone (see illustration). These types of oil and gas deposits are known as "conventional reservoirs" because the fluids can easily flow through the pores of the rock and into the extraction well. Although drilling can extract large amounts of oil and natural gas from the reservoir rock, much of it remains trapped within the shale. This oil and gas is very difficult to remove because it is trapped within tiny pore spaces or adsorbed onto clay mineral particles that make up the shale.

Unconventional Oil and Gas Reservoir: This drawing illustrates the new technologies that enable the development of unconventional oil and natural gas fields. In these gas fields, the oil and gas are held in shales or another rock unit that is impermeable. To produce that oil or gas, special technologies are needed. One is horizontal drilling, in which a vertical well is deviated to horizontal so that it will penetrate a long distance of reservoir rock. The second is hydraulic fracturing. With this technique, a portion of the well is sealed off and water is pumped in to produce a pressure that is high enough to fracture the surrounding rock. The result is a highly fractured reservoir penetrated by a long length of well bore.

Unconventional Oil and Natural Gas In the late 1990s, natural gas drilling companies developed new methods for liberating oil and natural gas that is trapped within the tiny pore spaces of shale. This discovery was significant because it unlocked some of the largest natural gas deposits in the world. The Barnett Shale of Texas was the first major natural gas field developed in a shale reservoir rock. Producing gas from the Barnett Shale was a challenge. The pore spaces in shale are so tiny that the gas has difficulty moving through the shale and into the well. Drillers discovered that they could increase the permeability of the shale by pumping water down the well under pressure that was high enough to fracture the shale. These fractures liberated some of the gas from the pore spaces and allowed that gas to flow to the well. This technique is known as "hydraulic fracturing" or "hydrofracing." Drillers also learned how to drill down to the level of the shale and turn the well 90 degrees to drill horizontally through the shale rock unit. This produced a well with a very long "pay zone" through the reservoir rock (see illustration). This method is known as "horizontal drilling." Horizontal drilling and hydraulic fracturing revolutionized drilling technology and paved the way for developing several giant natural gas fields. These include theMarcellus Shale in the Appalachians, the Haynesville Shale in Louisiana and the Fayetteville Shale in Arkansas. These enormous shale reservoirs hold enough natural gas to serve all of the United States' needs for twenty years or more.

Shale in brick and tile: Shale is used as a raw material for making many types of brick, tile, pipe, pottery, and other manufactured products. Brick and tile are some of the most extensively used and highly desired materials for building homes, walls, streets, and commercial structures. © iStockphoto / Guy Elliott.

Shale Used to Produce Clay Everyone has contact with products made from shale. If you live in a brick house, drive on a brick road, live in a house with a tile roof, or keep plants in "terra cotta" pots, you have daily contact with items that were probably made from shale. Many years ago these same items were made from natural clay. However, heavy use depleted most of the small clay deposits. Needing a new source of raw materials, manufacturers soon discovered that mixing finely ground shale with water would produce a clay that often had similar or superior properties. Today, most items that were once produced from natural clay have been replaced by almost identical items made from clay manufactured by mixing finely ground shale with water.

Shale Used to Produce Cement Cement is another common material that is often made with shale. To make cement, crushed limestone and shale are heated to a temperature that is high enough to evaporate off all water and break down the limestone into calcium oxide and carbon dioxide. The carbon dioxide is lost as an emission, but the calcium oxide combined with the heated shale makes a powder that will harden if mixed with water and allowed to dry. Cement is used to make concrete and many other products for the construction industry.

Oil shale: A rock that contains a significant amount of organic material in the form of solid kerogen. Up to 1/3 of the rock can be solid organic material. This specimen is approximately four inches (ten centimeters) across.

Oil Shale Oil shale is a rock that contains significant amounts of organic material in the form of kerogen. Up to 1/3 of the rock can be solid kerogen. Liquid and gaseous hydrocarbons can be extracted from oil shale, but the rock must be heated and/or treated with solvents. This is usually much less efficient than drilling rocks that will yield oil or gas directly into a well. Extracting the hydrocarbons from oil shale produces emissions and waste products that cause significant environmental concerns. This is one reason why the world's extensive oil shale deposits have not been aggressively utilized. Oil shale usually meets the definition of "shale" in that it is "a laminated rock consisting of at least 67% clay minerals." However, it sometimes contains enough organic material and carbonate minerals that clay minerals account for less than 67% of the rock.

Shale core samples: When shale is drilled for oil, natural gas, or mineral resource evaluation, a core is often recovered from the well. The rock in the core can then be tested to learn about its potential and how the resource might be best developed.

Composition of Shale Shale is a rock composed mainly of clay-size mineral grains. These tiny grains are usually clay minerals such as illite, kaolinite, and smectite. Shale usually contains other clay-size mineral particles such as quartz, chert, and feldspar. Other constituents might include organic particles, carbonate minerals, iron oxide minerals, sulfide minerals, and heavy mineral grains. These "other constituents" in the rock are often determined by the shale's environment of deposition, and they often determine the color of the rock.

Black shale: Organic-rich black shale. Natural gas and oil are sometimes trapped in the tiny pore spaces of this type of shale.

Colors of Shale Like most rocks, the color of shale is often determined by the presence of specific materials in minor amounts. Just a few percent of organic materials or iron can significantly alter the color of a rock.

Shale gas plays: Since the late 1990s, dozens of previously unproductive black organic shales have been successfully developed into valuable gas fields. See the article: "What is Shale Gas?"

Black and Gray Shale A black color in sedimentary rocks almost always indicates the presence of organic materials. Just one or two percent organic materials can impart a dark gray or black color to the rock. In addition, this black color almost always implies that the shale formed from sediment deposited in an oxygen-deficient environment. Any oxygen that entered the environment quickly reacted with the decaying organic debris. If a large amount of oxygen was present, the organic debris would all have decayed. An oxygen-poor environment also provides

the proper conditions for the formation of sulfide minerals such as pyrite, another important mineral found in most black shales. The presence of organic debris in black shales makes them the candidates for oil and gas generation. If the organic material is preserved and properly heated after burial, oil and natural gas might be produced. The Barnett Shale, Marcellus Shale, Haynesville Shale, Fayetteville Shale, and other gas-producing rocks are all dark gray or black shales that yield natural gas. The Bakken Shale of North Dakota and the Eagle Ford Shale of Texas are examples of shales that yield oil. Gray shales sometimes contain a small amount of organic matter. However, gray shales can also be rocks that contain calcareous materials or simply clay minerals that result in a gray color.

Utica and Marcellus Shale: Two black organic shales in the Appalachian Basin are thought to contain enough natural gas to supply the United States for several years. These are the Marcellus Shale and Utica Shale.

Red, Brown, and Yellow Shale Shales that are deposited in oxygen-rich environments often contain tiny particles of iron oxide or iron hydroxide minerals such as hematite, goethite, or limonite. Just a few percent of these minerals distributed through the rock can produce the red, brown, or yellow colors exhibited by many types of shale. The presence of hematite can produce a red shale. The presence of limonite or goethite can produce a yellow or brown shale. Green Shale Green shales are occasionally found. This should not be surprising because some of the clay minerals and micas that make up much of the volume of these rocks are typically a greenish color.

Natural gas shale well: In less than ten years, shale has skyrocketed to prominence in the energy sector. New drilling and well development methods such as hydraulic fracturing and horizontal drilling can tap the oil and natural gas trapped within the tight matrix of organic shales. © iStockphoto / Edward Todd.

Hydraulic Properties of Shale Hydraulic properties are characteristics of a rock such as permeability and porosity that reflect its ability to hold and transmit fluids such as water, oil, or natural gas. Shale has a very small particle size, so the interstitial spaces are very small. In fact they are so small that oil, natural gas, and water have difficulty moving through the rock. Shale can therefore serve as a cap rock for oil and natural gas traps, and it also is an aquiclude that blocks or limits the flow of groundwater. Although the interstitial spaces in a shale are very small, they can take up a significant volume of the rock. This allows the shale to hold significant amounts of water, gas, or oil but not be able to effectively transmit them because of the low permeability. The oil and gas industry overcomes these limitations of shale by using horizontal drilling and hydraulic fracturing to create artificial porosity and permeability within the rock. Some of the clay minerals that occur in shale have the ability to absorb or adsorb large amounts of water, natural gas, ions, or other substances. This property of shale can enable it to selectively and tenaciously hold or freely release fluids or ions.

Expansive soils map: The United States Geological Survey has prepared a generalized expansive soils map for the lower 48 states.

Engineering Properties of Shale Soils Shales and the soils derived from them are some of the most troublesome materials to build upon. They are subject to changes in volume and competence that generally make them unreliable construction substrates.

Landslide: Shale is a landslide-prone rock.

Expansive Soils The clay minerals in some shale-derived soils have the ability to absorb and release large amounts of water. This change in moisture content is usually accompanied by a change in volume which can be as much as several percent. These materials are called "expansive soils." When these soils become wet they swell, and when they dry out they shrink. Buildings, roads, utility lines, or other structures placed upon or within these materials can be weakened or damaged by the forces and motion of volume change. Expansive soils are one of the most common causes of foundation damage to buildings in the United States.

Shale delta: A delta is a sediment deposit that forms when a stream enters a standing body of water. The water velocity of the stream suddenly decreases and the sediments being carried settle to the bottom. Deltas are where the largest volume of Earth's mud is deposited. The image above is a satellite view of the Mississippi delta, showing its distributary channels and interdistributary deposits. The bright blue water surrounding the delta is laden with sediment.

Slope Stability Shale is the rock most often associated with landslides. Weathering transforms the shale into a clay-rich soil which normally has a very low shear strength - especially when wet. When these low-strength materials are wet and on a steep hillside, they can slowly or rapidly move down slope. Overloading or excavation by humans will often trigger failure.

Shale on Mars: Shale is also a very common rock on Mars. This photo was taken by the mast camera of the Mars Curiosity Rover. It shows thinly bedded fissile shales outcropping in the Gale Crater. Curiosity drilled holes into the rocks of Gale Crater and identified clay minerals in the cuttings. NASA image.

Environments of Shale Deposition An accumulation of mud begins with the chemical weathering of rocks. This weathering breaks the rocks down into clay minerals and other small particles which often become part of the local soil. A rainstorm might wash tiny particles of soil from the land and into streams, giving the streams a "muddy" appearance. When the stream slows down or enters a standing body of water such as a lake, swamp, or ocean, the mud particles settle to the bottom. If undisturbed and buried, this accumulation of mud might be transformed into a sedimentary rock known as "mudstone." This is how most shales are formed. The shale-forming process is not confined to Earth. The Mars rovers have found lots of outcrops on Mars with sedimentary rock units that look just like the shales found on Earth (see photo). Contributor: Hobart King

Sandstone A clastic sedimentary rock composed of sand-size grains of mineral, rock, or organic material.

Sandstone: The specimen shown is about two inches (five centimeters) across.

What is Sandstone? Sandstone is a sedimentary rock composed of sand-size grains of mineral, rock, or organic material. It also contains a cementing material that binds the sand grains together and may contain a matrix of silt- or clay-size particles that occupy the spaces between the sand grains. Sandstone is one of the most common types of sedimentary rock and is found in sedimentary basins throughout the world. It is often mined for use as a construction material or as a raw material used in manufacturing. In the subsurface, sandstone often serves as an aquifer for groundwater or as a reservoir for oil and natural gas.

What is Sand? To a geologist, the word "sand" in sandstone refers to the particle size of the grains in the rock rather than the material of which it is composed. Sand-size particles range in size from 1/16 millimeter to 2 millimeters in diameter. Sandstones are rocks composed primarily of sand-size grains.

Sandstone: Close-up view of the sandstone specimen shown above.

Weathering and Transport of Sand The grains of sand in a sandstone are usually particles of mineral, rock, or organic material that have been reduced to "sand" size by weathering and transported to their depositional site by the action of moving water, wind, or ice. Their time and distance of transport may be brief or significant, and during that journey the grains are acted upon by chemical and physical weathering. If the sand is deposited close to its source rock, it will resemble the source rock in composition. However, the more time and distance that separate the source rock from the sand deposit, the greater its composition will change during transport. Grains that are composed of easily-weathered materials will be modified, and grains that are physically weak will be reduced in size or destroyed. If a granite outcrop is the source of the sand, the original material might be composed of grains of hornblende, biotite, orthoclase, and quartz. Hornblende and biotite are the most chemically and physically susceptible to destruction, and they would be eliminated in the early stage of transport. Orthoclase and quartz would persist longer, but the grains of quartz would have the greatest chance of survival. They are more chemically inert, harder, and not prone to cleavage. Quartz is typically the most abundant type of sand grain present in sandstone. It is extremely abundant in source materials and is extremely durable during transport.

Types of Sand Grains The grains in a sandstone can be composed of mineral, rock, or organic materials. Which and in what percentage depends upon their source and how they have suffered during transport. Mineral grains in sandstones are usually quartz. Sometimes the quartz content of these sands can be very high - up to 90% or more. These are sands that have been worked and reworked by wind or water and are said to be "mature." Other sands can contain significant amounts of feldspar, and if they came from a source rock with a significant quartz content they are said to be "immature."

Siltstone A clastic sedimentary rock composed of silt-size grains.

Siltstone Colors: Siltstone occurs in a wide variety of colors. It is usually gray, brown, or reddish brown. It can also be white, yellow, green, red, purple, orange, black, and other colors. The colors are a response to the composition of the grains, the composition of the cement, or stains from subsurface waters. Specimens in the photo are about two inches across. Click for larger image.

What is Siltstone? Siltstone is a sedimentary rock composed mainly of silt-sized particles. It forms where water, wind, or ice deposit silt, and the silt is then compacted and cemented into a rock. Silt accumulates in sedimentary basins throughout the world. It represents a level of current, wave, or wind energy between where sand and mud accumulate. These include fluvial, aeolian, tidal, coastal, lacustrine, deltaic, glacial, paludal, and shelf environments. Sedimentary structures such as layering, cross-bedding, ripple marks, erosional contacts, and fossils provide evidence of these environments. Siltstone is much less common than sandstone and shale. The rock units are usually thinner and less extensive. Only rarely is one notable enough to merit a stratigraphic name.

What Is Silt? The word "silt" does not refer to a specific substance. Instead, it is a word used for loose granular particles in a specific size range. Silt-sized particles range between 0.00015 and 0.0025 inches in diameter, or between 0.0039 and 0.063 millimeters in diameter. They are intermediate in size between coarse clay on the small side and fine sand on the large side. Grains of coarse silt are large enough that most people can see them without magnification on a background of contrasting color. Most people are not able to sense them if they roll a few grains of silt between their thumb and index finger. Most people are able to detect a few grains of silt by biting them gently between their front

teeth. (This test is not recommended, but some experienced geologists and soil scientists use it for quick field identification of silt in sediment and soil.) Silt does not have a definite composition. It is usually a mixture of clay minerals, micas, feldspars, and quartz. The small-size fraction of silt is mostly clay. The coarse-size fraction is mostly grains of feldspar and quartz.

Siltstone Outcrop: An exposure of the Holtzclaw siltstone near Louisville, Kentucky. It shows the thinly bedded and differentially weathered character of the rock unit. Siltstones are rarely of sufficient thickness or lateral persistence to merit a stratigraphic name. Public domain photo by John Knouse.

What Color is Siltstone? Siltstone occurs in a wide range of colors. It is usually gray, brown, or reddish brown. White, yellow, green, red, purple, orange, black, and other colors occur. The color is caused by the composition of the grains, the composition of the cement that binds them together, and stains produced by contact with subsurface waters.

Field Identification Siltstone can be difficult to identify in the field without close examination. Weathered surfaces often appear to show sedimentary structures where none are present. Different layers weather at different rates. Siltstone is often interbedded with other lithologies. Identification requires breaking off a small piece and observing the grain size. Scraping the surface with a nail or knife blade will dislodge tiny silt grains instead of dislodging sand grains or producing a white effervescent powder.

Siltstone Uses and Economics Siltstone has very few uses. It is rarely the target of mining for use as a construction material or manufacturing feedstock. The intergranular pore spaces in siltstone are too small for it to serve as a good aquifer. It is rarely porous enough or extensive enough to serve as an oil or gas reservoir. Its main use is as a low-quality fill when better materials are not locally available.

What are Minerals? Minerals are the foundation of industries ranging from construction to manufacturing to agriculture to technology and even cosmetics.

Rhodochrosite: Specimen of rhodochrosite from the Sunnyside Mine, San Juan County, Colorado. Rhodochrosite is a manganese carbonate mineral (MnCO3) that is used as an ore of manganese and is also cut as a gemstone. USGS image.

We Use Minerals Many Times Every Day! Every person uses products made from minerals every day. The salt that we add to our food is the mineral halite. Antacid tablets are made from the mineral calcite. It takes many minerals to make something as simple as a wooden pencil. The "lead" is made from graphite and clay minerals, the brass band is made of copper and zinc, and the paint that colors it contains pigments and fillers made from a variety of minerals. A cell phone is made using dozens of different minerals that are sourced from mines throughout the world. The cars that we drive, the roads that we travel, the buildings that we live in, and the fertilizers used to produce our food are all made using minerals. In the United States, about three trillion tons of mineral commodities are consumed each year to support the standard of living of 300 million citizens. That is about ten tons of mineral materials consumed for every person, every year.

Common items made from minerals: Most of the things that we use in our daily life are either made from minerals or produced using mineral products. Antacid tablets are made from calcite, table salt is crushed halite, several minerals are used to make a wood pencil, and dozens of minerals from many different countries are used to make a cell phone.

Did You Know? The white "m" on a piece of M&M's candy is a titanium oxide pigment, most likely produced from the mineral rutile.

Structure of the mineral halite: The mineral "halite" has a chemical composition of NaCl. That means it contains equal numbers of sodium and chloride atoms. In this case they are electrically charged atoms, known as ions. Those ions are arranged in a cubic pattern that repeats in all directions. The small sodium ions are positioned between the larger chloride ions.

What are Minerals? To meet the definition of "mineral" used by most geologists, a substance must meet five requirements:     

naturally occurring inorganic solid definite chemical composition ordered internal structure

"Naturally occurring" means that people did not make it. Steel is not a mineral because it is an alloy produced by people. "Inorganic" means that the substance is not made by an organism. Wood and pearls are made by organisms and thus are not minerals. "Solid" means that it is not a liquid or a gas at standard temperature and pressure. "Definite chemical composition" means that all occurrences of that mineral have a chemical composition that varies within a specific limited range. For example: the mineral halite (known as "rock salt" when it is mined) has a chemical composition of NaCl. It is made up of an equal number of atoms of sodium and chlorine.

"Ordered internal structure" means that the atoms in a mineral are arranged in a systematic and repeating pattern. The structure of the mineral halite is shown in the illustration on this page. Halite is composed of an equal ratio of sodium and chlorine atoms arranged in a cubic pattern.

Did You Know? Although liquid water is not a mineral, it is a mineral when it freezes. Ice is a naturally occurring, inorganic solid with a definite chemical composition and an ordered internal structure. Learn more.

The Word "Mineral" The word "mineral" is used in many different ways. The definition given above is a formal definition preferred by geologists. The word also has a nutritional meaning. It is used in reference to the many inorganic chemicals that organisms need to grow, repair tissue, metabolize, and carry out other body processes. Mineral nutrients for the human body include: iron, calcium, copper, sulfur, phosphorus, magnesium and many others. An archaic use of the word "mineral" comes from the Linnaean taxonomy in which all things can be assigned to the animal, vegetable, and mineral kingdoms. The word "mineral" is also used inconsistently in geology. In mining, anything obtained from the ground and used by man is considered to be a "mineral commodity" or a "mineral material." These include: crushed stone, which is a manufactured product made from crushed rocks; lime, which is a manufactured product made from limestone or marble (both composed of the mineral calcite); coal which is organic; oil and gas which are organic fluids; rocks such asgranite that are mixtures of minerals; and, rocks such as obsidian which do not have a definite composition and ordered internal structure.

Minerals in rocks: Most rocks are aggregates of minerals. This rock, a granite pegmatite, is a mixture of mineral grains. It contains pink orthoclase, milky quartz, black hornblende and black biotite.

Mineral Commodities in Industry 2010 United States Mineral Commodity Consumption Mineral Commodity

Millions of Metric Tons

Crushed Stone

1,200.0

Sand and Gravel

786.1

Salt

55.8

Iron Ore

48.0

Phosphate Rock

30.5

Gypsum

22.5

Clays

21.3

Dimension Stone

14.0

Lime

18.5

Sulfur

11.30

Bauxite

8.4

Potash

5.6

Soda Ash

5.2

Barite

2.66

Copper

1.74

Lead

1.40

Values above are estimates of mineral commodity consumption from the United States Geological Survey. Many other commodities could be added to this table.

The construction industry is the largest consumer of mineral commodities. Crushed stoneis used for foundations, road base, concrete, and drainage. Sand and gravel are used in concrete and foundations. Clays are used to make cement, bricks, and tile. Iron ore is used to make reinforcing rods, steel beams, nails, and wire. Gypsum is used to make drywall. Dimension stone is used for facing, curbing, flooring, stair treads, and other architectural work. These are just a few of the many uses for these commodities in construction. In agriculture, phosphate rock and potash are used to make fertilizer. Lime is used as an acid-neutralizing soil treatment. Mineral nutrients are added to animal feed. The chemical industry uses large amounts of salt, lime, and soda ash. Large amounts of metals, clay, and mineral fillers/extenders are used in manufacturing.

Mohs Hardness Scale is a set of reference minerals used for classroom hardness testing. Determining the hardness of a mineral is one of the most important tests used in mineral identification.

The Acid Test: Geologists use dilute hydrochloric acid to identify carbonate minerals. Carbonate minerals will effervesce with various levels of vigor in contact with the acid.

Physical Properties of Minerals There are approximately 4000 different minerals, and each of those minerals has a unique set of physical properties. These include: color, streak, hardness, luster, diaphaneity, specific gravity, cleavage, fracture, magnetism, solubility, and many more. These physical properties are useful for identifying minerals. However, they are much more important in determining the potential industrial uses of the mineral. Let's consider a few examples. The mineral talc, when ground into a powder, is perfectly suited for use as a foot powder. It is a soft, slippery powder so it will not cause abrasion. It has the ability to absorb moisture, oils, and odor. It adheres to the skin and produces an astringent effect - yet it washes off easily. No other mineral has a set of physical properties that are as suitable for this purpose. The mineral halite, when crushed into small grains, is perfectly suited for flavoring food. It has a salty taste that most people find pleasing. It dissolves quickly and easily, allowing its flavor to spread through the food. It is soft, so if some does not dissolve it will not damage your teeth. No other mineral has physical properties that are better suited for this use. The mineral gold is perfectly suited for use in jewelry. It can be easily shaped into a custom item of jewelry by a craftsperson. It has a pleasing yellow color that most people enjoy. It has a bright luster that does not tarnish. Its high specific gravity gives it a nice "heft" that is preferred by most people over lighter metals. Other metals can be used to make jewelry, but these properties make gold an overwhelming favorite. (Some people might add that gold's rarity and value are two additional properties that make it desirable for jewelry. However, rarity is not a property, and its value is determined by supply and demand.)

Star sapphire: A deep blue star sapphire 8 mm x 6 mm cabochon from Thailand. Inclusions of rutile within the stone align with the crystallographic axis of the corundum to produce the star - which is only clearly visible and centered when the back of the stone is cut at 90 degrees to the C-axis of the crystal. This stone has been heat treated to darken the stone and enhance visibility of the star. Mineral Information [1] Minerals on Geology.com: Links to content about minerals on the geology.com website. [2] USGS Mineral Information: Links to information about minerals on the United States Geological Survey website. [3] State and National Geological Surveys: Links to geological survey websites, most of which contain information about minerals found and produced in their location. [4] USGS Mineral Resources Program: The sole Federal source of scientific information on mineral potential, production, consumption, and environmental effects. [5] The Mineral and Locality Database: The largest mineralogy database and reference website on the internet.

Physical Properties: Determining Factors The primary characteristics of a mineral that determine its physical properties are its composition and the strength of the bonds in its ordered internal structure. Here are some examples: Galena, a lead sulfide, has a much higher specific gravity than bauxite, an aluminum hydroxide. This difference is because of their composition. Lead is much heavier than aluminum. Diamond and graphite both consist of pure carbon. Diamond is the hardest natural mineral, and graphite is one of the softest. This difference occurs because of the types of bonds connecting the carbon atoms in their mineral structures. Each carbon atom in diamond is bonded to four other carbon atoms with strong covalent bonds. Graphite has a sheet structure in which atoms within the sheets are bonded to one another with strong covalent bonds, but the bonds between the sheets are weak electrical bonds. When graphite is scratched the weak bonds fail easily, making it a soft mineral.

The gemstones ruby and sapphire are color variations of the mineral corundum. These color differences are caused by composition. When corundum contains trace amounts of chromium, it exhibits the red color of a ruby. However, when it contains trace amounts of iron or titanium, it exhibits the blue color of sapphire. If, at the time of crystallization, enough titanium is present to form tiny crystals of the mineral rutile, a star sapphire may form. This occurs when tiny crystals of rutile align systematically within the crystalline structure of the corundum to give it a silky luster that might produce a "star" that aligns with the primary crystallographic axis (see photo). Contributor: Hobart King

Andalusite and Chiastolite

Andalusite: A scatter of faceted andalusite. If you look closely at these gems, you can see that many of them appear to be composed of a mosaic of color. This is an expression of the strong pleochroism of andalusite. Image by cobalt123, used here under a Creative Commons license.

What is Andalusite? Andalusite is a rock-forming mineral that is mined for use in high-temperature refractories. Gem-quality specimens are cut into faceted gems and cabochons. Andalusite forms during the regional metamorphism of shale. It is found in schist and gneiss at some present and ancient convergent plate boundaries where the rocks have been exposed to the temperatures and pressures needed for its formation. In these rocks, andalusite is often associated with kyanite and sillimanite. Andalusite also forms during the contact metamorphism of argillaceous rocks. In this situation, it can form within the metamorphosed rock or in veins and cavities within the igneous rock. It can be associated with cordierite in hornfels, granite, and granitic pegmatite.

Chiastolite: A cabochon cut from a specimen of the chiastolite variety of andalusite. This specimen exhibits a sharp cross, formed from graphite particles that were pushed out of the way during crystal growth. The diagonal fiber of this specimen is a result of needle-like crystals (possibly rutile crystals) that have grown within the andalusite crystal.

What is Chiastolite? Chiastolite is a variety of andalusite that contains black particles of graphite arranged in geometric patterns. The graphite is pushed aside by crystal growth within a rock that is being metamorphosed. As growth occurs, the particles become concentrated at crystal interfaces. The result can be a cross-shaped pattern within the mineral - similar to the "cross-stone" shown in the photo here. People have known about these cross stones for centuries and have valued them for their perceived religious or spiritual meaning. Attractive specimens are often cut and polished for use as amulets, charms, and novelty gems.

Twinned andalusite crystals: Twinned crystals of andalusite (chiastolite) in a piece of black micaceous schist. Photo by Moha112100, used here under a Creative Commons license.

Physical Properties of Andalusite Chemical Classification

Silicate

Color

Reddish brown, olive green, white to gray

Streak

White

Luster

Vitreous

Diaphaneity Cleavage

Transparent to nearly opaque Good

Mohs Hardness

6.5 to 7.5

Specific Gravity

3.17

Diagnostic Properties

Crystal form, associated minerals, strongly pleochroic, symmetrical inclusions

Chemical

Al2SiO5

Composition Crystal System

Uses

Orthorhombic Used to manufacture high-temperature porcelain of spark plugs; used to make high-temperature ceramics used in furnaces, kilns, incinerators; high-quality crystals are often used as gemstones.

Physical Properties and Uses of Andalusite Andalusite has a number of useful physical properties. It has the ability to withstand high temperatures without alteration. For that reason it is used to make high-temperature ceramics and refractories. The white porcelain of many spark plugs is made using andalusite. Andalusite is one of a small number of minerals that commonly forms prismatic crystals with a square crosssection. This can be important information for identification in the field. Transparent specimens of andalusite are often strongly pleochroic. This makes them have different apparent colors when viewed from different directions. This pleochroic effect allows andalusite to be cut into unique gemstones. Although twinning is not common in andalusite, nicely crystallized specimens that possess twinning can be distinctive. Twinning can produce cross-shaped structures perpendicular to the crystallographic c-axis, similar to what is shown in the rock in the photo above.

Andalusite: Crystals of andalusite showing their prismatic habit and square cross-section. These crystals are from Lisens Valley, Austria. Specimen and photo by Arkenstone / www.iRocks.com.

Andalusite as an Indicator Mineral

Stability fields: This chart shows the temperature / pressure ranges where andalusite, kyanite, and sillimanite are stable.

Andalusite, kyanite, and sillimanite all share the chemical composition of Al2SiO5. However, they have different crystal structures. Their crystal structure differs because they form under extremely different conditions of temperature and pressure. The phase diagram at left summarizes the conditions under which these minerals form. Andalusite is the low-temperature mineral of the three. Sillimanite is the high-temperature mineral, and kyanite forms at high pressures and lower temperatures. Information from a phase diagram can be useful during mineral exploration. If a geologist finds andalusite in the field, the phase diagram reveals the possible range of temperatures and pressures that the rocks were subjected to when the andalusite crystallized. If the mineral being sought has a dramatically different temperature and pressure of crystallization, then it might not be present in those rocks. If the pressure range of the target mineral is higher, then it is possible that it exists at depth. If the temperature range of the target mineral is higher, then exploration should move toward a heat source or toward greater depth. That is a simplified example of how the phase diagram can be used. Contributor: Hobart King

Anhydrite An evaporite mineral used as a soil treatment and to produce construction materials

Anhydrite: Anhydrite from Balmat, New York. This specimen of massive anhydrite has a typical gray color and a sugary appearance on broken surfaces caused by exposure of cleavage faces. Specimen is approximately 4 inches (10 centimeters) across.

What is Anhydrite? Anhydrite is an evaporite mineral that occurs in extensive layered deposits in sedimentary basins where large volumes of sea water have been evaporated. It is typically interbedded with halite, gypsum, and limestone in accumulations that can be up to hundreds of feet thick. On a much smaller scale, anhydrite can form in shoreline or tidal flat sediments from the evaporation of sea water. Anhydrite also occurs as a vein-filling mineral in hydrothermal deposits. It is deposited from solution, often along with calcite and halite, as gangue in sulfide mineral deposits. Anhydrite is also found in the cap rock of salt domes and in cavities of trap rock. Anhydrite is an anhydrous calcium sulfate with a composition of CaSO4. It is closely related to gypsum, which has a chemical composition of CaSO4.2H2O. The worldwide abundance of gypsum greatly exceeds the abundance of anhydrite. Anhydrite receives its name from the Greek "anhydrous" which means "without water." It readily converts to gypsum under humid conditions or in contact with groundwater. This transition involves the absorption of water and a significant change in volume. That expansion can cause deformation in the rock units. If gypsum is heated to about 200 degrees Celsius, it will yield water and be converted to anhydrite. This reaction occurs much less often.

Physical Properties of Anhydrite Chemical Classification

Sulfate

Color

Colorless, white, and light shades of brown, red, gray, pink, blue, violet

Streak

White

Luster

Vitreous to pearly

Diaphaneity Cleavage

Transparent to translucent Perfect cleavage in three directions to form cubic-shaped cleavage fragments

Mohs Hardness

3 to 3.5

Specific Gravity

2.9 to 3

Diagnostic Properties

Cubic cleavage, harder than gypsum, higher specific gravity than calcite, no acid reaction.

Chemical Composition

CaSO4

Crystal System Uses

Orthorhombic Soil treatment. Ingredient in plaster and other construction materials.

Anhydrite: Massive anhydrite from Balmat, New York showing sedimentary layering and sugary appearance on broken surfaces. Specimen is approximately 4 inches (10 centimeters) across.

Physical Properties and Identification One of the most distinctive properties of anhydrite is its cubic cleavage. It cleaves in three directions at right angles. This can easily be seen in coarsely crystalline specimens or with a hand lens in fine-grained specimens. This distinctive cleavage has earned anhydrite the nickname of "cube spar."

Anhydrite can be a small challenge to identify when it occurs in massive form. It can be confused with gypsum, calcite, or halite - which it is almost always associated with. Compared to gypsum, anhydrite exhibits cleavage in three directions at right angles and has a greater hardness. Its right angle cleavage and lack of acid reaction allows it to be distinguished from calcite. Compared to halite, anhydrite is insoluble and slightly harder.

Anhydrite: Anhydrite from Mound House, Nevada with a very fine texture that could be confused with lithographic limestone. Specimen is approximately 4 inches (10 centimeters) across.

Uses of Anhydrite Anhydrite can be substituted for gypsum in some of its uses. Both minerals are crushed for use as a soil treatment, and in this purpose anhydrite is superior. One ton of anhydrite has more calcium than one ton of gypsum - because gypsum is about 21% water by weight. This yields more calcium per ton in a soil application. Anhydrite also has a higher solubility, which helps it benefit the soil quickly. Small amounts of anhydrite are used as drying agents in plaster, paint, and varnish. It is also used along with gypsum to produce plaster, joint compound, wallboard, and other products for the construction industry. Anhydrite has also been used as a source of sulfur in the production of sulfuric acid.

Synthetic Anhydrite Hydrofluoric acid is produced using fluorite and sulfuric acid. For every ton of hydrofluoric acid produced, about 3 1/2 tons of synthetic anhydrite is produced. For decades this synthetic anhydrite was considered to be a nuisance product that had a disposal expense. However, much of it is now dried in a kiln and used as a raw material for producing cement, plaster, and flooring. It is also used as a filler in the production of plastics and paper products. Contributor: Hobart King

Apatite, Phosphorite and Phosphate Rock

Apatite Crystals: A scatter of small greenish yellow apatite crystals from Cerro del Mercado, Durango, Mexico. These hexagonal crystals are small, mostly about 8 millimeters in length. Apatite crystals of this clarity are not often found in large sizes. Image copyright by Geology.com.

What is Apatite? Apatite is the name of a group of phosphate minerals with similar chemical compositions and physical properties. They are an important constituent of phosphorite, a rockmined for its phosphorus content and used to make fertilizers, acids, and chemicals. Apatite has a relatively consistent hardness and serves as the index mineral for a hardness of five in the Mohs Hardness Scale. Specimens with excellent clarity and color are sometimes cut as faceted gemstones. Those with good color and translucence are cut as cabochons. Physical Properties of Apatite Chemical Classification

Color

Phosphate

Green, brown, blue, yellow, violet, pink, colorless. Transparent specimens with excellent clarity and vivid color are used as gemstones.

Streak

White

Luster

Vitreous to subresinous

Diaphaneity Cleavage

Transparent to translucent Poor to indistinct

Mohs Hardness

5

Specific Gravity

3.1 to 3.3

Diagnostic Properties

Color, crystal form, and hardness. Brittle, often highly fractured. Can be scratched with a steel knife blade.

A group of calcium phosphates. Fluorapatite: Ca5(PO4)3F

Chemical

Hydroxylapatite: Ca5(PO4)3(OH)

Composition

Chlorapatite: Ca5(PO4)3Cl Carbonate-rich apatite/francolite: Ca5(PO4,CO3)3(F,O)

Crystal System

Uses

Hexagonal Fertilizer, phosphoric acid, hydrofluoric acid, gemstones, ore of rare earth elements, pigments, gemstone. Serves as a hardness of 5 on the Mohs Hardness Scale.

Physical Properties Apatite is best known for its use as an index mineral with a hardness of 5 in the Mohs Hardness Scale. It is usually green in color, but can be yellow, brown, blue, purple, pink, or colorless. These colors are often so vivid that apatite has frequently been cut as a gemstone. Apatite is a brittle material. It breaks by both fracture and cleavage, but the cleavage is generally indistinct. Hexagonal apatite crystals are sometimes found in igneous and metamorphic rocks.

Faceted Apatite: Five faceted stones from Madagascar in various colors. Clockwise from left: a green 8 x 6.2 millimeter oval of 1.23 carats; a yellow 8 x 6.3 millimeter oval of 1.37 carats; a bluish green 8.1 x 6.2 millimeter oval of 1.38 carats; a blue 7.1 x 5 millimeter oval of 0.91 carats (heat treated); and, a bluish green 7.1 x 5.2 millimeter oval of 1.05 carats (heat treated).

Geologic Occurrence Apatite forms under a wide variety of conditions and is found in igneous, metamorphic, and sedimentary rocks. The most important deposits of apatite are in sedimentary rocks formed in marine and lacustrine environments. There, phosphatic organic debris (such as bones, teeth, scales, and fecal material) had accumulated and was mineralized during diagenesis. Some of these deposits contain enough phosphorus that they can be mined and used to produce fertilizers and chemical products.

Apatite occasionally occurs as well-formed hexagonal crystals in hydrothermal veins and pegmatite pockets. These crystals often have a very high clarity and a vivid color and have been cut into gems for collectors. Mineral collectors also enjoy these well-formed apatite crystals, and the prices paid for them often exceeds their value as gem rough.

Phosphate rock from the Upper Zhujiaqing Formation of the Yunnan Province of Southwestern China with a fossiliferous peloidal texture. Photograph by James St. John, used here under a Creative Commons License.

Phosphate Rock and Phosphorite Phosphate rock and phosphorite are names used for sedimentary rocks that contain at least 15% to 20% phosphate on the basis of weight. The phosphorous content in these rocks is mainly derived from the presence of apatite minerals. Determining which apatite-group minerals are contained in the rock cannot be determined without laboratory testing because their particle sizes are so small. Most phosphate rock has a non-detrital origin similar to limestone. Some of the phosphate is deposited by precipitation from solution; some is the remains and waste products of organisms; and, some is deposited by groundwater during diagenesis. Like limestone, phosphate rock is deposited in sedimentary basins where the influx of detrital material is relatively low. That allows the phosphate to accumulate with very little dilution from other materials. Where the dilution rate is high, phosphatic shales, mudstones, limestones, and sandstones will form instead of phosphate rock.

Phosphate rock from the Simplot Mine, Phosphoria Formation of southeastern Idaho, with a peloidal texture. Photograph by James St. John, used here under a Creative Commons License.

Uses of Apatite as Phosphate Rock Most of the phosphate rock mined throughout the world is used to produce phosphate fertilizer. It is also used to produce animal feed supplements, phosphoric acid, elemental phosphorous, and phosphate compounds for the chemical industry. China is the largest producer of phosphate rock, producing approximately 100 million tons in 2014. The United States, Russia, Morocco, and Western Sahara are also major phosphate producers. Over 75% of the world's reserves of phosphate rock are located in Morocco and Western Sahara. Phosphate rock is the only material that can be used to produce enough fertilizer to satisfy world demand. Without it, farmers would not be able to produce enough food to feed the world's population. It is surprising that one type of rock, a rock that most people know nothing about, is so important to keeping the world fed and alive.

Cat's-Eye Apatite: Two nice cabochons with a chatoyanceproduced by a fine silk of rutile needles. The stone on the left is a yellow 7 x 5.9 millimeter oval of 1.82 carats cut from apatite mined in Kenya. The stone on the right is a greenish golden (heattreated) 9.3 x 6.9 millimeter oval of 2.77 carats cut from apatite mined in Kenya.

Gemology Transparent specimens of apatite with vivid green, blue, yellow, or pink color and excellent clarity are often cut into faceted gemstones. Some stones are heat treated to improve their color. Attractive translucent stones of excellent color are cut en cabochon. Rarely, translucent apatite contains a fine silk of parallel rutile crystals. When cut en cabochon with the silk oriented parallel to the bottom of the stone, these specimens will often exhibit a chatoyance known as "cat's eye." As a gemstone, apatite is more popular with gem collectors than it is with jewelry buyers. The mineral has a Mohs hardness of 5, breaks with parting, and is very brittle. These characteristics make it too fragile for use in most types of jewelry. Contributor: Hobart King

Arsenopyrite Mineral Properties and Uses

What is Arsenopyrite? Arsenopyrite from Gold Hill, Utah. Specimen is approximately 4 inches (10 centimeters) across

Arsenopyrite is an iron arsenic sulfide. It is the most common arsenic mineral and the primary ore of arsenic metal. Arsenopyrite is most often found as a hydrothermal vein mineral and sometimes as a mineral of contact metamorphism. It is sometimes referred to in old texts as "mispickel".

Physical Properties of Arsenopyrite Chemical Classification

sulfide

Color

silver white to steel gray

Streak

dark grayish black

Luster

metallic

Diaphaneity

opaque

Cleavage

poor

Mohs Hardness

5.5 to 6

Specific Gravity

5.9 to 6.2

Diagnostic Properties Chemical Composition Crystal System Uses

smells like garlic when crushed, crystal form iron arsenic sulfide, FeAsS monoclinic poison, preservative, pigment

Augite A common rock-forming mineral of dark-colored igneous rocks.

Augite: A specimen of the "jeffersonite" variety of augite. Approximately 11 x 6.3 x 4.3 centimeters in size. From the Franklin Mining District of Sussex County, New Jersey. Specimen and photo by Arkenstone / www.iRocks.com.

Igneous rock composition chart: This chart illustrates the generalized mineral composition of igneous rocks. Augite, as the most abundant pyroxene mineral, can play an important role in the composition of gabbro, basalt, diorite, and andesite.

What is Augite? Augite is a rock-forming mineral that commonly occurs in mafic and intermediate igneous rocks such as basalt, gabbro, andesite, and diorite. It is found in these rocks throughout the world, wherever they occur. Augite is also found in ultramafic rocks and in some metamorphic rocksthat form under high temperatures. Augite has a chemical composition of (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 with many paths of solid solution. Commonly associated minerals include orthoclase, plagioclase, olivine, and hornblende. Augite is the most common pyroxene mineral and a member of the clinopyroxene group. Some people use the names "augite" and "pyroxene" interchangeably, but this usage is strongly discouraged. There are a large number of pyroxene minerals, many of which are distinctly different and easy to identify.

Augite, diopside, jadeite, spodumene, and hypersthene are just a few of the distinctly different pyroxene minerals. Physical Properties of Augite Chemical

A single chain inosilicate

Classification Color

Streak

Luster Diaphaneity Cleavage

Dark green, black, brown White to gray to very pale green. Augite is often brittle, breaking into splintery fragments on the streak plate. These can be observed with a hand lens. Rubbing the debris with a finger produces a gritty feel with a fine white powder beneath. Vitreous on cleavage and crystal faces. Dull on other surfaces. Usually translucent to opaque. Rarely transparent. Prismatic in two directions that intersect at slightly less than 90 degrees.

Mohs Hardness

5.5 to 6

Specific Gravity

3.2 to 3.6

Diagnostic Properties

Two cleavage directions intersecting at slightly less than 90 degrees. Green to black color. Specific gravity.

Chemical Composition Crystal System Uses

A complex silicate. (Ca,Na)(Mg,Fe,Al)(Si,Al)2O6 Monoclinic No significant commercial use.

Physical Properties of Augite Augite is usually green, black, or brown in color with a translucent to opaque diaphaneity. It usually exhibits two distinct cleavage directions that intersect at slightly less than 90 degrees. A hand lens is often needed to properly observe the cleavage, especially in fine-grained rocks. Light reflecting from cleavage surfaces and crystal faces of augite produces a vitreous luster, while light striking other surfaces produces a dull luster. Augite has a Mohs hardness of 5.5 to 6. Its specific gravity of 3.2 to 3.6 is higher than most other minerals in the rocks in which it occurs.

Augite: A specimen of the "fassaite" variety of augite. Approximately 5 x 3.1 x 1.4 centimeters in size. From the Skardu District of Pakistan. Specimen and photo by Arkenstone / www.iRocks.com.

Uses of Augite Augite does not have any physical, optical, or chemical properties that make it especially useful. It is therefore one of the few minerals that has no commercial use. The calcium content of augite has been found to be of limited use in studies of the temperature history of igneous rocks.

Extraterrestrial Augite Augite is a mineral that has been found beyond Earth. It is a common mineral of lunar basalts. It has also been identified in many stone meteorites. Some of these meteorites are thought to be pieces of Mars or the Moon that were launched into space by large impact events.

Azurite A deep blue mineral, gem material, ornamental stone, ore of copper, and pigment.

Azurite with Malachite Nodule: A specimen of nodular azurite sawn and polished to reveal its beautiful blue structures. A specimen like this would be a superb gem material or ornamental stone. Approximately 8.6 x 7.5 x 3.1 centimeters in size. From the Bisbee area of Cochise County, Arizona. Specimen and photo by Arkenstone / www.iRocks.com.

What is Azurite? Azurite is a copper carbonate hydroxide mineral with a chemical composition of Cu 3(CO3)2(OH)2. It is best known for its characteristic deep blue to violet-blue color. The blue color, known as "azure," is like the deep blue evening skies often seen above deserts and winter landscapes. Azurite is not a common or abundant mineral, but it is beautiful and its blue color attracts attention. It has been used by people in many parts of the world for thousands of years. Ancient people used it as an ore of copper, as a pigment, as a gemstone, and as an ornamental stone. It is still used for all of these purposes today.

Azurite and Chrysocolla: A slab of rock showing fractures filled with azurite and chrysocolla. Slab is about 8 centimeters long. Photographed wet to show full color. From near Bisbee, Arizona.

Geologic Occurrence Azurite is a secondary mineral that usually forms when carbon-dioxide-laden waters descend into the Earth and react with subsurface copper ores. The carbonic acid of these waters dissolves small amounts of copper from the ore. The dissolved copper is transported with the water until it reaches a new geochemical environment. This new environment could be a location where water chemistry or temperature changes, or where evaporation occurs. If conditions are right, the mineral azurite might form. If these conditions persist for a long time, a significant accumulation of azurite might develop. This has occurred in many parts of the world. Azurite precipitation occurs in pore spaces, fractures, and cavities of the subsurface rock. The resulting azurite is usually massive or nodular. In rare situations, azurite is found as stalactitic and botryoidal growths. Wellformed monoclinic crystals are infrequently found. These can only occur if azurite precipitates unrestricted in a fracture or cavity and is not disrupted by later crystallization or rock movements. Malachite is another copper carbonate mineral that forms under conditions similar to azurite. These minerals are often found in the same deposit and are often intergrown with one another. This produces a material known as azurmalachite, which, when of high quality, can be used as a beautiful lapidary material. In the United States, Arizona, New Mexico, and Utah are the notable locations for finding azurite. More important deposits have been found in France and Namibia. Noteworthy occurrences have been found in Mexico, Chile, Australia, Russia, and Morocco.

Azurite Nodules in Sandstone: Small azurite nodules about one centimeter in size in a matrix of fine-grained sandstone. From the Nacimiento Mine, New Mexico.

Physical Properties of Azurite Chemical Classification

Carbonate

Color

Deep blue to violet blue; "azure"

Streak

Light blue

Luster

Vitreous, earthy

Diaphaneity

Opaque, translucent, transparent

Cleavage

Two distinct directions, one perfect, one poor

Mohs Hardness

3.5 to 4

Specific Gravity

3.7 to 3.9

Diagnostic Properties

Deep blue color, effervescence in dilute HCl, high specific gravity, low hardness.

Chemical Composition

Copper carbonate. Cu3(CO3)2(OH)2

Crystal System Uses

Monoclinic Minor ore of copper, gem material, ornamental stone, pigment.

Physical Properties of Azurite The most diagnostic property of azurite is it distinctive deep blue color. It is also soft with a Mohs hardness of only 3.5 to 4. It contains copper, which gives its blue color and a specific gravity of 3.7 to 3.9, which is exceptionally high for a non-metallic mineral. Azurite is a carbonate mineral and produces a slight effervescence with dilute hydrochloric acid, producing a light blue liquid. Azurite produces a light blue streak on unglazed porcelain.

Uses of Azurite While azurite is not an extremely abundant mineral and is rarely found in large deposits, it has been used in a number of ways. Some of these are explained below.

Azurite "Blueberries": Small nodules of azurite, between five and ten millimeters across, that weathered out of a poorlycemented sandstone near La Sal, Utah.

Copper Prospecting and Mining Geologists know that abundant azurite is often found in the rocks above deposits of copper ore. That enables them to use azurite as an indicator mineral in the search for subsurface copper deposits. The presence of abundant azurite indicates the possibility of finding some form of copper ore below, nearby, or up a contemporary or ancient hydraulic gradient. Azurite has been used as an ore of copper metal for thousands of years. The ancient Egyptians mined it on the Sinai Peninsula and smelted it to produce copper. Today, azurite deposits on their own are usually not large enough to be worth opening a copper mine. Where other copper ores are mined, azurite might be removed if it is of adequate grade and easy to mine.

Azurmalachite Cabochons: Azurite is frequently associated with malachite, and that association can produce very interesting gem materials. These cabochons were cut from a material known as "azurmalachite" produced at the Morenci Mine in Arizona. They were cut from thin vein material and have a natural wall-rock backing. Both cabs are about 25 millimeters tall.

Azurite Granite: A piece of "K2 Granite" or "Azurite Granite," an ornamental stone recently found along the border between China and Pakistan. It performs well as a lapidary material and gemstone. Photographed dry. This piece is about ten centimeters across, and the largest orb is about on centimeters across.

Jewelry and Ornamental Stone Azurite is easy to cut and shape into cabochons, beads, small carvings, and ornaments. It also accepts a bright polish. Unfortunately, azurite has problems that limit its use in jewelry. The greatest concern is the fact that azurite has a Mohs hardness of just 3.5 to 4.0. It also is brittle and can break along cleavage planes. This lack of durability makes it easily damaged if used in a ring, bracelet, or other jewelry item that is subject to abrasion. Azurite also slowly weathers to malachite. This results in a lightening and greening of the gemstone's deep blue color. Store azurite jewelry in darkness, away from heat, and where air circulation is limited. This might be in a closed jewelry box or drawer. Azurite jewelry is difficult to clean. A gentle cleaning with a soft damp cloth or with cool soapy water is best. Abrasive cleaners or excessive cleaning will damage the stone. Ultrasonic and steam cleaning can cause damage. If jewelry containing azurite needs repairs, the repairs should be done in a way that does not heat the stone. Hydroxide minerals are very sensitive to heat. Heating will cause azurite to green or blacken. Azurite is rarely treated to enhance its color. However, it is frequently treated with resins and other substances that impregnate and stabilize the rough. Much of the inexpensive material sold as "azurite" is a composite made of crushed azurite in a binder of resin or other substance. Often, chrysocolla, malachite, or other minerals are blended in. An interesting ornamental stone found near the border between China and Pakistan has recently appeared in the lapidary market. It is a white granite with orbs of bright blue azurite dispersed through the stone. Most people who see it initially think that it is fake, but it can be sawn to reveal round azurite areas inside, and xray diffraction reveals azurite. This azurite granite is commonly called "K2 granite" after the second-highest mountain in the world, because the rock was first discovered near the base of the mountain.

Azurite Pigment: High-purity azurite finely ground into a powder and ready for use as a pigment. Azurite has been used as a pigment for thousands of years. Today, synthetic pigments are used more than natural pigments. They are lower in cost and standardized in their properties.

Azurite Pigments Azurite was ground and used as a pigment in blue paint as early as ancient Egypt. Through time, its use became much more common. During the Middle Ages and Renaissance, it was the most important blue pigment used in Europe. Much of the azurite used to make the pigment was mined in France. Making pigment from azurite was costly. During the Middle Ages it was difficult to mine, transportation was slow, and grinding and processing were slow and difficult. Azurite pigment was gradually replaced, starting in the 18th century, when man-made pigments such as "Prussian blue" and "blue verditer" were invented. These synthetic pigments are standardized products with uniform properties. That makes them predictable in their use. They are also less costly to produce. Many paintings done during the Middle Ages, before azurite was replaced with Prussian blue, show deterioration of the blue color. Over time and exposure to the atmosphere and light, azurite slowly weathers to malachite. Much of the blue azurite pigment used during the Middle Ages now shows obvious signs of green malachite as a weathering product. This is another reason why man-made pigments are now used instead of azurite. Azurite pigment and paints are still available today and are easy to find. But they are mainly used by painters who want to employ historical methods in their work.

Azurite Crystals: Well-formed crystals of azurite are popular with mineral collectors because of their rarity and beauty. This small cluster of blade-shaped azurite crystals is from the Tsumeb Mine in Namibia. This specimen is small, about 1.4 x 1.4 x 0.4 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Mineral Collecting Azurite is popular with mineral collectors. They appreciate its deep blue monoclinic crystals, nodular habit with interesting structures, and representative examples of its botryoidal and stalactitic habits. Excellent specimens can sell for hundreds, thousands, or tens of thousands of dollars depending upon their quality and size. The instability of azurite is a problem for collectors. If exposed to heat or high humidity, specimen surfaces will begin to weather to malachite. This causes a dull, faded or greenish appearance depending upon the severity of alteration. Valuable specimens are best stored in closed collection drawers where there is limited air circulation, darkness, and cool, stable temperatures.

Barite The nonmetallic mineral with an incredible specific gravity.

Barite: Barite from Kings Creek, South Carolina. Specimen is approximately 4 inches (10 centimeters) across.

What is Barite? Barite is a mineral composed of barium sulfate (BaSO4). It receives its name from the Greek word "barys" which means "heavy." This name is in response to barite's high specific gravity of 4.5, which is exceptional for a nonmetallic mineral. The high specific gravity of barite makes it suitable for a wide range of industrial, medical, and manufacturing uses. Barite also serves as the principal ore of barium.

Barite Rose: This "barite rose" is a cluster of bladed barite crystals that have grown in sand, incorporating many of the sand grains within each crystal. Specimen and photo by Arkenstone / www.iRocks.com.

Barite Occurrence Barite often occurs as concretions and void-filling crystals in sediments and sedimentary rocks. It is especially common as concretions and vein fillings in limestone and dolostone. Where these carbonate rock units have been heavily weathered, large accumulations of barite are sometimes found at the soil-bedrock contact. Many of the commercial barite mines produce from these residual deposits. Barite is also found as concretions in sand and sandstone. These concretions grow as barite crystallizes within the interstitial spaces between sand grains. Sometimes crystals of barite grow into interesting shapes within the sand. These structures are known as "barite roses" (see photo). They can be up to several inches in length and incorporate large numbers of sand grains. Occasionally barite is so abundant in a sandstone that it serves as the "cement" for the rock. Barite is also a common mineral in hydrothermal veins and is a gangue mineral associated with sulfide ore veins. It is found in association with ores of antimony, cobalt, copper, lead, manganese, and silver. In a few locations barite is deposited as a sinter at hot springs. Physical Properties of Barite Chemical Classification

Sulfate

Color

Colorless, white, light blue, light yellow, light red, light green

Streak

White

Luster

Vitreous to pearly

Diaphaneity

Transparent to translucent

Cleavage

Very good, basal, prismatic

Mohs Hardness

2.5 to 3.5

Specific Gravity

4.5

Diagnostic Properties

High specific gravity, three cleavage directions at right angles

Chemical Composition

Barium sulfate, BaSO4

Crystal System Uses

Orthorhombic Drilling mud; high-density filler for paper, rubber, plastics

Physical Properties of Barite Barite is generally easy to identify. It is one of just a few nonmetallic minerals with a specific gravity of four or higher. Combine that with its low Mohs hardness (2.5 to 3.5) and its three directions of right-angle cleavage, and the mineral can usually be reliably identified with just three observations. In the classroom, students often have difficulty identifying specimens of massive barite with fine-grained crystals. They look at the specimen, see the sugary appearance, correctly attribute it to cleavage, and apply a drop of dilute hydrochloric acid. The mineral effervesces and they think that they have calcite or a piece of marble. The problem is that the effervescence is caused by contamination. The students tested the hardness

of the barite with a piece of calcite from their hardness kit. Or the specimen of barite can naturally contain calcite. However, any student who tests the specific gravity will discover that calcite or marble are incorrect identifications. Barite is also a good mineral to use when teaching about specific gravity. Give students several white mineral specimens that are about the same size (we suggest calcite, quartz, barite, talc, gypsum). Students should be able to easily identify barite using the "heft test" (placing Specimen "A" in their right hand and Specimen "B" in their left hand and "hefting" the specimens to determine which one is heaviest). Students in third or fourth grade are capable of using the heft test to identify barite.

Gas well site: Barite is used to make high-density drilling mud for wells. Aerial photo of a gas well site. © iStockphoto / Edward Todd.

Barite from Canada: Barite from Madoc, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across.

Uses of Barite Most barite produced is used as a weighting agent in drilling muds. This is what 99% of the barite consumed in the United States is used for. These high-density muds are pumped down the drill stem, exit through the cutting bit and return to the surface between the drill stem and the wall of the well. This flow of fluid does two things: 1) it cools the drill bit; and, 2) the high-density barite mud suspends the rock cuttings produced by the drill and carries them up to the surface.

Barite is also used as a pigment in paints and as a weighted filler for paper, cloth and rubber. The paper used to make some playing cards has barite packed between the paper fibers. This gives the paper a very high density that allows the cards to be "dealt" easily to players around a card table. Barite is used as a weighting filler in rubber to make "anti-sail" mudflaps for trucks. Barite is the primary ore of barium, which is used to make a wide variety of barium compounds. Some of these are used for x-ray shielding. Barite has the ability to block x-ray and gamma-ray emissions. Barite is used to make high-density concrete to block x-ray emissions in hospitals, power plants, and laboratories. Barite compounds are also used in diagnostic medical tests. If a patient drinks a small cup of liquid that contains a barium powder in a milkshake consistency, the liquid will coat the patient's esophagus. An x-ray of the throat taken immediately after the "barium swallow" will image the soft tissue of the esophagus (which is usually transparent to x-rays) because the barium is opaque to x-rays and blocks their passage. A "barium enema" can be used in a similar way to image the shape of the colon.

Barite from Australia: Barite from Edith River, Northern Territory, Australia. Specimen is approximately 2 inches (5 centimeters) across.

Barite from Utah: Barite from Mercur, Utah. Specimen is approximately 4 inches (10 centimeters) across.

Barite Production 2015 Barite Production Country

Thousand Metric Tons

China

3,000

India

900

Iran

300

Kazakhstan

300

Mexico

220

Morocco

900

Pakistan

120

Peru

100

Thailand

130

Turkey

200

Vietnam

90

United States

700

Other Countries

500

Barite production is from the USGS Mineral Commodity Summary.

The oil and gas industry is the primary user of barite worldwide. There it is used as a weighting agent in drilling mud. This is a growth industry, as global demand for oil and natural gas has been on a long-term increase. In addition, the long-term drilling trend is more feet of drilling per barrel of oil produced. This has caused the price of barite to increase. Price levels during 2012 were between 10% and 20% higher than 2011 in many important markets. The typical price of drilling mud barite is about $150 per metric ton at the mine. Substitutes for barite in drilling mud include celestite, ilmenite, iron ore, and synthetic hematite. None of these substitutes have been effective at displacing barite in any major market area. They are too expensive or do not perform competitively. China and India are the leading producers of barite, and they also have the largest reserves. The United States does not produce enough barite to supply its domestic needs. In 2011 the United States produced about 700,000 metric tons of barite and imported about 2,300,000 metric tons.

Bauxite Almost all of the aluminum that has ever been produced has been made from bauxite

Bauxite from Little Rock, Arkansas, exhibiting a pisolitic structure and characteristic red iron staining. Specimen is approximately 4 inches (10 centimeters) across.

What is Bauxite? Bauxite is not a mineral. It is a rock formed from a laterite soil that has been severely leached of silica and other soluble materials in a wet tropical or subtropical climate. It is the primary ore of aluminum. Almost all of the aluminum that has ever been produced has been extracted from bauxite.

What is Bauxite's Composition? Bauxite does not have a specific composition. It is a mixture of hydrous aluminum oxides, aluminum hydroxides, clay minerals, and insoluble materials such as quartz, hematite, magnetite, siderite, and goethite. The aluminum minerals in bauxite can include: gibbsite Al(OH)3, boehmite AlO(OH), and, diaspore, AlO(OH). Physical Properties of Bauxite Color

White, gray, sometimes stained yellow, orange, red, pink, or brown by iron or included iron minerals

Streak

Usually white, but iron stain can discolor

Luster

Dull, earthy

Diaphaneity

Opaque

Cleavage

None

Mohs Hardness

1 to 3

Specific Gravity

2 to 2.5

Diagnostic Properties

Often exhibits pisolitic structure; color

Chemical Composition

Variable but always rich in aluminum oxides and aluminum hydroxides

Crystal System Uses

n/a Primary ore of aluminum, also used as an abrasive

Physical Properties of Bauxite Bauxite is typically a soft (H:1-3), white to gray to reddish brown material with a pisolitic structure, earthy luster and a low specific gravity (SG: 2.0-2.5). These properties are useful for identifying bauxite; however, they have nothing to do with bauxite's value or usefulness. This is because bauxite is almost always processed into another material with physical properties that are distinctly different from bauxite.

Pisolites in bauxite: Close-up view of the bauxite specimen in the photo at top of page. This photo shows detail of the pisolites.

Bauxite Used for Aluminum Production Bauxite is the principal ore of aluminum. The first step in producing aluminum is to crush the bauxite and purify it using the Bayer Process. In the Bayer Process, the bauxite is washed in a hot solution of sodium hydroxide, which leaches aluminum from the bauxite. The aluminum is precipitated out of solution in the form of aluminum hydroxide, Al(OH)3. The aluminum hydroxide is then calcined to form alumina, Al2O3. Aluminum is smelted from the alumina using the Hall-Heroult Process. In the Hall-Heroult Process, the alumina is dissolved in a molten bath of cryolite (Na3AlF6). Molten aluminum is removed from the solution by electrolysis. This process uses an enormous amount of electricity. Aluminum is usually produced where electricity costs are very low.

Bauxite without pisolites: Bauxite from Demerara, Guyana. Some specimens of bauxite do not have the pisolitic structures. Specimen is approximately 4 inches (10 centimeters) across.

Use of Bauxite as an Abrasive Calcined alumina is a synthetic corundum, which is a very hard material (9 on the Mohs Hardness Scale). Calcined alumina is crushed, separated by size, and used as an abrasive. Aluminum oxide sandpaper, polishing powders, and polishing suspensions are made from calcined alumina. Sintered bauxite is often used as a sand-blasting abrasive. It is produced by crushing bauxite to a powder and then fusing it into spherical beads at a very high temperature. These beads are very hard and very durable. The beads are then sorted by size for use in different types of sandblasting equipment and for different sandblasting applications. Their round shape reduces wear on the delivery equipment.

Use of Bauxite as a Proppant Sintered bauxite is also used as an oil field proppant. In drilling for oil and natural gas, the reservoir rock is often fractured by pumping fluids into the well under very high pressures. The pressure builds up to very high levels that cause the reservoir rock to fracture. When fracturing occurs, water and suspended particles known as "proppants" rush into the fractures and push them open. When the pumps are turned off, the fractures close, trapping the proppant particles in the reservoir. If an adequate number of crush-resistant particles remain in the reservoir, the fractures will be "propped" open, allowing for a flow of oil or natural gas out of the rocks and into the well. This process is known as hydraulic fracturing. Powdered bauxite can be fused into tiny beads at very high temperatures. These beads have a very high crush resistance, and that makes them suitable as a proppant. They can be produced in almost any size and in a range of specific gravity. The specific gravity of the beads and their size can be matched to the viscosity of the hydraulic fracturing fluid and to the size of fractures that are expected to develop in the rock. Manufactured proppants provide a wide selection of grain size and specific gravity compared to a natural proppant known as frac sand.

Substitutes for Bauxite World bauxite resources are adequate for decades of production at current rates. Other materials could be used instead of bauxite for alumina production. Clay minerals, alunite, anorthosite, power plant ash, and oil shale could be used to produce alumina but at higher costs, using different processes. Silicon carbide could

be used in place of bauxite-based abrasives. Synthetic mullite could be used in place of bauxite-based refractories.

Bauxite Localities 2010 Estimated Bauxite Production Country

Thousand Metric Tons

Australia

68,414

China

44,000

Brazil

28,100

India

18,000

Guinea

17,400

Jamaica

8,540

Russia

5,475

Kazakhstan

5,310

Suriname

4,000

Greece

2,500

The values above are estimated bauxite production for 2010. Data from the USGS Mineral Commodity Summary.

Bauxite is found in abundance at many locations around the world. In 2010 the ten leading bauxite producing countries were: Australia, China, Brazil, India, Guinea, Jamaica, Russia, Kazakhstan, Suriname, and Greece. Each of these countries has enough reserves for many years of continued production. Some have reserves for over 100 years of production. The United States has small amounts of bauxite in Arkansas, Alabama, and Georgia; however, there is very little mining of bauxite in the United States, and at least 99% of consumption is imported. Contributor: Hobart King

Benitoite Douglas Sterrett's 1911 USGS report on the mineral's discovery, geology, mining, and properties.

Faceted benitoite: Five tiny gemstones of faceted benitoite in a color gradient set from nearly colorless to violetishblue. Each stone is a round brilliant of about 3.5 millimeters and weighing about .20 carat. Photo by TheGemTrader.com.

Benitoite and neptunite crystals: This specimen is a plate of translucent blue benitoite crystals and black neptunite crystals on a background of white natrolite. (This association is typical and an important characteristic of the mineral.) The crystals are about 2 centimeters in length and the plate measures about 15 x 11 x 2 centimeters in size. The specimen is from the Dallas Gem Mine, San Benito River headwaters area, New Idria District, Diablo Range, San Benito County, California. Specimen and photo by Arkenstone / www.iRocks.com.

Information Sources [1] Douglas B. Sterrett (1911). Benitoite, Gems and Precious Stones, Mineral Resources of the United States, Calendar Year 1909, Part II Nonmetals, pages 742-748, published 1911.

Description of Benitoite An excellent description of the new California gem mineral, benitoite, has recently been given by G. D. Louderback, of the University of California. The locality was visited during the summer of 1909 by the present writer, and every facility was given for the examination of the deposit by the Dallas Mining Company through the kindness of Mr. Thomas Hayes, at that time acting superintendent. The following description has been abstracted in part from Doctor Louderback's report and notes supplied from personal observation have been added.

Who Discovered Benitoite? The difficulty mentioned by Doctor Louderback in learning who was the original discoverer of the benitoite property was encountered by the writer. It is evident that J. M. Couch, of Coalinga, grubstaked by R. W. Dallas, was instrumental in finding the deposit. Whether he discovered it while out alone or on a second trip with L. B. Hawkins, of Los Angeles, is a point in dispute. Material taken to Los Angeles by Mr. Hawkins was pronounced volcanic glass and valueless. According to Mr. Couch, specimens given to Harry U. Maxfield, of Fresno, were shown to G. Eacret, of Shreve & Co., San Francisco, and to G. D. Louderback. Specimens cut by Mr. Eacret were thought to be sapphire. Doctor Louderback found the material to be a new mineraland named it benitoite after the county in which it was found.

Benitoite mine map: Map showing the location in San Benito County in central California.

Location of the Benitoite Deposit The benitoite mine is in the southeastern part of San Benito County, near the Fresno County line. The deposit is about 35 miles by road northwest of Coalinga in the Diablo Range, about three-fourths of a mile south of Santa Rita Peak, and on one of the tributaries of San Benito River. The elevation of the mine is about 4,800 feet above sea level; the elevation of Santa Rita Peak is 5,161 feet. The mine is in the end of one of the branching ridges from the south side of Santa Rita Peak. The end of the southward extension of this ridge is a low knob about 160 feet above the creek. This knob is called the apex, and from it a small spur extends to the west down to the creek. The benitoite mine is in the south side of this spur, about 50 feet lower than the apex and 250 feet west of it.

Geology of the Benitoite Deposit The benitoite deposit occurs in a large area of serpentinewhich extends many miles northward past the New Idria quicksilver mine and a few miles southward, and forms the summit of an anticlinal ridge pitching down to Coalinga. This serpentine is of the usual type of the Coast Ranges and presents different phases from hard dark-green and greenish-black material to softer lighter-colored rock containing more or less talcose and chloritic minerals. Slickenside seams and lentil-shaped blocks and masses are common through the serpentine, much of which is decomposed near the surface and breaks down to light grayish-green soil which has a greasy feeling when rubbed between the fingers. Inclusions of masses of schists and other rocks of the Franciscan formation occur in the serpentine. These schists may be micaceous or more basic, having common hornblende, actinolite, or glaucophane as characteristic minerals. The benitoite deposit is located in one of these basic inclusions, a portion of which has a somewhat schistose structure, while the rest is nearly massive. These phases were probably originally different adjacent formations that have been metamorphosed. Part of the massive form is a dark-gray to greenish-gray rock that might be called trap. In some specimens the following minerals are determinable under the microscope: augite, plagioclase crushed and recrystallized and containing clinozoisite prisms, secondary albite, yellow serpentine, and a little titanite and pyrite. The rock is therefore a partly metamorphosed diabase or gabbro. The more schistose phases are grayish-blue to blue and grade into vein material. They are composed of one or more varieties of hornblende, some partially chloritized, with albite, and, near the vein, with natrolite. The hornblende occurs in minute needles, felted masses of needles, blades, and stouter prisms. These have a bluish to yellowish green to nearly colorless pleochroism, and are in part probably actinolite and in part glaucophane or allied hornblende. The natrolite fails and the albite is also less abundant in the hornblende rock at some distance from the vein. The vein is a highly mineralized shattered zone in the schistose rock. The fractures and joints with the vein filling are about parallel with the schistosity of the rock, which averages nearly east and west in strike with local variations and has a varying dip of 20° to 70° N. A sketch map of a small area on the benitoite mine hill giving the outcrops with their dips and strikes and the formations encountered in the mine workings shows the schist and gabbro inclusion in the serpentine to be quite irregular in shape. The width at the mine between the serpentine walls is about 150 feet and at a distance of 150 feet east of the mine it is only about 90 feet; about 80 feet farther east at the apex it is over 100 feet. This schist inclusion has been described by Kalph Arnold as 150 feet wide at its widest point and at least 1,200 feet long. The metamorphism of the schist inclusion has been of two kinds — first mashing and sheeting of the original basic rock producing schistosity and opening channels for solutions and then a passage of mineral-bearing solutions recrystallizing and replacing the minerals of the rock with albite. The albite permeated the rock for many feet each side of the fracture zone. The conditions of temperature or pressure of the solutions became changed, so that natrolite was next deposited. The natrolite did not permeate far into the rock, but formed a coating on the walls of the fissures. Neptunite and benitoite were formed with the natrolite at this stage in the fissures and openings but did not penetrate the wall rock. This whole mineralized zone containing many bands and masses of natrolite with gem minerals in the joints, fissures, and open spaces in the brecciated hornblende rock may be called the vein.

The unfilled cavities and seams in the vein zone aided by later fractures and faults has offered an easy passage for more recent decomposing meteoric waters. The latter have leached portions of hornblende schist along and included in the vein, have removed part of the minerals of the vein, and have stained the natrolite on the walls of the cavities and seams with iron and manganese oxides. The rock, leached of albite, has a more or less porous texture and is composed principally of fine fibrous blue hornblende and actinolite.

Benitoite crystal structure: Crystal structure of benitoite, BaTiSi3O9, P-6c2, projected onto the (a,c) plane. Public domain image by Perditax.

Development of the Benitoite Mine Development work at the benitoite mine at the time of the writer's visit consisted of a large and a small open cut, a prospect drift or tunnel with a crosscut tunnel, and an incline shaft. The large open cut or "glory hole" was 20 to 45 feet wide, 85 feet long, and from a few feet to 35 feet deep; it had a north of east direction into the hillside. The smaller open cut was to the north side of the entrance of the larger cut and at a lower level, it was about 60 feet long and 10 to 15 feet deep. The prospect tunnel was driven 120 feet in a direction N. 70° E. from the end of the large open cut. The crosscut tunnel was 45 feet long and driven to the north at a right angle from the main tunnel at a distance of 50 feet from the mouth. The incline shaft was sunk 35 feet deep from the north side of the open cut at about the middle. The prospect tunnel cut through the hornblende schist formation into decomposed serpentine. The contact was evidently a fault line, and near it the serpentine contained much talcose and scaly asbestiform material. The fault was directly across the schistosity with a north-south strike and a. dip of 45° W. This prospect tunnel encountered a little natrolite (vein material) in the hornblende schist in its upper west side, 15 feet beyond the crosscut tunnel, which crossed a small streak of vein material containing a little benitoite about 10 feet from the main tunnel. Vein material formed the roof of the prospect tunnel for several feet near its mouth. The "glory hole" was excavated in a very large pocket or bulge in the vein, a portion of which may still be seen along the north wall of the open cut. The incline shaft was apparently sunk in the lower part of this outcrop and did not encounter benitoite. The smaller open cut exposed vein material with benitoite, which was more plentiful near the east end of the cut than at the west end. The vein and the schist in this cut were much blackened and stained with films and seams of manganese dioxide. About 30 feet S. 60° E. of the upper end of the huge open cut a ledge of altered blue hornblende schist outcrops prominently. This ledge also carries a streak of natrolite with benitoite. Benitoite has been found in bowlders a few hundred yards west of the mine

on the hillside and in the creek. These bowlders have evidently rolled from the outcrop on the hill above and probably from near the mine. Doctor Louderback states that benitoite lias been found for a distance of about 230 feet at the surface along the mineral zone and in very small quantity at its extremes. The writer observed benitoite in place through a distance of about 170 feet in an east and west direction. The strike of the ledge outcropping to the east of the open cut was about N. 60° W., with a high northerly dip. The strike encountered in the tunnel, about 30 feet lower and to the north, was nearly cast and west with a dip of about 40° N. In the upper part of the face of the open cut the dip was high, about 65° N., and below the middle of the face it was low, 15° to 25° N. Along the north side of the open cut and in the lower cut the strike was about east and west and the dip was probably rather low, 20° to 30° N. These measurements do not agree closely with those of Doctor Louderback, especially in regard to the dip of the vein. Jointing of the rock and the irregular nature of the vein, however, make accurate measurements difficult. Doctor Louderback places the dip at 65° to 69° N., but the dip measured by the writer is much lower, probably 15° to 30° N. in the lower part of the cut. The evidence for this measurement is found in the position of the vein at the outcrop and in the tunnel, of the layers of blue schist and natrolite in the end of the cut, and of the ledge along the north side of the open cut and in the lower cut. Such a low dip would account for the failure of the incline to cut the mineralized zone. The failure might also be due to the pinching out of the vein a short distance below the large pocket opened in the "glory hole." The impression gained by a study of the deposit and by plotting the location of the vein where encountered in different places was that the deposit consists of an ore shoot pitching to the west and lying in a fracture zone in hornblende schist with an irregular east and west strike and north dip. This shoot had a lenticular cross section with a thickness of more than 25 feet in the thickest part but pinching out on the sides. The upper edge of the shoot has been removed by erosion. A portion of the lover edge was encountered in the tunnel. The eastern extension of such a shoot would have been removed by erosion and the western extension would be underground, to the north of, west of, and below, the open cut. Doctor Louderback mentions the outcrop of spheroidal gabbro on the southeast of the benitoite deposit on the hillside. The outcrop of rock on the north side of the vein zone, on the summit of the ridge, is of a similar nature and has been mentioned above as diabase or gabbro. The same rock was encountered in the crosscut tunnel 40 feet below the surface and 30 feet north of the main tunnel. Underground this rock occurred in large loose spheroidal bowlders ranging up to several feet in thickness, with large openings between them. This material was difficult to mine and required careful timbering. The open spaces evidently extended to the surface above, as a strong draft of air came through them. The spheroidal shape of the blocks and the open spaces between them were doubtless formed by decomposition and leaching along fracture planes.

Fluorescent benitoite: This is a photograph of small benitoite crystals under ultraviolet light. The mineral exhibits a brilliant blue color under ultraviolet radiation. Public domain photo by Parent Géry.

Mineralogy of the Benitoite Zone The benitoite occurs with neptunite in crusts, seams, and thicker deposits of white natrolite on the walls of geode-like cavities and fissures in the hornblende schist. These deposits occur in both irregularly shaped masses and in seams with more definite directions. They inclose fragments of hornblende schist which has been heavily impregnated with natrolite. In some of the inclusions the gradation from the hornblende rock containing much natrolite to natrolite containing acicular inclusions of hornblende is complete. The benitoite is embedded in or attached to natrolite, being in some places completely, in other places partly, enveloped by it. In the latter places the benitoite projects into the cavities along with the coarse drusy surfaces of the natrolite. Natrolite with or without benitoite and neptunite fills some of the fissures and former cavities completely. The benitoite is always in contact with natrolite and has not been found embedded in the hornblende rock alone. It is in many places attached to hornblende impregnated with natrolite and is partly or completely inclosed in natrolite on the remaining sides. The neptunite is subject to the same relations with the natrolite and is, in places, partly surrounded by benitoite. These facts point to the same period of formation for the three minerals with the power of crystallization arranged in the following order: neptunite, benitoite, and natrolite.

Obtaining Benitoite Specimens The benitoite is obtained by breaking open masses of vein rock and carefully chiseling or working the crystals out of the inclosing natrolite. Many gems are injured or ruined by this method. The removal of the natrolite by acid has been tried with partial success. Large slabs of rock 2 to 3 or more feet across are obtained coated with natrolite and carrying benitoite and neptunite. The last two minerals are either visible on the drusy surface of the natrolite or are completely covered by natrolite. The position of the benitoite and neptunite is often marked by lumps or a thickening of the natrolite crust. By carefully cutting into these lumps beautiful crystals are sometimes uncovered. Often the inclosing crust or shell of white natrolite can be split from a crystal of neptunite or benitoite in two or three large pieces, so that the covering can readily be replaced over the crystal. Such material makes beautiful specimens. Slabs of bluish hornblende rock with a drusy pure white crust of natrolite containing brilliant reddish-black neptunite and blue benitoite in fine crystals are excellent for the same purpose. The minerals associated with benitoite are described and analyses are given in the paper of Louderback and Blasdale. Neptunite is titanium silicate containing iron, manganese, potassium, sodium, and magnesium. It occurs in black to reddish- black prismatic crystals of the monoclinic system, the length commonly being several times the thickness. It has a prismatic cleavage and the thin splinters or powder show a deep reddishbrown color. The hardness is between 5 and 6 and the specific gravity 3.18 to 3.19. Neptunite is practically insoluble in hydrochloric acid. The natrolite, with which the benitoite and neptunite are associated, does not generally occur in distinct crystals of any size. It forms massive granular white aggregates of crystallized material with curved ridge-like or cockscomb-like groups of crystals and drusy botryoidal masses in the cavities. Natrolite is a hydrous silicate of sodium and aluminum crystallizing in the orthorhombic system. Other minerals occurring in smaller quantity in the cavities are emerald-green copper stain, amphibole needles, albite, aegirine, and psilomelane. The amphiboles are actinolite, a variety intermediate between crossite and crocidolite, and a little glaucophane.

Physical Properties of Benitoite Chemical Classification

Color

Barium Titanium Silicate

Most specimens are violetish-blue. Some specimens are colorless. A range of color and saturation between colorless and deep violetish-blue occurs. Rare orange heat-treated specimens are known.

Streak

White

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent Poor

Mohs Hardness

6 to 6.5

Specific Gravity

3.6

Diagnostic

Tabular dipyramidal crystals. Intense blue fluorescence under short-wave ultraviolet light. Associated with serpentine

Properties

and albite, but more importantly with rare minerals such as natrolite, joaquinite, and neptunite.

Chemical Composition Crystal System Uses

BaTiSi3O9

Hexagonal Gemstone, collector mineral, Official State Gem of California.

Chemical and Physical Properties of Benitoite The chemical and physical properties of benitoite and its associated minerals have been described by Louderback and Blasdale, and the following notes are taken from their description. The chemical analyses show it to be an acid barium titano-silicate corresponding to the formula BaTiSi3O9 . Benitoite is insoluble in ordinary acids, but is attacked by hydrofluoric acid and dissolves in fused sodium carbonate. Alone, it fuses quietly to a transparent glass at about 3. The color of benitoite is not affected by heating the stone to redness and allowing to cool. The hardness is greater than orthoclase and less than peridot, or about 6 to 6 1/2, and the specific gravity is 3.64 to 3.67. Benitoite crystallizes in the trigonal division of the hexagonal system. The common forms observed are the base c(0001), trigonal prisms m(1010), and n(0110), and the trigonal pyramids p(1011) and π(0111). Other forms are rather rare and of small importance. Of these faces the pyramid π generally has the largest development. This gives the crystal a triangular aspect with the corners truncated by smaller planes. The prism faces are narrow, though generally present. Many of the crystals are naturally etched on one or more sets of faces. Such faces are a little dulled or slightly pitted. Benitoite has an imperfect pyramidal cleavage and a conchoidal fracture.

Faceted benitoite: Three blue stones of faceted benitoite. Benitoite is often cut into round brilliants because of its high refractive index and dispersion. Cutters must orient benitoite carefully to take full advantage of its pleochroism. Photo by TheGemTrader.com.

Benitoite Gemology The mean refractive index of benitoite is greater than that of sapphire, and measures 1.757 to 1.804 (sapphire 1.759 to 1.767). The birefringence is high and the pleochroism very strong. The crystals are generally transparent with a pale to deep-blue and bluish-violet color. Color variations are common in the same crystal, and the change from dark to light blue or colorless may be sharp or gradual. The pleochroism of benitoite is pale to dark-blue or purplish and colorless. The richest colors are seen when the crystals are viewed parallel to the base. The intensity of the blue diminishes as the light ray penetrates the crystal at other angles until perpendicular to the base, when the crystal is colorless. Care is necessary, therefore, in cutting the gem so as to secure the best effects. Pale-colored stones should be cut with the table perpendicular to the base or parallel to the vertical axis of the crystal to secure the full color value. Deeper colored stones may be cut in the same way or with the table in an intermediate position, if the color is very strong. By cutting intensely colored stones with the table only slightly out of parallel to the base, the color may be reduced to a desirable shade. The dichroscope may be used to determine the position of the vertical axis and accordingly of the base perpendicular to it. When viewed perpendicular to the vertical axis with a dichroscope the twin colors or two rays of light are very intense to pale blue (depending on the depth of color of the crystal) and colorless. When viewed parallel to the vertical axis, or perpendicular to the base, the two rays are colorless and remain so while the dichroscope is rotated. The color of one of the rays becomes stronger as the crystal is rotated from this position. Benitoite crystals exhibiting two shades of color, as dark and light blue or blue and colorless in different parts of the same crystal, may be cut so as to show these variations, or sometimes in such, a way that the resulting color is of nearly uniform intensity. Benitoite has been cut as a brilliant, with the step or trap cut, and "en cabochon." The brilliant cut is especially suitable to show the brilliancy and fire of the gem. The brilliancy is due to the high refractive index and the fire or red flash, often seen in dull or artificial light is, in part at least, caused by the dispersion of the mineral. Of the colors produced by dispersion during the refraction of light in benitoite yellow and green are largely absorbed in the colored gems so that principally red and violet-colored lights are seen. These flashes of colored lights along with the natural fine blue of benitoite render the gem particularly beautiful. The step cut displays the color of benitoite to advantage, with only slight loss of brilliancy. Cabochon-cut gems from crystals with color variations or partially flawed material have some beauty. The size of the gems cut from benitoite range in weight from a small fraction of a carat to several carats. According to Doctor Louderback the largest perfect stone so far cut weighs over 7 carats and is about three

times as heavy as the next largest flawless gem so far obtained. The majority of larger cut stones weigh from 1 1/2 to 2 carats. The principal production is in stones weighing less than 1 1/2 carats. The use of benitoite in rings or jewelry subjected to hard wear is limited by its comparative softness. The beautiful color, brilliancy, and fire of the gem, however, adapt it to other classes of fine jewelry. Since the supply of benitoite is thought to be limited and a fairly large demand has already arisen for the gem, it is probable the price will be kept high, possibly as high as that of sapphire, its nearest rival in color.

Other Benitoite Deposits? So far benitoite has been found at one place only. J. M. Couch, one of the original discoverers of the benitoite deposit, has located several prospects in formations resembling that at the benitoite mine. In one of these, three-fourths of a mile to the north on the east side of Santa Rita Peak, cavities lined with natrolite crusts and crystals have been found in a bluish hornblende schist rock very similar to that at the original mine. The schist near the vein is composed of bluish hornblende and actinolite needles penetrating granular masses of albite. This rock also incloses crystals of natrolite showing that part of it was formed later than or during the crystallization of the natrolite. In the cavities the natrolite occurs in simple well-developed white columnar crystals up to a centimeter or more in thickness and several times as long. Neither benitoite nor neptunite have been found associated with this natrolite.

Beryl This minor ore of beryllium is one of the most important gem minerals.

Aquamarine beryl: A spectacular crystal of aquamarine from the Shigar Valley of northern Pakistan. This specimen clearly shows the hexagonal form with terminations and a vivid blue color. The specimen is approximately 15 x 11 x 7.5 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What is Beryl? Beryl is a relatively rare silicate mineral with a chemical composition of Be3Al2Si6O18. It is found in igneous and metamorphic rocks in many parts of the world. Before 1969, beryl served as the only important ore of berylliummetal. Since then, most of the world's supply of beryllium is refined from bertrandite, a beryllium silicate hydroxide, mined at Spor Mountain, Utah. Small amounts of beryl, mostly produced as a by-product of gemstone mining, are still used to produce beryllium. The major economic interest in beryl today is its use as a gemstone. It is one of the most important gem minerals, and the gems are named by their color as emerald (green), aquamarine (greenish blue to blue), morganite (pink to orange), red beryl (red), heliodor (yellow to greenish yellow), maxixe (deep blue), goshenite (colorless), and green beryl (light green). Emerald and aquamarine are the most popular. Compared to other gemstones, emeralds are second only to diamonds in terms of the dollar value imported into the United States. Occasionally, chatoyantspecimens of beryl are found that can be cut into cabochons to produce interesting cat's-eye gemstones.

Emerald beryl: Vivid green beryl crystals from the Cosquez Mine in Colombia. The cluster measures 5 x 4.2 x 3 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Geologic Occurrence of Beryl Beryl is a mineral that contains a significant amount of beryllium. Beryllium is a very rare metal, and that limits the occurrence of beryl to a few geological situations where beryllium is present in sufficient amounts to form minerals. It mainly occurs in granite, rhyolite, and granite pegmatites; in metamorphic rocks associated with pegmatites; and, in veins and cavities where hydrothermal activity is associated with rocks of granitic composition. These different types of deposits are often found together and serve as an exploration model for finding beryl. Beryl is also found where carbonaceous shale, limestone, and marble have been acted upon by regional metamorphism. The famous emerald deposits of Colombia and Zambia have been formed under these conditions. The carbonaceous material is thought to provide the chromiumor vanadium needed to color the emerald.

Physical Properties of Beryl Chemical Classification

Silicate

Color

Green, yellow, blue, red, pink, orange, colorless

Streak

Colorless (harder than the streak plate)

Luster

Vitreous

Diaphaneity Cleavage Mohs Hardness

Translucent to transparent Imperfect 7.5 to 8

Specific Gravity

Diagnostic Properties

2.6 to 2.8 Crystals are prismatic with flat terminations, hexagonal, and without striations. Hardness and relatively low specific gravity.

Chemical

Be3Al2Si6O18

Composition Crystal System

Hexagonal (occurs in prismatic to tabular crystals)

Uses

Gemstones, a minor ore of beryllium.

Physical Properties of Beryl The most important physical properties of beryl are those that determine its usefulness as a gem. Color and clarity are very important. Beryl occurs in a diversity of colors, with some of those colors being highly desirable. It also occurs in transparent crystals that have clarity and size that are sufficient for faceting. Many beryls have a color that can be improved by heating. Beryl's durability is generally good. It has a Mohs hardness of 7.5 to 8, which helps it resist scratches when worn in jewelry. Beryl breaks by cleavage and it is also brittle. Many specimens, especially of emerald, are fractured or highly included. These weaknesses can make it vulnerable to damage by impact, pressure or temperature change. Beryl can be difficult to identify. When it occurs as a crystal, its prismatic, hexagonal form with flat terminations and lack of striations is a good aid in identification. Beryl's high hardness and relatively low specific gravity can be helpful for identifying massive specimens.

Uses of Beryl (ore of beryllium) Beryl was once the only important ore of beryllium metal, but today the mining of bertrandite at Spor Mountain, Utah supplies about 80% of the world's beryllium. The extraction of beryllium from beryl is very costly, and as long as bertrandite is available in large amounts, beryl will be a minor ore of that metal.

Red beryl: A specimen of red beryl on matrix from the Wah Wah Mountains of Utah. The specimen measures 4.7 x 3.8 x 3.1 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Gem Beryls The primary economic use of beryl today is as a gemstone. It occurs in a wide variety of colors that appeal to many consumers. A brief description of popular gem beryl varieties is presented in the sections below.

Lab-created emerald: Synthetic emeralds can be created in a lab, and these stones are usually superior to natural emerald in their clarity and color. The emeralds in this photo were made by Chatham Created Gems. The faceted stone measures 5.1 x 3 mm and weighs 0.23 carats. The emerald crystal on the right measures about 8 x 6 x 5 mm and weighs 2 carats.

Cesium-bearing beryl from Madagascar. Specimen is approximately 1 inch (2.5 centimeters) across.

Emerald Emeralds are gem-quality specimens of beryl that are defined by their green color. To be considered an "emerald," a stone must have a rich, distinct color in the bluish green to green to yellowish green range. If the color is not a rich saturated green, the stone should be called a "green beryl" instead of an "emerald." There are some disagreements between buyers and sellers on judging the color boundary between emerald and green beryl. Some also believe that the name "emerald" is reserved for stones with a green color caused by chromium rather than by vanadium. Material colored by iron is almost always too light to be called emerald and usually lacks the distinct green color typically associated with emerald. Emerald is the most popular beryl. Excellent specimens are also quite valuable. Emerald, sapphire and ruby are considered to be the "big three" of colored stones. More money is spent on these in the United States than all other colored stones combined. In many years, the United States imports a higher dollar value of emerald than of ruby and sapphire combined. Colombia, Zambia, Brazil, and Zimbabwe are major producers of gem-quality emerald. A small amount of emerald is sporadically mined in the United States near Hiddenite, North Carolina. Emerald is a beautiful stone, but it is often fractured or highly included. Most of the emerald entering the retail market has been treated in some way. Fractures are often impregnated with glass or resins to stabilize the stone and make the fractures less visible. Stones are often waxed or oiled to hide fractures and surface-reaching inclusions. Heating and drilling are often done to reduce the visibility of inclusions. Even after these treatments, a person with a small amount of knowledge can usually look into a display case at the typical mall jewelry store and with reasonable success identify natural stones and lab-created stones by their clarity. Lab-created stones have a bright green color and are transparent. Natural stones are usually translucent or have visible inclusions and fractures. Natural stones without these characteristics are extremely rare and have a very high price. Many people prefer natural stones and their visible flaws. Others prefer the clarity and color of lab-created stones and their significantly lower price. Lab-created emeralds account for a significant percentage of the stones on display and being sold in many department stores and mall jewelry stores. Aquamarine Aquamarine is the second most popular gem beryl. Like emerald, its identity is defined by its color. Aquamarine has a distinct greenish blue to blue color. Unlike emerald, light-colored stones in this color range

are still called aquamarine. The stones that are richly colored are the most desirable, and the stones with a very pale color are made into inexpensive jewelry. Aquamarine differs from emerald in another way - it normally has far fewer inclusions and fractures. So, most of the aquamarine seen in mall jewelry stores is usually eye clean and without visible fractures. The color of aquamarine can usually be improved by heating. Most stones entering the retail market have been heated. Many of the greenish blue stones offered for sale were distinctly bluish green or even yellow before treatment.

Morganite beryl: An interesting specimen of morganite with tourmaline crystals from the Pederneira Mine in Minas Gerais, Brazil. This specimen has been nicknamed the "Sword in the Stone." Approximately 13.8 x 8.0 x 11.7 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Morganite Morganite, also known as "pink beryl" and "rose beryl," is a rare variety of beryl that ranges in color between yellowish orange, orange, pink, and lilac. "Rose," "salmon," and "peach" are words often used to describe the gem's color. Trace amounts of manganese are thought to cause the pink color. Morganite is the third most commonly seen variety of beryl in jewelry stores, but the selection is usually limited, and stones with top color are very hard to find. Most morganite sold in jewelry has been heat treated to improve its color. Heating generally removes traces of yellow from the stone and converts orange or yellowish stones into a more desirable pink color. Some morganite has been irradiated to deepen its color. Synthetic morganite has been produced but has not been widely marketed because morganite is not well known to consumers. Three things have limited the popularity of morganite: 1) most specimens have a very light color; 2) jewelry manufacturers are hesitant to make a large commitment to the gem because they usually do not have a steady source of supply; and, 3) consumers are not familiar with morganite because it has never been strongly promoted.

Heliodor beryl: A highly etched greenish yellow heliodor crystal of gem quality from Ukraine. Approximately 4.4 x 2.5 x 2.0 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Yellow Beryl Yellow beryl, also called "golden beryl" or "heliodor," is a yellow to greenish yellow beryl. A few vendors call it "yellow emerald" but many believe that name is inappropriate. Yellow beryl is a durable stone that often has a beautiful yellow color and a relatively low price. The public is not familiar with the gem, and as a result the demand is very low and so is the price. People who enjoy yellow gems and want an item of jewelry with a yellow beryl will have a hard time finding it at most jewelry stores. It is most often seen in the inventory of a jeweler who does custom designs. Small amounts of iron are thought to produce the color of yellow beryl, which can often be changed with heating or irradiation. Despite the fact that many specimens of yellow beryl depreciate with treatment to less valuable colors, some specimens can be heated to a greenish blue similar to aquamarine, while others can be irradiated to produce a more desirable yellow color. Those with plans to treat yellow beryl must experiment because treatment success is variable.

Beryl gems: Faceted beryl gems, clockwise from bottom left: aquamarine, morganite, and heliodor, all from Madagascar; green beryl from unknown locality.

Green Beryl "Green beryl" is the name given to light green specimens of beryl that do not have a tone and saturation dark enough to merit the name "emerald." Some of this light green beryl is colored by iron and lacks the distinct green color associated with emerald. Some is colored by chromium or vanadium and does not have the proper hue, tone, and saturation to be called "emerald." The price difference between green beryl and emerald is significant, so some buyers or sellers hope to have specimens judged in their favor. This can lead to problems because a precise color boundary between emerald and green beryl has not been defined with industry-wide agreement. Green beryl can be an attractive gem, but it is rarely seen in jewelry.

Natural red beryl: The photo above shows a faceted red beryl with a beautiful medium red color. It measures about 5.2 x 3.9 millimeters in size. From the Wah Wah Mountains of Utah. Photo by TheGemTrader.com.

Lab-created red beryl: Synthetic red beryl has the same composition and physical properties as a naturally occurring stone. The gem in the photo weighs 1.23 carats and measures 7.4 x 5.4 mm. Finding a red beryl of this size and clarity in nature would be nearly impossible.

Red Beryl Red beryl is one of the world's rarest gem materials. Gem-quality material that is large enough to facet has been found in very modest amounts in the Wah Wah Mountains and Thomas Range of Utah. Occurrences of red beryl have been found in the Black Range of New Mexico, but crystals there are just a few millimeters in length and are generally too small to facet. Red beryl is usually a very strong and attractive red color. It has a high enough saturation that even small gems have a very strong color. This is fortunate because most gems cut from red beryl are very small and only suitable for cutting into melee. Gems over one carat in size are very rare and sell for thousands of dollars per carat. The material is often included and fractured, and these characteristics are accepted just as they are accepted in emerald. In Utah, the host rocks of red beryl are rhyolitic lava flows. Here, crystals of red beryl formed in small vugs and shrinkage cracks long after the rhyolite crystallized. It is thought that ascending beryllium-rich gases encountered descending mineral-rich groundwater to create the geochemical environment needed to form red beryl. Trace amounts of manganese are thought to cause the color. Beryl is a relatively rare mineral because beryllium rarely occurs in large enough quantities to produce minerals. Red beryl is extremely rare because the conditions needed to supply the color-producing manganese at the proper time to a beryl-forming environment is improbable. So, the formation of red beryl requires the coincidence of two nearly impossible events. Red beryl was initially named "bixbite" after Maynard Bixby, who first discovered the material. That name has been mostly abandoned because it was so often confused with bixbyite, a manganese iron oxide mineral also named after Mr. Bixby. Some people call it "red emerald," but that name is rejected by many in the trade because it causes confusion with another variety of beryl named "emerald."

Faceted Goshenite: This specimen displays the excellent clarity and transparency that are frequently seen in goshenite. Image by DonGuennie, used here under a Creative Commonslicense.

Goshenite Goshenite is the name used for colorless beryl. In most cases, color in beryl is caused by trace amounts of certain metals that impart a color. That is often the case for goshenite, but color-inhibiting elements can keep goshenite colorless.

Goshenite is often found in large hexagonal crystals with exceptional clarity and transparency. In the Middle Ages these crystals were cut and polished into lenses for hand magnifiers, telescopes, and some of the earliest eyeglasses. With a Mohs hardness of 7.5 to 8.0, these were some of the earliest scratch-resistant lenses. Goshenite is sometimes cut into gemstones. These gems are mainly of interest to collectors. They are rarely used in jewelry, because they lack color and their appearance is inferior to other colorless gems such as diamond and white sapphire. Maxixe Another rare beryl is a very dark blue material known as "maxixe" (pronounced mashish). The dark blue color is thought to be developed in the ground by exposure to natural radiation. Maxixe is an unfortunate material because the wonderful blue color quickly fades in daylight to a pale brownish yellow color. The color can be restored with additional irradiation, but that color is also quickly lost with exposure to light. Maxixe was first found in 1917 in a mine in the Minas Gerais area of Brazil. It has since been found in small amounts at a few other locations.

Cat's-eye beryl: This yellow heliodor was mined in Madagascar and cut into a 10 x 8 millimeter chatoyant oval. It has a beautiful translucent color and a faint eye.

Chatoyant Beryl Beryl occasionally contains a fine silk that allows it to be cut into a chatoyant gem. Aquamarine, golden beryl, and emerald are the most likely beryls to be found with chatoyance. When properly oriented and cut en cabochon, these gems usually produce a weak cat's eye, but occasionally a strong cat's eye is produced. The most desirable chatoyant beryls are those with a highly desirable color and a bright, thin eye that perfectly bisects the gem. Contributor: Hobart King

Biotite Biotite is group of common rock-forming minerals found in igneous and metamorphic rocks.

Biotite: Biotite from Bancroft, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across.

What is Biotite? Biotite is a name used for a large group of black mica minerals that are commonly found in igneous and metamorphic rocks. These include annite, phlogopite, siderophyllite, fluorophlogopite, fluorannite, eastonite, and many others. These micas vary in chemical composition but are all sheet silicate minerals with very similar physical properties. A generalized chemical composition for the biotite group is: K(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2

The name "biotite" is used in the field and in entry-level geology courses because these minerals generally cannot be distinguished without optical, chemical, or x-ray analysis. Biotite is a primary mineral found in a wide range of crystalline igneous rocks such as granite, diorite, gabbro, peridotite, and pegmatite. It also forms under metamorphic conditions when argillaceous rocks are exposed to heat and pressure to form schist and gneiss. Although biotite is not very resistant to weathering and transforms into clay minerals, it is sometimes found in sediments and sandstones.

Physical Properties of Biotite Chemical Classification

Dark mica

Color

Black, dark green, dark brown

Streak

White to gray, flakes often produced

Luster

Vitreous

Diaphaneity

Thin sheets are transparent to translucent, books are opaque.

Cleavage

Basal, perfect

Mohs Hardness

2.5 to 3

Specific Gravity

2.7 to 3.4

Diagnostic Properties

Dark color, perfect cleavage

Chemical Composition

K(Mg,Fe)2-3Al1-2Si2-3O10(OH,F)2

Crystal System

Monoclinic

Uses

Very little industrial use

Properties of Biotite Biotite is very easy to identify, and with a little experience a person will be able to recognize it on sight. It is a black mica with perfect cleavage and a vitreous luster on the cleavage faces. When biotite is separated into thin sheets, the sheets are flexible but will break upon severe bending. When held up to the light, the sheets are transparent to translucent with a brown, gray, or greenish color. Experienced observers can sometimes recognize phlogopite by its brown color.

Biotite angled view: Biotite from Bancroft, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across.

Biotite Minerals As noted above, biotite is a name used for a number of black mica minerals that have different chemical compositions but very similar physical properties. These minerals generally cannot be distinguished from one another without laboratory analysis. A small list of the biotite minerals is given below with their chemical compositions. Mineral

Chemical Composition

Annite

KFe3(AlSi3)O10(OH)2

Phlogopite

KMg3(AlSi3)O10(OH)2

Siderophyllite

KFe2Al(Al2Si2)O10(F,OH)2

Eastonite

KMg2Al(Al2Si3)O10(OH)2

Fluorannite

KFe3(AlSi3)O10F2

Fluorophlogopite

KMg3(AlSi3)O10F2

Biotite side view: An edge view of the biotite specimen from the photo above. Specimen is approximately 3/8 inch (.95 centimeter) thick.

Uses of Biotite Biotite has a small number of commercial uses. Ground mica is used as a filler and extender in paints, as an additive to drilling muds, as an inert filler and mold-release agent in rubber products, and as a non-stick surface coating on asphalt shingles and rolled roofing. It is also used in the potassium-argon and argon-argon methods of dating igneous rocks.

Biotite in sandstone: Core samples of biotitic sandstone from the Apple Creek Formation, Copper Queen Mine, near Salmon, Idaho. USGS image.

The Other "Fool's Gold" Biotite has been known to cause excitement in inexperienced gold panners. A few tiny flakes of biotite swishing in a gold pan can produce bright bronze-colored reflections in the pan when struck by sunlight. These reflections can fool the inexperienced panner into thinking that he has found gold. If the panner regains his composure, removes one of these flakes from the pan and pokes it with a pin, it will break. First-time panners quickly learn to do some testing before shouting "gold" - which probably isn't a good idea even when gold is found because it can attract unwanted visitors to your panning spot. Small flakes of biotite have also been known to cause excitement when they are observed in rocks. Their bronze-colored reflections can fool the inexperienced observer into thinking that tiny flakes of gold are present. Again, the pin test or a hand lens will usually yield a quick answer. Contributor: Hobart King

Bornite Mineral Properties and Uses What is Bornite? Bornite is a copper iron sulfide mineral commonly found in hydrothermal veins, contact metamorphic rocks and in the enriched zone of sulfide copper deposits. It is a common ore of copper and is easily recognized because it tarnishes to iridescent shades of blue, purple, green and yellow. It is often mined as an ore of copper.

Physical Properties of Bornite Chemical Classification

sulfide

Color

brownish bronze on a fresh surface, iridescent purple, blue, and black on a tarnished surface

Streak

grayish black

Luster

metallic

Diaphaneity

opaque

Cleavage

poor

Mohs Hardness

3

Specific Gravity

5.0 to 5.1

Diagnostic Properties Chemical Composition Crystal System Uses

color copper iron sulfide, Cu5FeS4 tetragonal Primarily an ore of copper.

Bornite from Musina, South Africa. Specimen is approximately 3/4 inch (1.9 centimeters) across.

Calcite The unique properties of calcite make it suitable for a variety of uses.

Calcite: Calcite in the form of white marble was the primary stone used in the Supreme Court building. © iStockphoto / Gary Blakeley.

Calcite as pink marble: Calcite in the form of a pink marble from Tate, Georgia. This specimen is approximately four inches (ten centimeters) across.

What is Calcite? Calcite is a rock-forming mineral with a chemical formula of CaCO3. It is extremely common and found throughout the world in sedimentary, metamorphic, and igneous rocks. Some geologists consider it to be a "ubiquitous mineral" - one that is found everywhere. Calcite is the principal constituent of limestone and marble. These rocks are extremely common and make up a significant portion of Earth's crust. They serve as one of the largest carbon repositories on our planet.

The properties of calcite make it one of the most widely used minerals. It is used as a construction material, abrasive, agricultural soil treatment, construction aggregate, pigment, pharmaceutical and more. It has more uses than almost any other mineral.

Calcite in the form of oolitic limestone from Bedford, Indiana. Specimen is about four inches (ten centimeters) across.

Calcite as Limestone and Marble Limestone is a sedimentary rock that is composed primarily of calcite. It forms from both the chemical precipitation of calcium carbonate and the transformation of shell, coral, fecal and algal debris into calcite during diagenesis. Limestone also forms as a deposit in caves from the precipitation of calcium carbonate. Marble is a metamorphic rock that forms when limestone is subjected to heat and pressure. A close examination of a broken piece of marble will usually reveal obvious cleavage faces of calcite. The size of the calcite crystals is determined by the level of metamorphism. Marble that has been subjected to higher levels of metamorphism will generally have larger calcite crystals.

Calcite in concrete used in a high-rise building: Calcite in the form of limestone is used to make cement and also used as the aggregate in most concrete. A concrete slurry can be pumped or hoisted from the ground and poured into forms to produce the structural elements of buildings. © iStockphoto / Frank Leung.

Uses of Calcite in Construction The construction industry is the primary consumer of calcite in the form of limestone and marble. These rocks have been used as dimension stones and in mortar for thousands of years. Limestone blocks were the primary construction material used in many of the pyramids of Egypt and Latin America. Today, rough and polished limestone and marble are still an important material used in prestige architecture. Modern construction uses calcite in the form of limestone and marble to produce cement and concrete. These materials are easily mixed, transported, and placed in the form of a slurry that will harden into a durable construction material. Concrete is used to make buildings, highways, bridges, walls, and many other structures.

Calcite with cleavage: Transparent calcite from Baxter Springs, Kansas, showing characteristic cleavage. Specimen is approximately four inches (10 centimeters) across.

Calcite as agrilime: Acid-neutralizing qualities of calcite make finely crushed limestone a preferred material for soil treatment. © iStockphoto / Krzch-34.

Physical Properties of Calcite Chemical Classification Color

Carbonate Usually white but also colorless, gray, red, green, blue, yellow, brown, orange

Streak

White

Luster

Vitreous

Diaphaneity

Transparent to translucent

Cleavage

Perfect, rhombohedral, three directions

Mohs Hardness

3

Specific Gravity

2.7

Diagnostic Properties

Rhombohedral cleavage, powdered form effervesces weakly in dilute HCl, curved crystal faces and frequent twinning

Chemical Composition

CaCO3

Crystal System Uses

Hexagonal Acid neutralization, a low-hardness abrasive, soil conditioner, heated for the production of lime

Calcite as an antacid: The acid-neutralizing ability of calcite is used in medicine. High-purity calcite was used to make these antacid tablets. © iStockphoto / Rudi Tapper.

Uses in Acid Neutralization Calcite has numerous uses as a neutralizer of acids. For hundreds of years, limestones and marbles have been crushed and spread on fields as an acid-neutralizing soil treatment. They are also heated to produce lime that has a much faster reaction rate in the soil. Calcite is used as an acid neutralizer in the chemical industry. In areas were streams are plagued with acid mine drainage, crushed limestone is dispensed into the streams to neutralize their waters. Calcium carbonate derived from high-purity limestones or marbles is used in medicine. Mixed with sugar and flavoring, calcium carbonate is made into chewable tablets used in the neutralization of stomach acids. It is also an ingredient in numerous medications used to treat digestive and other ailments.

Calcium Carbonate Sorbents Sorbents are substances that have the ability to "capture" another substance. Limestone is often treated and used as sorbent material during the burning of fossil fuels. Calcium carbonate reacts with sulfur dioxide and other gases in the combustion emissions, absorbs them, and prevents them from escaping to the atmosphere.

Calcite as marble blocks: White marble blocks for monuments or statuary, awaiting transport from a quarry in Portugal. © iStockphoto / Manuel Ribeiro.

Monuments and Statuary Marble is an attractive and easily worked rock that has long been used for monuments and sculptures. Its lack of significant porosity allows it to stand up well to freeze-thaw action outdoors, and its low hardness makes it an easy stone to work. It has been used in projects as large as the pyramids and as small as a figurine. It is widely used as cemetery markers, statues, mantles, benches, stairways, and much more.

Calcite as chalk: Calcite in the form of chalk from Dover, England. Specimen is about 4 inches (10 centimeters) across.

Many Other Uses In a powdered form, calcite often has an extremely white color. Powdered calcite is often used as a white pigment or "whiting." Some of the earliest paints were made with calcite. It is a primary ingredient in whitewash, and it is used as an inert coloring ingredient of paint.

Pulverized limestone and marble are often used as a dietary supplement in animal feed. Chickens that produce eggs and cattle that produce milk need to consume a calcium-rich diet. Small amounts of calcium carbonate are often added to their feeds to enhance their calcium intake. Calcite has a hardness of three on the Mohs scale, and that makes it suitable as a low-hardness abrasive. It is softer than the stone, porcelain, and plastic surfaces found in kitchens and bathrooms but more durable than dried food and other debris that people want to remove. Its low hardness makes it an effective cleaning agent that does not damage the surface being cleaned. Pulverized limestone is also used as a mine safety dust. This is a nonflammable dust that is sprayed onto the walls and roofs of underground coal mines to reduce the amount of coal dust in the air (which can be an explosion hazard). The mine safety dust adheres to the wall of the mine and immobilizes the coal dust. Its white color aids in illumination of the mine. It is the perfect material for this use.

Calcite as travertine cave formations: Calcite cave formations of Luray Caverns, Virginia, USA. © iStockphoto / Daniel Yost.

Calcite: A Carbon Dioxide Repository Carbon dioxide is an important gas in Earth's environment. In the atmosphere it serves as a greenhouse gas that works to trap and hold heat near the surface of the planet. The process of limestone formation removes carbon dioxide from the atmosphere and stores it away for long periods of time. This process has been occurring for millions of years - producing enormous volumes of stored carbon dioxide. When these rocks are weathered, used to neutralize acids, heated to make cement or metamorphosed severely, some of their carbon dioxide is released and returned to the atmosphere. All of these processes of limestone formation and destruction have an impact on Earth's climate. Contributor: Hobart King

Calcite as lithographic limestone: Calcite in the form of lithographic limestone from Solnhofen, Bavaria, Germany. Note the fine, uniform texture that is characteristic of lithographic limestone. Specimen is about 4 inches (ten centimeters) across.

Calcite as oolitic limestone: Calcite in the form of oolitic limestone from Tyrone, Pennsylvania. This specimen is approximately four inches (ten centimeters) across.

Calcite as translucent onyx: Calcite in the form of translucent onyx from Tecali, Mexico. Specimen is about four inches (ten centimeters) across.

Double refraction in calcite: Transparent calcite (known as "Iceland Spar") from Chihuahua, Mexico. This specimen shows excellent double refraction. Specimen is about four inches (ten centimeters) across.

Calcite as calcareous tufa: Calcite in the form of calcareous tufa from Mumford, New York. This specimen is approximately four inches (ten centimeters) across.

Calcite as travertine: Calcite in the form of travertine from Tivoli, Italy. Specimen is about four inches (ten centimeters) across.

Picasso Stone: A variety of marble with brown and black markings is known as "Picasso Stone." It is frequently cut and polished as cabochons or used to produce tumbled stones. It is popular for jewelry and ornamental crafts.

White calcite as marble: Calcite in the form of white, coarsely crystalline marble from Tate, Georgia. Specimen is about four inches (ten centimeters) across.

Calcite sand crystals: Calcite in the form of siliceous crystals from the Badlands, South Dakota. The calcite grew as crystals in a sand, including the sand grains within its crystal structure. Specimen is about five inches (twelve centimeters) across.

Cassiterite Mineral Properties and Uses

What is Cassiterite? Cassiterite is a tin oxide mineral that is found in vein deposits, granitic rocks, pegmatites, areas of contact metamorphism and the altered zone of ore deposits. It is also found in placer deposits where it is most commonly mined and given the name of "stream tin". It is the most widely mined ore of tin.

Uses of Cassiterite Cassiterite is the principle ore of tin. In the past, much of the tin was used to produce "tin cans" (actually steel plated with tin) for food containers. However, this use is being rapidly replaced by containers made of glass, plastic, paper, aluminum and other materials. Small amounts are also used to produce solder and polishing compounds.

Cassiterite sand from Plateau State, Nigeria, Africa. Placer-mined tin is often called "stream tin". Silt- to sand-size particles of cassiterite.

Physical Properties of Cassiterite Chemical Classification

oxide

Color

brown, black, reddish brown, brownish black

Streak

colorless

Luster

adamantine, splendent to submetallic

Diaphaneity Cleavage

opaque to translucent imperfect

Mohs Hardness

6 to 7

Specific Gravity

6.8 to 7.1

Diagnostic Properties Chemical Composition

high specific gravity, luster, streak, fibrous appearance tin oxide, SnO2

Particles of cassiterite from a placer deposit near Tinton, South Dakota. Specimens are approximately 1/8 inch to 3/8 inch (.3 centimeter to .95 centimeter) across.

Crystal System Uses

tetragonal an ore of tin

Cassiterite from near Keystone, South Dakota. Specimen is approximately 4 inches (10 centimeters) across.

Chalcocite Mineral Properties and Uses What is Chalcocite? Chalcocite is an iron sulfide mineral and an important ore of copper. It is most commonly found as a supergene mineral in the enriched zones of sulfide deposits. It is also occurs in hydrothermal veins.

Physical Properties of Chalcocite Chemical Classification

sulfide

Color

black, blackish lead gray

Streak

grayish black

Luster

metallic

Diaphaneity

opaque

Cleavage

poor

Mohs Hardness

2.5 to 3

Specific Gravity

5.5 to 5.8

Diagnostic Properties

color, soft, sooty appearance

Chemical Composition

copper sulfide, Cu2S

Crystal System Uses

Chalcocite from Butte, Montana. This specimen is approximately 3 inches (7.6 centimeters) across.

monoclinic Used as an ore of copper

Chalcocite from Musina, South Africa. Specimen is approximately 1 inch (2.5 centimeters) across.

Chalcopyrite The world's most important ore of copper for at least five thousand years.

Auriferous Chalcopyrite: A specimen of chalcopyrite with pyrrhotite from the Rouyn District, Quebec, Canada. Some chalcopyrite contains enough gold or silver that it can be an ore of those metals without considering the copper content. This specimen is about ten centimeters across.

What is Chalcopyrite? Chalcopyrite is a brass-yellow mineral with a chemical composition of CuFeS2. It occurs in most sulfide mineral deposits throughout the world and has been the most important ore of copper for thousands of years. The surface of chalcopyrite loses its metallic luster and brass-yellow color upon weathering. It tarnishes to a dull, gray-green color, but in the presence of acids the tarnish can develop a red to blue to purple iridescence. The iridescent colors of weathered chalcopyrite attract attention. Some souvenir shops sell chalcopyrite that has been treated with acid as "peacock ore." But, "peacock ore" is a more appropriate name for the mineral bornite.

Related: Interesting Facts About Copper

Copper Uses, Resources, Supply, Demand and Production Information Republished from USGS Fact Sheets released in June, 2009 and January 2014

Copper - A Metal Used Through The Ages

Copper was one of the first metals ever extracted and used by humans, and it has made vital contributions to sustaining and improving society since the dawn of civilization. Copper was first used in coins and ornaments starting about 8000 B.C., and at about 5500 B.C., copper tools helped civilization emerge from the Stone Age. The discovery that copper alloyed with tin produces bronze marked the beginning of the Bronze Age at about 3000 B.C. Copper is easily stretched, molded, and shaped; is resistant to corrosion; and conducts heat and electricity efficiently. As a result, copper was important to early humans and continues to be a material of choice for a variety of domestic, industrial, and high-technology applications today.

How Do We Use Copper Today?

In 1886, the Statue of Liberty represented the largest use of copper in a single structure. To build the statue, about 80 tons of copper sheet was cut and hammered to a thickness of about 2.3 millimeters (3/32 inch), or about that of two U.S. pennies placed together. Photo copyright iStockphoto / A. Harris.

Presently, copper is used in building construction, power generation and transmission, electronic product manufacturing, and the production of industrial machinery and transportation vehicles. Copper wiring and plumbing are integral to the appliances, heating and cooling systems, and telecommunications links used every day in homes and businesses. Copper is an essential component in the motors, wiring, radiators, connectors, brakes, and bearings used in cars and trucks. The average car contains 1.5 kilometers (0.9 mile) of copper wire, and the total amount of copper ranges from 20 kilograms (44 pounds) in small cars to 45 kilograms (99 pounds) in luxury and hybrid vehicles.

Ancient Uses of Copper As in ancient times, copper remains a component of coinage used in many countries, but many new uses have been identified. One of copper's more recent applications includes its use in frequently touched surfaces (such as brass doorknobs), where copper's antimicrobial properties reduce the transfer of germs and disease. Semiconductor manufacturers have also begun using copper for circuitry in silicon chips, which enables microprocessors to operate faster and use less energy. Copper rotors have also recently been found to increase the efficiency of electric motors, which are a major consumer of electric power.

This graph shows how copper was used in the United States during 2011 by industry sector. As an example: copper used in building construction could have been used for wiring, plumbing, weatherproofing and many other individual types of use. Data for this chart is from the United States Geological Survey Mineral Commodity Summary for 2011.

What Properties Make Copper Useful?

The excellent alloying properties of copper have made it invaluable when combined with other metals, such as zinc (to form brass), tin (to form bronze), or nickel. These alloys have desirable characteristics and, depending on their composition, are developed for highly specialized applications. For example, copper-nickel alloy is applied to the hulls of ships because it does not corrode in seawater and reduces the adhesion of marine life, such as barnacles, thereby reducing drag and increasing fuel efficiency. Brass is more malleable and has better acoustic properties than pure copper or zinc; consequently, it is used in a variety of musical instruments, including trumpets, trombones, bells, and cymbals.

Types of Copper Deposits

Copper is an essential component in the motors, wiring, radiators, connectors, brakes, and bearings used in cars and trucks. The average car contains 1.5 kilometers (0.9 mile) of copper wire, and the total amount of copper ranges from 20 kilograms (44 pounds) in small cars to 45 kilograms (99 pounds) in luxury and hybrid vehicles. Photo copyright iStockphoto / Rawpixel.

Copper occurs in many forms, but the circumstances that control how, when, and where it is deposited are highly variable. As a result, copper occurs in many different minerals. Chalcopyrite is the most abundant and economically significant of the copper minerals. Research designed to better understand the geologic processes that produce mineral deposits, including copper deposits, is an important component of the USGS Mineral Resources Program. Copper deposits are broadly classified on the basis of how the deposits formed. Porphyry copper deposits, which are associated with igneous intrusions, yield about two-thirds of the world's copper and are therefore the world's most important type of copper deposit. Large copper deposits of this type are found in mountainous regions of western North and South America. Another important type of copper deposit-the type contained in sedimentary rocks-accounts for approximately one-fourth of the world's identified copper resources. These deposits occur in such areas as the central African copper belt and the Zechstein basin of Eastern Europe. Individual copper deposits may contain hundreds of millions of tons of copper-bearing rock and commonly are developed by using open-pit mining methods. Mining operations, which usually follow ore discovery by many years, often last for decades. Although many historic mining operations were not required to conduct their mining activities in ways that would reduce their impact on the environment, current Federal and State regulations do require that mining operations use environmentally sound practices to minimize the effects of mineral development on human and ecosystem health. USGS mineral environmental research helps characterize the natural and human interactions between copper deposits and the surrounding aquatic and terrestrial ecosystems. Research helps define the natural baseline conditions before mining begins and after mine closure. USGS scientists are investigating

Copper was one of the first metals used to make coins and that practice began in about 8000 BC. The coin shown above is a Roman follis featuring an image of Constantinus I. Photo copyright iStockphoto / craetive.

climatic, geologic, and hydrologic variables to better understand the resource-environment interactions.

Copper Supply, Demand and Recycling

The world's production (supply) and consumption (demand) of copper have increased dramatically in the past 25 years. As large developing countries have entered the global market, demand for mineral commodities, including copper, has increased. In the past 20 years, the Andean region of South America has emerged as the world's most productive copper region. In 2007, about 45 percent of the world's copper was produced from the Andes Mountains; the United States produced 8 percent. Virtually all copper produced in the United States comes from, in decreasing order of production, Arizona, Utah, New Mexico, Nevada, or Montana. The risk of disruption to the global copper supply is considered to be low because copper production is globally dispersed and is not limited to a single country or region. Because of its importance in construction and power transmission, however, the impact of any copper supply disruption would be high. Copper is one of the most widely recycled of all metals; approximately one-third of all copper consumed worldwide is recycled. Recycled copper and its alloys can be remelted and used directly or further reprocessed to refined copper without losing any of the metal's chemical or physical properties.

How Do We Ensure Adequate Supplies of Copper for the Future?

To help predict where future copper resources might be located, USGS scientists study how and where known copper resources are concentrated in the Earth's crust and use that knowledge to assess the potential for undiscovered copper resources. Techniques to assess mineral resource potential have been developed and refined by the USGS to support the stewardship of Federal lands and to better evaluate mineral resource availability in a global context.

Copper is an important element in a number of gemstones such as turquoise, azurite, malachite and chrysocolla. It gives these minerals their green or blue color and their high specific gravity. The cabochons shown above are some of the many gemstones mined in Arizona.

Did You Know? At least 160 copper-bearing minerals have been identified in nature; some of the more familiar minerals are chalcopyrite, malachite, azurite, and turquoise.

Arizona produces more copper than any other state. This brief history shows how Arizona's copper mining built a state and changed a nation.

Did You Know? Copper is necessary for human health; the best sources of dietary copper include seafood, organ meats, whole grains, nuts, raisins, legumes, and chocolate.

In the 1990s, the USGS conducted an assessment of U.S. copper resources and concluded that nearly as much copper remained to be found as had already been discovered. Specifically, the USGS found that about 350 million tons of copper had been discovered and estimated that about 290 million tons of copper remained undiscovered in the United States.

Visible from space, the Bingham Canyon copper mine in Utah has produced more than 12 million tons of porphyry copper. The mine is more than 4 kilometers (2.5 miles) across at

Global Copper Resource Assessment

The USGS assessed undiscovered copper in two deposit types that account for about 80 percent of the world's copper supply. Porphyry copper deposits account for about 60 percent of the world's copper. In porphyry copper deposits, copper ore minerals are disseminated in igneous intrusions. Sedimenthosted stratabound copper deposits, in which copper is concentrated in layers in sedimentary rocks, account for about 20 percent of the world's identified copper resources. Globally, mines in these two deposit types produce about 12 million tons of copper per year.

the top and 800 meters (0.5 mile) deep and is one of the engineering wonders of the world. Photograph by C.G. Cunningham, USGS.

Did You Know? The United States was the world's largest copper producer until 2000; beginning in 2000, Chile became the world's leading copper producer.

This study considered potential for exposed and concealed deposits within 1 kilometer of the surface for porphyry deposits and up to 2.5 kilometers of the surface for sediment-hosted stratabound deposits. For porphyry deposits, 175 tracts were delineated; 114 tracts contain 1 or more identified deposits. Fifty tracts were delineated for sediment-hosted stratabound copper deposits; 27 contain 1 or more identified deposits. Results of the assessment are provided by deposit type for 11 regions (table 1). The mean total undiscovered resource for porphyry deposits is 3,100 million tons, and the mean total undiscovered resource for sediment-hosted deposits is 400 million tons, for a global total of 3,500 million tons of copper. The ranges of resource estimates (between the 90th and 10th percentiles) reflect the geologic uncertainty in the assessment process. Approximately 50 percent of the global total occurs in South America, South Central Asia and Indochina, and North America combined. South America has the largest identified and undiscovered copper resources (about 20 percent of the total undiscovered amount). The world's largest porphyry deposits are mined in this region. Central America and the Caribbean host two undeveloped giant (>2 million ton copper) porphyry copper deposits in Panama. Most of the undiscovered resources are in a belt that extends from Panama to southwestern Mexico. North America hosts highly mineralized porphyry copper tracts that include supergiant (>25 million tons copper) porphyry deposits in northern Mexico, the western United States, and Alaska, as well as giant deposits in western Canada. The estimated undiscovered porphyry copper resources are approximately equal to the identified resources. In the United States, undiscovered sediment-hosted stratabound copper deposits in Michigan, Montana, and Texas are estimated to contain about three times as much copper as has been identified. Two giant deposits are known, in Michigan and Montana. Northeast Asia is relatively underexplored, with modest identified porphyry copper resources and only one identified giant porphyry copper deposit. However, the mean undiscovered resources are estimated to be quite large. This region has the largest ratio of undiscovered to identified

The qualities of copper that have made it the material of choice for a variety of domestic, industrial, and hightechnology applications have resulted in a steady rise in global copper consumption. USGS studies of copper consumption show some interesting trends for the 1990 to 2012 time period. Copper consumption in emerging economies, such as China and India, rose considerably, whereas the consumption rate in the United States, fell slightly. Until 2002, the United States was the leading copper consumer and annually used about 16 percent of total world refined copper (about 2.4 million tons). In 2002, the United States was overtaken by China as the world's leading user of refined copper. The booming economy in China contributed to a quadrupling of its annual refined copper consumption during the 12 years from 2000 to 2012. Graph by USGS.

Did You Know? Before 1982, the U.S. penny was made entirely of copper; since 1982, the U.S. penny has been only coated with copper.

resources in the study. North Central Asia has 35 porphyry copper deposits, including a supergiant deposit in Mongolia and a giant deposit in Kazakhstan. The tract area is estimated to contain about three times the amount of identified porphyry copper resource. This region also hosts three giant sediment-hosted stratabound copper deposits, in Kazakhstan and Russia. The USGS estimates that as much sediment-hosted stratabound copper as has already been discovered may be present. South Central Asia and Indochina are less thoroughly explored than many other parts of the world; however, four giant porphyry copper deposits have been identified to date in the Tibetan Plateau. Undiscovered porphyry copper deposits may contain eight times the identified amount of copper. Southeast Asia Archipelagos host world-class, gold-rich porphyry copper deposits such as a supergiant in Indonesia and about 16 giant deposits in Indonesia, Papua New Guinea, and the Philippines. Although parts of the region are well explored, undiscovered porphyry resources are likely to exceed identified resources. Eastern Australia has one giant porphyry copper deposit and several small porphyry deposits. Modest undiscovered resources are expected under cover. Eastern Europe and Southwestern Asia have been mined for copper since ancient times, and giant porphyry copper deposits have recently been identified. Undiscovered copper is predicted to be about twice the identified resources, both for porphyry deposits along a belt from Romania through Turkey and Iran and for sediment-hosted stratabound deposits in Afghanistan. Western Europe has the largest sediment-hosted stratabound copper deposit in the world, in Poland. Undiscovered sedimenthosted stratabound copper resources in southwestern Poland are estimated to exceed identified resources by about 30 percent. Africa and the Middle East have the world's largest accumulation of sediment-hosted stratabound copper deposits, with 19 giant deposits in the Central African Copperbelt in the Democratic Republic of Congo and Zambia. Significant undiscovered copper resources remain to be discovered.

Contributors

The information in these USGS Fact Sheets was prepared by Jeff Doebrich, Kathleen Johnson, Jane Hammarstrom, Michael Zientek, and Connie Dicken.

Distribution of known copper deposits in 2008. Red indicates copper associated with igneous intrusions (porphyry copper deposits) and blue indicates copper contained in sedimentary rocks (sediment-hosted copper deposits). Map by USGS. Enlarge Map

Did You Know? Copper is one of the few metals that occur in nature in native form. Because of this, it was one of the first metals used by ancient peoples and it continues to be an important metal today.

Physical Properties of Chalcopyrite Chemical Classification

Sulfide

Color

Brass yellow. Tarnishes to gray green, sometimes iridescent.

Streak

Greenish black

Luster

Metallic

Diaphaneity

Opaque

Cleavage

Poor

Mohs Hardness

3.5 to 4

Specific Gravity

4.1 to 4.3

Diagnostic Properties

Color, greenish streak, softer than pyrite, brittle.

Chemical Composition

Copper iron sulfide, CuFeS2.

Crystal System Uses

Tetragonal The most important ore of copper for thousands of years.

Chalcopyrite on Dolomite: Tetragonal crystals of chalcopyrite on dolomite from Baxter Springs, Kansas. This specimen is about 10 centimeters across.

Physical Properties of Chalcopyrite The most obvious physical properties of chalcopyrite are its brassy yellow color, metallic luster, and high specific gravity. These give it a similar appearance to pyrite and gold. Distinguishing these minerals is easy. Gold is soft, has a yellow streak and has a much higher specific gravity. Chalcopyrite is brittle and has a

greenish gray streak. Pyrite is hard enough that it cannot be scratched with a nail, but chalcopyrite is easily scratched with a nail. The name "fool's gold" is most often associated with pyrite because it is more common and more often confused with gold. Chalcopyrite is also confused with gold, so the name "fool's gold" is also applied and appropriate.

Chalcopyrite: Chalcopyrite from Ajo, Arizona. Specimen is approximately 10 centimeters across.

Chalcopyrite: Specimen of chalcopyrite from Rouyn District, Quebec, Canada. Specimen is approximately 10 centimeters across.

Geologic Occurrence of Chalcopyrite Chalcopyrite forms under a variety of conditions. Some is primary, crystallizing from melts as accessory minerals inigneous rocks. Some forms by magmatic segregation and is in the stratified rocks of a magma chamber. Some occurs in pegmatite dikes and contact metamorphic rocks. Some is disseminated through schist and gneiss. Many volcanogenic massive sulfide deposits containing chalcopyrite are known.

The most significant chalcopyrite deposits to be mined are hydrothermal in origin. In these, some chalcopyrite occurs in veins and some replaces country rock. Associated ore minerals include pyrite, sphalerite, bornite, galena, and chalcocite. Chalcopyrite serves as the copper source for many secondary mineral deposits. Copper is removed from chalcopyrite by weathering or solution, transported a short distance, then redeposited as secondary sulfide, oxide, or carbonate minerals. Many malachite, azurite, covellite, chalcocite, and cuprite deposits contain this secondary copper.

Uses of Chalcopyrite The only important use of chalcopyrite is as an ore of copper, but this single use should not be understated. Chalcopyrite has been the primary ore of copper since smelting began over five thousand years ago. Some chalcopyrite ores contain significant amounts of zincsubstituting for iron. Others contain enough silver or goldthat the precious metal content more than pays the costs of mining.

Chlorite A group of common sheet silicate minerals

Chlorite: Chlorite from Quebec, Canada. This specimen is approximately 3 inches (7.6 centimeters) across.

What is Chlorite? "Chlorite" is the name of a group of common sheet silicate minerals that form during the early stages of metamorphism. Most chlorite minerals are green in color, have a foliated appearance, perfect cleavage, and an oily to soapy feel. They are found in igneous, metamorphic and sedimentary rocks. Chlorite minerals are found in rocks altered during deep burial, plate collisions, hydrothermal activity, or contact metamorphism. They are also found as retrograde minerals in igneous and metamorphic rocks that have been weathered. Rocks that commonly contain abundant chlorite include greenschist, phyllite, chlorite schist, and greenstone. Chlorite Minerals Mineral

Composition

Baileychlore

(Zn,Fe+2,Al,Mg)6(Al,Si)4O10(O,OH)8

Borocookeite

Li1+3xAl4-x(BSi3O10)(OH)8

Chamosite

(Fe+2,Mg,Al,Fe+3)6(Si,Al)4O10(OH,O)8

Clinochlore

(Mg,Fe)5Al(Si3Al)O10(OH)8

Cookeite Donbassite Franklinfurnaceite

(Al2Li)Al2(Si3AlO10)(OH)8 Al13(Al3Si9O30)(OH)24 Ca2Fe+3Mn+23Mn+3(Zn2Si2O10)(OH)8

Glagolevite

Na(Mg,Al)6(AlSi3O10)(OH,O)8

Gonyerite

(Mn,Mg)5Fe+3(Fe+3Si3O10)(OH)8

Nimite

(Ni,Mg,Al)6(Si,Al)4O10(OH)8

Odinite

(Fe+2,Mg,Al,Fe+3,Ti,Mn)12(Al,Si)10O25OH20

Orthochamosite

(Fe+2,Mg,Fe+3)5Al(Si3AlO10)(O,OH)8

Pennantite

(Mn5Al)(Si3Al)O10(OH)8

Ripidolite

(Mg,Fe,Al)6(Al,Si)4O10(OH)8

Sudoite

Mg2(Al,Fe)3(Si3AlO10)(OH)8

Chlorite Minerals Chlorite minerals have a generalized chemical composition of (X,Y)4-6(Si,Al)4O10(OH,O)8. The "X" and "Y" in the formula represent ions, which might include: Fe+2, Fe+3, Mg+2, Mn+2, Ni+2, Zn+2, Al+3, Li+1, or Ti+4. The composition and physical properties of chlorites vary as these ions substitute for one another in solid solution. The most common chlorite minerals are clinochlore, pennantite, and chamosite. A more comprehensive list of chlorite minerals and their chemical compositions is shown in the green table on this page.

Chlorite: A side view of chlorite showing its foliated appearance. Specimen is from Quebec, Canada and is approximately 3 inches (7.6 centimeters) across.

Where Does Chlorite Form? Chlorite minerals most often form in rock environments where minerals are altered by heat, pressure, and chemical activity. These generally have a temperature less than a few hundred degrees and are within a few miles of Earth's surface. Chlorite minerals often form in clay-rich sedimentary rocks that are buried in deep sedimentary basins or subjected to regional metamorphism at a convergent plate boundary. Chlorite that forms here is usually associated with biotite, muscovite, garnet, staurolite, andalusite, or cordierite. Metamorphic rocks rich in chlorite might include phyllite and chlorite schist.

Another environment of chlorite mineral formation is in oceanic crust descending into subduction zones. Here, amphiboles, pyroxenes, and micas are altered into chlorite. Chlorite minerals also form during the hydrothermal, metasomatic, or contact metamorphism. These chlorite minerals are often found in fractures, solution cavities, or the vesicles of igneous rocks. Physical Properties of Chlorite Chemical

Silicate

Classification Color

Various shades of green. Rarely yellow, white, pink, black

Streak

Greenish to greenish gray

Luster

Vitreous, pearly, dull

Diaphaneity

Transparent, translucent, opaque

Cleavage

Perfect in one direction

Mohs Hardness

2 to 3

Specific Gravity

2.6 to 3.3

Diagnostic

Color, hardness, foliated appearance, feels slightly greasy

Properties

A generalized formula: (X,Y)4-6(Si,Al)4O10(OH,O)8 Chemical Composition

The "X" and "Y" in the formula represent ions, which might include: Fe+2, Fe+3, Mg+2, Mn+2, Ni+2, Zn+2, Al+3, Li+1, or Ti+4. The composition and physical properties of chlorites vary as these ions substitute for one another in solid solution.

Crystal System Uses

Monoclinic Very few industrial uses. Used as a filler and as a constituent of clay.

Physical Properties of Chlorites Members of the chlorite mineral group are typically green in color, have a foliated appearance, perfect cleavage, and an oily or soapy feel. Their variable chemical composition gives them a range of hardness and specific gravity. This makes them difficult to differentiate in hand specimen. Recognizing a mineral as a member of the chlorite group is usually easy. However, placing a specific name on it can be difficult. Detailed optical, chemical, or x-ray analysis is usually required for positive identification. The name "chlorite" is often used in classrooms and the field because the minerals are difficult or impossible to identify. As a result, the individual chlorite minerals are poorly known.

Uses of Chlorite Chlorite is a mineral with a low potential for industrial use. It does not have physical properties that make it suited for a particular use, and it does not contain constituents that make it a target of mining. When found, chlorite is usually intimately intermixed with other minerals, and the cost of separation would be high. As a result, chlorite is not mined and processed for any specific use. Its major use is as a coincidental constituient in crushed stone.

Chromite The only ore of chromium, the metal used to make stainless steel, nichrome, and chrome plating.

Chromite: Chromite from the Transvaal area of South Africa. Specimen is approximately 4 inches (10 centimeters) across.

What is Chromite? Chromite is an oxide mineral composed of chromium, iron, and oxygen (FeCr2O4). It is dark gray to black in color with a metallic to submetallic luster and a high specific gravity. It occurs in basic and ultrabasic igneous rocks and in the metamorphic and sedimentary rocks that are produced when chromite-bearing rocks are altered by heat or weathering. Chromite is important because it is the only economic ore of chromium, an essential element for a wide variety of metal, chemical, and manufactured products. Many other minerals contain chromium, but none of them are found in deposits that can be economically mined to produce chromium. Physical Properties of Chromite Chemical Classification

Oxide

Color

Dark gray to black, rarely brownish black

Streak

Dark brown

Luster

Metallic to submetallic

Diaphaneity Cleavage

Opaque None

Mohs Hardness

5.5 to 6

Specific Gravity

4.0 to 5.1 (variable)

Diagnostic Properties

Luster, streak

Chemical Composition FeCr2O4 with magnesium substituting for iron in significant amounts Crystal System Uses

Isometric An ore of chromium

Properties of Chromite Chromite can be challenging to identify. Several properties must be considered to differentiate it from other metallic ores. Hand specimen identification of chromite requires a consideration of: color, specific gravity, luster, and a characteristic brown streak. The most important clue to identifying chromite is its association with ultrabasic igneous rocks and metamorphic rocks such as serpentinite. Chromite is sometimes slightly magnetic. This can cause it to be confused with magnetite. Chromite and ilmenitehave very similar properties. Careful observations of hardness, streak, and specific gravity are required to distinguish these minerals in hand specimens.

Chromite and Solid Solution

Did You Know? The color of many gemstones is derived from trace amounts of chromium. The red color of rubies, the pink of some sapphires, and the green color of emeralds are derived from chromium. Image © iStockphoto / ProArtWork.

Magnesium frequently substitutes for iron in chromite. A solid solution series exists between the mineral chromite (FeCr2O4) and the isomorphous mineral magnesiochromite (MgCr2O4). Intermediate specimens can be rich in iron ((Fe,Mg)Cr2O4) or magnesium ((Mg,Fe)Cr2O4). For convenience in communication, these minerals are often referred to collectively as "chromite." Some mineralogists give a generalized chemical composition of (Mg,Fe)(Cr,Al) 2O4 for chromite. This composition recognizes multiple solid solution paths between chromite and hercynite (FeAl2O4), spinel (MgAl2O4), magnesiochromite (MgCr2O4), magnetite (Fe3O4), and magnesioferrite (MgFe2O4). Because of the many different compositions in these solid solution series, geologists and metallurgists often consider "chromite" to be any member of the solid solution series that has a significant Cr2O3 content.

Bushveld stratiform chromite deposit: A field photo of the Bushveld LG6 chromite seam. This clearly shows the stratiform nature of the deposit. USGS photo by Klaus Schulz.

Stratiform, Podiform, and Beach Sands Small amounts of chromite are found in many types of rock. However, chromite deposits that are large enough for mining are generally found in: 1) stratiform deposits (large masses of igneous rock such as norite or peridotite that slowly crystallized from subsurface magma); 2) podiform deposits (serpentines and other metamorphic rocks derived from the alteration of norite and peridotite); and, 3) beach sands (derived from the weathering of chromite-bearing rocks).

Chromite from South Africa: Chromite from the Transvaal area of South Africa. This specimen is approximately 3.5 inches (9 centimeters) across.

STRATIFORM DEPOSITS Stratiform deposits are large masses of igneous rock that cooled very slowly in subsurface magma chambers. During this slow cooling, chromite and associated minerals crystallized early while the magma was still at a

very high temperature. Their crystals then settled to the bottom of the magma chamber to form a layered deposit. Some of the layers in these deposits can contain 50% or more chromite on the basis of weight. Most of the world's known chromite occurs in two stratiform deposits: the Bushveld Complex in South Africa and the Great Dyke in Zimbabwe. Other important stratiform deposits include: the Stillwater Complex in Montana, the Kemi Complex of Finland, the Orissa Complex of India, the Goias in Brazil, the Mashaba Complex of Zimbabwe and small deposits in Madagascar. Nearly all of these are Precambrian in age.

Chromite from Zimbabwe: Chromite from Shurugwi, Zimbabwe. Specimen is approximately 4 inches (10 centimeters) across.

PODIFORM DEPOSITS Podiform deposits are large slabs of oceanic lithosphere that have been thrust up onto a continental plate. These slabs of rock, also known as "ophiolites," can contain significant amounts of chromite. In these deposits the chromite is disseminated through the rock and not highly concentrated in easy-to-mine layers. Podiform deposits are known in Kazakhstan, Russia, the Philippines, Zimbabwe, Cyprus, and Greece. The first discoveries of podiform chromite deposits were made near Baltimore, Maryland in the early 1800s. These deposits supplied nearly all of the world's chromite until about 1850. These deposits were small and are no longer in production. BEACH SANDS Chromite is found in beach sands derived from the weathering of chromite-bearing rocks and laterite soils that developed over peridotite. Beach sand rich in chromite and other heavy minerals is sometimes mined, processed to remove heavy minerals, and returned to the environment. Two facts allow these chromite sands to occasionally contain economic deposits of chromite. First, chromite is one of the more weathering-resistant minerals of peridotite. That causes it to be concentrated in residual soils that form in the weathering zone above chromite-rich rocks. Second, chromite has a higher specific gravity than other minerals in peridotite. This causes it to be selectively transported and deposited by wave and current actions, concentrating it in certain locations at streams and beaches. These deposits are sometimes rich enough and large enough that they can be mined for chromite.

Uses of Chromite and Chromium

Did You Know? School buses and yellow lines on highways are often painted with "chrome yellow" paint. The "chrome" means chromium was used as an ingredient. Image © iStockphoto / 2windspa.

Chromium is a metal used to induce hardness, toughness, and chemical resistance in steel. The alloy produced is known as "stainless steel." When alloyed with iron and nickel, it produces an alloy known as "nichrome" which is resistant to high temperatures and used to make heating units, ovens, and other appliances. Thin coatings of chromium alloys are used as platings on auto parts, appliances, and other products. These are given the name "chrome plated." It is also used to make superalloys that can perform well in the hot, corrosive, and high-stress environment of jet engines. Chromium's name comes from the Greek word "chroma" which means "color." Chromium is used as a pigment in paint. The familiar yellow lines painted down the center of highways and the yellow paint used on school buses are often "chrome yellow" - a color produced from chromium pigment. Chromium is an important pigment in many types of paint, ink, dye, and cosmetics. Trace amounts of chromium produce the color in many minerals and gemstones. The red color of ruby, the pink of some sapphires, and the green color of emerald are caused by tiny amounts of chromium. Chromite Production and Reserves Country

2011 Mine Production

2012 Mine Production (estimated)

Reserves

India

3,850

3,800

54,000

Kazakhstan

3,800

3,800

210,000

South Africa

10,200

11,000

200,000

5,450

5,300

NA

0

0

620

Other Countries United States

The values above are estimated chromite production and reserves in thousands of metric tons. Data from USGS Mineral Commodity Summaries. [2]

Chromium Production and Recycling in the United States Chromium is not mined in the United States. The chromium consumed by United States industry comes from: A) other countries in the form of chromite ore, ferrochromium or chromium metal; or, B) chromium recovered from recycled metals. Over half of the chromium used in the United States today is from recycling.

Because chromium is essential for the defense and prosperity of the United States, the federal government maintains a stockpile of chromite ore, ferrochromium, and chromium metal for use in a national emergency. This type of emergency could occur if the United States was involved in a war and the enemy prevented the delivery of chromite and chromium products by sea transport. In addition, small chromite deposits have been located in the United States which could be mined if they are needed. Chromite Information [1] Stratiform Chromite Deposit Model: Ruth F. Schulte, Ryan D. Taylor, Nadine M. Piatak, and Robert R. Seal II; Chapter E of Mineral Deposit Model for Resource Assessment; Scientific Investigations Report 2010-5070-E; 131 pages; November 2012. [2] Chromium: John F. Papp, United States Geological Survey, Mineral Commodity Summaries, January 2013. [3] Chromium: John F. Papp, United States Geological Survey, 2011 Minerals Yearbook, April 2013. [4] Chromium Makes Stainless Steel Stainless: S. J. Kropschot and Jeff Doebrich, United States Geological Survey, Fact Sheet 2010-3089, September 2010. [5] How a Rogue Geologist Discovered a Diamond Trove in the Canadian Arctic: Carl Hoffman, Wired Magazine, Issue 16.12, last accessed June 2016.

Chromite and Diamond Exploration Kimberlite, the type of rock that holds many of the world's most important diamond deposits, usually contains small amounts of chromite, ilmenite, and certain types of garnet. Although these minerals occur in very small amounts, they are much more common in the rock than diamonds. Because these minerals do not occur together in most other types of rocks, they can be a valuable indicator of a nearby kimberlite body if they are found in stream sediments, glacial tills, residual soils, core samples, or well cuttings. Some of the greatest diamond deposits on Earth were discovered using the geology of indicator minerals. Contributor: Hobart King

Chrysoberyl An extreme gem: The third-hardest gem mineral. The gem with the finest "cat's-eye." The color-change mineral alexandrite.

Chrysoberyl: Three faceted chrysoberyls showing a range of yellow and yellow-green color. These stones were produced in Sri Lanka and are about 4.3 millimeters in diameter and weigh about 0.52 carats each - a very high weight for stones of this size, caused by chrysoberyl's high specific gravity.

What Is Chrysoberyl? Chrysoberyl is a beryllium-aluminum oxide mineral with a chemical composition of BeAl2O4. It is distinctly different from the beryllium-aluminum silicate (Be3Al2(SiO3)6 mineral known as "beryl," although the similar names can cause confusion. Chrysoberyl is not found in deposits that are large enough to allow it to be used as an ore of beryllium. Its only important use is as a gemstone; however, it excels in that use because of its very high hardness and its special properties of chatoyance and color change.

The Diverse Gems of Chrysoberyl Chrysoberyl is best known for its use as a gem. There are multiple varieties of gem chrysoberyl, each with its own name and unique physical properties. Ordinary chrysoberyl is a yellow to yellow-green to green gemstone with a translucent to transparent diaphaneity. Transparent specimens are usually cut into faceted stones. Specimens that are translucent or with silk are usually cut into cabochons. A photo of ordinary chrysoberyl is shown at the top of this page.

Cat's-Eye Chrysoberyl: Chrysoberyl that contains large numbers of fibrous inclusions can produce a "cat's-eye," a line of light across the surface of the stone that orients perpendicular to the included fibers. Chrysoberyl is the gem that exhibits the finest cat's-eyes, and when the term "cat's-eye" is used without a mineral name as a modifier, the speaker is most likely referring to chrysoberyl. This specimen exhibits the "milk-and-honey" effect - when properly oriented, the stone has two apparently different colors on each side of the cat's-eye line. This green cat's-eye chrysoberyl was produced in Sri Lanka and is about 5.6 x 4 millimeters in size.

Cat's-Eye Chrysoberyl is the gemstone that produces the most distinct "cat's-eye," or chatoyance. If a person uses the name "cat's-eye" without the name of another gemstone (for example, "cat's-eye tourmaline"), then he is most likely referring to chatoyant chrysoberyl. Cat's-eye chrysoberyl has also been called "cymophane." The phenomenon of cat's-eye occurs in cabochon-cut stones that contain a high density of parallel fibrous inclusions. The "cat's-eye" is a line of light that reflects from the dome of the cabochon at right angles to the parallel inclusions. The line of light is very similar to how a spool of silk thread will produce a line of reflection across the top of the spool as it is moved back and forth under a source of light. Some specimens of cat's-eye will appear to have a different color on each side of the cat's-eye line when illuminated from the proper direction with respect to the observer's eye. It gives the illusion that the stone is made of two different materials, a light material on one side of the line and a dark material on the other. This phenomenon is known as the "milk-and-honey" effect. A photo of cat's-eye chrysoberyl showing the milkand-honey effect is shown on this page.

Alexandrite: A faceted specimen of color-change alexandrite of 26.75 carats from Tanzania, showing a blue-green color in daylight and a purple-red color under incandescent light. Photographed by David Weinberg for Alexandrite.net and published here under a GNU Free Document License.

Alexandrite Alexandrite is the color-change variety of chrysoberyl. The most distinctive specimens appear to have a green to blue-green color in daylight but change to a red to purplish-red color under incandescent light. Specimens with strong and distinct color-change properties are rare, highly desirable, and sell at very high prices. Stones over five carats are especially rare. A photo pair of an alexandrite gem in daylight and incandescent light is shown on this page. The change in color is thought to occur only in specimens where chromium substitutes for aluminum in the mineral's atomic structure. The chrysoberyl in which this phenomenon was first observed was named "alexandrite" after Tsar Alexander II of Russia. Since then the "alexandrite effect" has been observed in other gems, which include color-change garnet, spinel, tourmaline, sapphire, and fluorite. Alexandrite is a rare material, only found in very small deposits. It was first discovered in the Ural Mountains of Russia in the late 1800s. Although that deposit has been mined out, small deposits have since been discovered in Brazil, India, Sri Lanka, Myanmar, China, Zimbabwe, Tanzania, Madagascar, Tasmania, and the United States. Alexandrite can also be strongly pleochroic (a stone that has a different apparent hue when viewed from different directions). It is a trichroic stone (exhibiting three different hues from three different directions) with a green, red, or yellow-orange hue depending upon the direction of observation. The pleochroism of chrysoberyl is not apparent in all specimens and varies under different types of light. It is not as distinctive as the color-change effect. Physical Properties of Chrysoberyl Chemical Classification

Oxide

Color

Usually ranges from brown to pale yellow, yellow-green and green

Streak

Colorless

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent Poor, prismatic

Mohs Hardness

8.5

Specific Gravity

3.7 to 3.8

Diagnostic Properties Chemical Composition Crystal System

Hardness, color

Beryllium aluminum oxide, BeAl2O4

Orthorhombic

Uses

As a gemstone: chrysoberyl when transparent, "cat's-eye" when chatoyant, and "alexandrite" in specimens that exhibit color change.

Physical Properties of Chrysoberyl One of the most distinctive properties of chrysoberyl is its exceptional hardness. With a Mohs hardness of 8.5, it is the third-hardest gemstone and the third-hardest mineral that is even occasionally found at Earth's surface. Although chrysoberyl is extremely hard, it does break with distinct cleavage in one direction and indistinctly or poorly in two others. It also has a brittle tenacity. Most specimens of chrysoberyl are nearly colorless or fall into the brown to yellow to green color range. Red specimens are occasionally found. Chrysoberyl often occurs in tabular or prismatic crystals with distinct striations (see photo below). It also occurs in twinned crystals with distinct star and rosette shapes. These crystals usually persist well and retain their shape during stream transport because of the mineral's exceptional hardness. This makes them easy to identify in gem gravels, but the twinning often interferes with their usefulness as gems.

Chrysoberyl crystal: A beautiful chrysoberyl twinned crystal from Minas Gerais, Brazil. Photo by Yaiba Sakaguchi, used here under public domain.

Geologic Occurrence As a beryllium mineral, chrysoberyl only forms under those conditions where large amounts of beryllium are present. This limits its abundance and geographic distribution. High concentrations of mobile beryllium most often occur on the margins of magma bodies during the final stages of their crystallization. Thus, chrysoberyl usually forms in pegmatites and in metamorphic rocksassociated with pegmatites. These include mica schistsand dolomitic marbles. Chrysoberyl is also found along with other gem minerals in placer deposits. It is a hard, weathering-resistant mineral with a high specific gravity. These properties allow it to survive in sediments after other minerals have been destroyed by abrasion and chemical weathering. Contributor: Hobart King

Cinnabar A toxic mercury sulfide mineral. The primary ore of mercury, once used as a pigment.

Cinnabar: Massive cinnabar showing its characteristic red color and dull luster. Some contamination by clay. Photograph by H. Zell, used here under a GNU Free Documentation License.

What is Cinnabar? Cinnabar is a toxic mercury sulfide mineral with a chemical composition of HgS. It is the only important ore of mercury. It has a bright red color that has caused people to use it as a pigment and carve it into jewelry and ornaments for thousands of years in many parts of the world. Because it is toxic, its pigment and jewelry uses have almost been discontinued.

Cinnabar in sediment porosity: Cinnabar sometimes precipitates from fluids moving through the porosity of a sediment or a sedimentary rock. In those cases it can infill the pore spaces as a weak "cement." Photo © iStockphoto, only_fabrizio.

Geologic Occurrence of Cinnabar Cinnabar is a hydrothermal mineral that precipitates from ascending hot waters and vapors as they move through fractured rocks. It forms at shallow depths where temperatures are less than about 200 degrees Celsius. It usually forms in rocks surrounding geologically recent volcanic activity but can also form near hot springs and fumaroles. Cinnabar precipitates as coatings on rock surfaces and as fracture fillings. Less often, cinnabar can be deposited in the pore spaces of sediments. It is usually massive in habit and is rarely found as well-formed crystals. Other sulfide minerals are generally found associated with cinnabar. These can include pyrite, marcasite, realgar, and stibnite. Gangue minerals associated with cinnabar include quartz, dolomite, calcite, and barite. Small droplets of liquid mercury are sometimes present on or near cinnabar.

Cinnabar crystals: Bright red cinnabar crystals on a dolomite matrix. Crystals are about 1.3 centimeters in height, from Hunan, China. Specimen and photo by Arkenstone / www.iRocks.com.

Properties of Cinnabar The most striking property of cinnabar is its red color. Its bright color makes it easy to spot in the field and is a fascination for those who discover it. It has a Mohs hardness of 2 to 2.5 and is very easily ground into a very fine powder. It has a specific gravity of 8.1, which is extremely high for a nonmetallic mineral. The luster of cinnabar ranges from dull to adamantine. Specimens with a dull luster are usually massive, contain abundant impurities and do not have the brilliant red color of pure cinnabar. Adamantine specimens are usually the rarely-found crystals.

Physical Properties of Cinnabar Chemical

Sulfide

Classification Color

Bright red to brownish red, sometimes gray

Streak

Red

Luster

Adamantine to dull

Diaphaneity

Transparent, translucent, or opaque

Cleavage

Perfect, prismatic

Mohs Hardness

2 to 2.5

Specific Gravity

8 to 8.2

Diagnostic Properties

Specific gravity, color, streak, cleavage, association with volcanic activity.

Chemical

Mercury sulfide, HgS

Composition Crystal System

Uses

Trigonal The only important ore of mercury. Its use as a pigment, gem, and ornamental carving material has declined due to toxicity.

Metacinnabar: Crystals of metacinnabar on a rock surface. Specimen is from the Mount Diablo mine, Contra Costa County, California. Specimen is about 3.3 x 2.1 x 2.0 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Metacinnabar Metacinnabar is a polymorph of cinnabar. It has the same chemical composition (HgS) as cinnabar but a different crystal structure. Cinnabar is trigonal, while metacinnabar is isometric. The two minerals should not be confused with one another because metacinnabar has a metallic gray color, a gray-to-black streak and a metallic-to-submetallic luster.

Chinese red (cinnabar) lacquer box: A carved wooden box with a red lacquer finish from China's Ming Dynasty Period (box c. 1522-1566). Boxes like this were frequently painted with a lacquer containing a cinnabar pigment.

Mercury still: Textbook sketch of a still used for the distillation of mercury from cinnabar. Public domain image from Alchimia, Anonymous, 1570.

Uses of Cinnabar Cinnabar is the only important ore of mercury. For thousands of years, cinnabar has been mined and heated in a furnace. The mercury escapes as a vapor that can be condensed into liquid mercury. People began using cinnabar for pigments thousands of years ago in Italy, Greece, Spain, China, Turkey, and the Mayan countries of South America. Through time, people in almost every country where volcanoes are present discovered cinnabar and realized its utility as a pigment. Cinnabar is one of a very small number of minerals that was independently discovered, processed and utilized by ancient people in many parts of the world.

Cinnabar was mined at the volcano, ground into a very fine powder and then mixed with liquids to produce many types of paint. The bright red pigments known as "vermilion" and "Chinese red" were originally made from cinnabar. Cinnabar has been especially popular for making red lacquer in China. Its use in lacquer has declined because of its toxicity, but some use of cinnabar in lacquer continues. Cinnabar has also been used in powdered form for ritual blessings and burials. Powdered cinnabar was used as a cosmetic in many parts of the world for thousands of years. Eventually it was discovered that cinnabar is toxic, and its use in pigments, paints, and cosmetics began to decline. Today most, but not all, items made and sold under the name "cinnabar" have been made with less toxic and nontoxic imitation materials. Antique items made with toxic mineral cinnabar are still found in the marketplace.

Mercury switch: Mercury has the ability to conduct electricity and flow under the influence of gravity. This switch is currently in the "off" position, but if it is moved so that the mercury runs to the right, surrounding the two wires, the circuit will be connected and the switch will be in the "on" position. Photo by Medvedev, used here under a Creative Commons license.

Uses of Mercury Because cinnabar is the only important ore of mercury, the demand for mercury has driven mining activity. Mercury has many uses, but its toxicity has reduced its use in any application where reasonable substitutes can be found. Large amounts of mercury are currently used in the chemical industry in the production of chlorine and caustic soda during the electrolysis of brine. Mercury was widely used in temperature- and pressure-measuring instruments such as thermometers and barometers. It was often used in gravity switches because it flowed easily as a liquid and conducted electricity. Most of these uses have been discontinued. Mercury is currently used in some batteries and light bulbs, but their disposal is often regulated. Because it is toxic, it was once widely used to protect seed corn from fungus and to deworm materials used to make felt. It was used in dental amalgam but is being replaced by polymer resins and other materials. In almost all of its use, mercury is being replaced with less toxic and nontoxic substitutes. Mercury has been widely used in mining to separate goldand silver from ores and stream sediments. Large amounts of mercury were spilled during these operations, and today, mercury used in the 1800s is still being recovered from streams. Contributor: Hobart King

Zoisite and Clinozoisite Two very similar minerals with the same chemical composition but different crystal structures

Blue zoisite - Tanzanite: Tanzanite is the most widely known zoisite and one of the world's most popular gemstones. This violetish blue tanzanite is an exceptional faceted oval weighing 8.14 carats and measuring 14.4 x 10.5 x 7.6 millimeters in size. On the basis of its color and clarity, it would be rated in the top 1% of the tanzanite produced by TanzaniteOne Mining Ltd., the leading producer of tanzanite. Photo copyright by Richland Gemstones and used here with permission.

What are Zoisite and Clinozoisite? Zoisite and clinozoisite are minerals that form during the regional metamorphism and hydrothermal alteration ofigneous, metamorphic, and sedimentary rocks. In those environments they are found in massive form and as prismatic crystals in veins that cut schists and marbles. They are also found as crystals in pegmatites that form on the margins of igneous bodies. The two minerals are dimorphs - they share the same chemical composition but have a different crystal structure. Zoisite is the orthorhombic form of Ca2Al3(SiO4)(Si2O7)O(OH) and clinozoisite is the monoclinic form. The minerals have extremely similar physical properties and can be very difficult to tell apart in hand specimens unless the specimens are well-formed crystals. Clinozoisite forms a solid solution series with the mineral epidote in which iron can substitute for aluminum.

Zoisite: Shown above are 4 specimens of zoisite in unusual colors. Top row: pink and yellow crystals with orthorhombic crystal habit. Bottom row: (left) a parti-colored specimen with shades of green and pink in the same crystal; (right) a blue-green crystal with nice termination. Specimens and images copyright by Lapigems.

Uses of Zoisite and Clinozoisite Zoisite and clinozoisite are minerals that are usually found in small quantities. They have not been used in significant amounts by industry. Transparent and colorful specimens of both minerals have been used as gemstones. Zoisite is the mineral of some very diverse gem materials, one being the extremely popular tanzanite which was discovered in the 1960s and immediately became one of the world's most popular gems.

Thulite is a pink, opaque variety of zoisite that is often cut into cabochons or used to produce small sculptures. It can be an attractive material but is rarely seen in commercial use because the supply is limited and the public is unfamiliar with the gem.

Ruby in Zoisite: Anyolite, also known as "ruby in zoisite," is a rock composed of zoisite, with red corundum crystals (ruby) and often accented by black crystals of the hornblende, tschermakite. It is a rock that attracts attention and is cut into attractive cabochons and used to produce small sculptures. Image © iStockphoto / MarcelC.

Tanzanite Tanzanite is the most famous zoisite. It is a transparent blue zoisite that is colored by the presence of vanadium. Some blue zoisite is found naturally, but most is produced by heat-treating brown zoisite. The heat changes the oxidation state of vanadium to produce the blue color. Tanzanite is the second most popular blue stone, after sapphire. It is a rare gem only found in one small area in northern Tanzania. Thulite Thulite is an opaque pink variety of zoisite that is cut into cabochons and used to produce small sculptures. It is also a rare material, found in Norway, Namibia, Australia, North Carolina, and a few other locations. It is rarely seen in commercial use. Anyolite Anyolite is a very colorful rock composed mainly of zoisite. It is also known as "ruby in zoisite" because it is composed of green zoisite with bright red ruby crystals, sometimes accompanied by black crystals of the hornblendetschermakite. It is used to produce cabochons, tumbled stones, small sculptures and ornamental objects. Nice pieces of rough material are also sold as specimens. A material with a similar appearance, "ruby in fuchsite" is often misidentified as ruby in zoisite. Careful testing can easily differentiate these materials because the green fuchsite has a hardness of only 2 to 3, while the green zoisite has a hardness of at least 6. In addition, most specimens of ruby in fuchsite exhibit blue kyanite alteration rims around the ruby crystals, and this does not occur around ruby crystals in zoisite. Clinozoisite Gem-quality crystals of clinozoisite are sometimes cut into faceted stones. It is considered to be a "collectors" stone because it is rarely seen in jewelry.

Physical Properties of Zoisite and Clinozoisite Zoisite

Clinozoisite

Silicate

Silicate

Color

Colorless, gray, yellow, brown, pink, green, blue, and violet

Usually gray, yellow, green, or pink

Streak

White

White

Luster

Vitreous to granular, sugary

Vitreous to granular, sugary

Diaphaneity

Translucent to transparent

Translucent to transparent

Perfect in one direction

Perfect in one direction

Mohs Hardness

6.5

6.5

Specific Gravity

3.2 to 3.4

3.2 to 3.4

Diagnostic Properties

Hardness, specific gravity, striated crystals

Hardness, specific gravity, striated crystals

Chemical Composition

Ca2Al3(SiO4)(Si2O7)O(OH)

Ca2Al3(SiO4)(Si2O7)O(OH)

Orthorhombic

Monoclinic

Gemstones (tanzanite, anyolite, and thulite) and small sculptures

Gemstones

Chemical Classification

Cleavage

Crystal System Uses

Clinozoisite: Two views of the same crystal of clinozoisite from the Haramosh Mountains of Pakistan. The specimen is about 3.2 centimeters tall. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Zoisite and Clinozoisite Zoisite and clinozoisite have the same chemical composition. This gives them very similar physical properties, as shown in the accompanying table. The difference between the two minerals is in their crystal structure. Zoisite is a member of the orthorhombic crystal system, and clinozoisite is monoclinic. They are difficult to tell apart in hand specimen unless well-formed crystals are present. Optical tests and x-ray diffraction are the best ways to make positive identification.

Copper Mineral Properties and Uses

Copper: Copper from Bisbee, Arizona. This specimen is approximately 2.5 inches (6.4 centimeters) across.

What is Copper? Native copper is an element and a mineral. It is found in the oxidized zones of copper deposits; in hydrothermal veins; in the cavities of basalt that have been in contact with hydrothermal solutions; and as pore fillings and replacements in conglomerates that have been in contact with hydrothermal solutions. It is rarely found in large quantities, thus it is seldom the primary target of a mining operation. Most copper produced is extracted from sulfide deposits.

Uses of Copper Native copper was probably one of the early metals worked by ancient people. Nuggets of the metal could be found in streams in a few areas, and its properties allowed it to be easily worked without a required processing step. Today most copper is produced from sulfide ores. Copper is an excellent conductor of electricity. Most copper mined today is used to conduct electricity - mostly as wiring. It is also an excellent conductor of heat and is used in cooking utensils, heat sinks, and heat exchangers. Large amounts are also used to make alloys such as brass (copper and zinc) and bronze (copper, tin, and zinc). Copper is also alloyed with precious metals such as gold and silver. Copper has many other uses. Physical Properties of Copper Chemical Classification

Native element

Color

Copper red on a fresh surface, dull brown on a tarnished surface

Streak

Metallic copper red

Luster

Metallic

Diaphaneity Cleavage

Opaque None

Mohs Hardness

2.5 to 3

Specific Gravity

8.9

Diagnostic Properties

Color, luster, specific gravity, malleability, ductility

Chemical Composition

Copper, Cu

Crystal System Uses

Isometric Conducts electricity and heat; wiring, electrical contacts and circuits; coinage, alloys

Cordierite and the Gem Known as "Iolite" Cordierite is the mineral known as "Iolite" when it is of gem quality.

Iolite: A blue-violet iolite faceted from material mined in Madagascar. This specimen is approximately 9.4 x 7.1 x 4.8 millimeters in size and weighs about 1.83 carats. A nice iolite like this one could easily serve as an alternative gem for sapphire or tanzanite at a much lower price.

Cordierite crystals: A cluster of cordierite crystals from the Richmond Soapstone Quarry in Cheshire County, New Hampshire. The crystals are short and prismatic with a square cross-section. The cluster is about 19 centimeters tall. Specimen and photo by Arkenstone / www.iRocks.com.

What is Cordierite? Cordierite is a silicate mineral that is found in metamorphic and igneous rocks. It is typically blue to violet in color and is one of the most strongly pleochroic minerals. Cordierite has a chemical composition of (Mg,Fe)2Al4Si5O18 and forms a solid solution series with sekaninaite, which has a chemical composition of (Fe,Mg)2Al4Si5O18.

"Cordierite" is a name used by geologists. When the mineral is transparent and of gem quality, it is known as "iolite" in the gem and jewelry trade. Two older names for the mineral are "dichroite" and "water sapphire." The name dichroite means "two color rock," inspired by cordierite's pleochroic property. The name water sapphire is also related to pleochroism. It was used because a specimen could have the color of a sapphire when viewed from one direction, but if the stone was rotated it could appear to be as clear as water.

Geologic Occurrence of Cordierite Most cordierite forms during regional metamorphism of shales and other argillaceous rocks. When formed under these conditions, it is found in schist and gneiss. Less often, it forms during contact metamorphism and is found in hornfels. Cordierite is also found as an accessory mineral in granitic igneous rocks and in pegmatites. When crystals of cordierite have the opportunity to grow without obstructions, they can form short prismatic crystals with a rectangular cross-section. In metamorphic rocks, cordierite is often found associated with sillimanite, kyanite, andalusite, and spinel. Most gem-quality iolite is produced from placer deposits, where it occurs in association with other gems even though its specific gravity is not high enough to cause a concentration. When exposed to weathering, cordierite alters to mica and chlorite.

Cordierite pleochroism: One piece of cordierite from the Tulear Province of Madagascar, viewed from two different angles that display its pleochroism. The top image shows the specimen from its angle of maximum violet color. The bottom image shows the same specimen rotated by an angle of 90 degrees to show a yellowish color. This specimen is about 4 centimeters in length. Photos by John Sobolewski, displayed here under a Creative Commons license.

Physical Properties of Cordierite Chemical Classification

Silicate

Color

Strongly pleochroic. Most specimens appear blue to violet in color but can be clear, gray, or yellow from other directions.

Streak

Colorless

Luster

Vitreous, greasy

Diaphaneity

Transparent to translucent

Cleavage

Fair to poor

Mohs Hardness

7 to 7.5

Specific Gravity

2.5 to 2.8

Diagnostic Properties

Blue to violet color, strong pleochroism, visually similar to quartz

Chemical Composition

(Mg,Fe)2Al4Si5O18

Crystal System Uses

Orthorhombic Very few uses; used rarely in ceramics. Transparent specimens are sometimes used as gems.

Industrial Uses of Cordierite Cordierite is a mineral with very few industrial uses. It can be used as an ingredient for making ceramic parts used in catalytic converters. However, synthetic cordierite is used instead because its supply is reliable and its properties are consistent. Many other natural materials are losing their place in industry to synthetic materials for these reasons. Pleochroism in iolite: This video demonstrates pleochroism in iolite. Pleochroic materials appear to be different colors when observed from different directions. In this video we watch a rotating piece of iolite change colors between blue and clear with every 90 degrees of rotation. The color of the specimen depends upon the angle of observation. People who facet iolite must study the stone and determine its direction of best color. Then the stone is cut with its table at right angles to the direction of best color observation. That will produce a finished gem that exhibits its best color when viewed in the face-up position.

Known as "Iolite" by Jewelers When transparent and of high clarity, cordierite is used as a gemstone. It is known as "iolite" in the gem and jewelry industry. Iolite is a blue pleochroic gem that has an appearance similar to sapphire and tanzanite. It can serve as an alternative stone to either of these gems and is much lower in price. Unlike sapphire and tanzanite, iolite in the gem market is not known to receive heat, irradiation, or other treatments to improve its color. That is appealing to many people. Iolite is a challenging material to facet because of its extreme pleochroism. The cutter must examine the stone carefully and have its axis of top-quality color oriented perpendicular to the plane of the gem's table. A gem of good color can only be obtained if these cutting rules are followed. Faceted iolite gems weighing more than five carats are rare. Most stones are two carats or smaller. These small stones often have the best color because iolite often has a dark tone.

Iolite has a Mohs hardness of 7 to 7 1/2, which is durable enough for many gem uses. Its main physical disadvantage is its distinct cleavage in one direction. This makes it vulnerable to breakage when used in rings or other items that could encounter rough use. Iolite is almost never seen in mass-merchant jewelry. It is a gem that is unknown to the average consumer because it is not being marketed. Jewelers do not order it or market it because they are not confident that an abundant supply of quality material will be available to support them. This is surprising because significant iolite resources exist in many countries. Its value in the gem trade has not been developed and thus its price is low.

Cordierite crystals in matrix: A photo of cordierite crystals in their rock matrix from Minas Gerais, Brazil. Photo by Parent Géry, used here under a Creative Commons license.

Pleochroism in Cordierite (Iolite) Pleochroic materials appear to be different colors when viewed from different directions. When viewed from the direction that produces its most attractive color, most cordierite is a distinct blue to violet in color. It is one of the most strongly pleochroic minerals. Specimens that produce a strong violet color can be rotated to produce light violet or dark yellow hues. Specimens that produce a strong blue color can be rotated to produce yellow or colorless hues. People who facet iolite must study the stone to determine its direction of best color. Then they must facet the stone so that the direction of best color observation is at right angles to the table of the stone. That will produce the best possible color in the finished gem. See the video on this page for a demonstration of pleochroism in iolite. Contributor: Hobart King

Corundum Corundum has historically been used as an abrasive, but it is most famous as the mineral of ruby and sapphire.

Corundum: Two corundum crystal segments from India showing the mineral's hexagonal crystal habit and basal parting. These specimens are red in color and might be called "ruby corundum." Image © iStockphoto / Lissart.

What is Corundum? Corundum is a rock-forming mineral that is found in igneous, metamorphic, and sedimentary rocks. It is an aluminum oxide with a chemical composition of Al2O3 and a hexagonal crystal structure. The mineral is widely known for its extreme hardness and for the fact that it is sometimes found as beautiful transparent crystals in many different colors. The extreme hardness makes corundum an excellent abrasive, and when that hardness is found in beautiful crystals, you have the perfect material for cutting gemstones. Natural and synthetic corundum are used in a wide variety of industrial applications because of their toughness, hardness, and chemical stability. They are used to make industrial bearings, scratch-resistant windows for electronic instruments, wafers for circuit boards, and many other products.

Corundum crystals: Photos of three corundum crystals. On the left is a common corundum from Transvaal, South Africa, that is about 6 centimeters in height. In the center is a gem-quality ruby corundum from Karnataka, India, that is about 1.6 centimeters in

height. On the right is a blue sapphire corundum from Sri Lanka that is about two centimeters in height. All three specimens and photos by Arkenstone / www.iRocks.com.

Made Famous by Rubies and Sapphires Most people are familiar with corundum; however, very few people know it by its mineral name - instead they know it by the names "ruby" and "sapphire." A gemstone-quality specimen of corundum with a deep red color is known as a "ruby." A gemstone-quality corundum with a blue color is called a "sapphire." Colorless corundum is known as "white sapphire." Corundum of any other color is known as "fancy sapphire."

Corundum parting: Hexagonal crystal segments of corundum that have been separated by parting. These specimens are about one centimeter across. USGS photo by Andrew Silver.

Corundum gneiss with sapphire: A specimen of corundum gneiss from Gallatin Valley, Montana. This specimen is about twelve centimeters across and has a round blue sapphire crystal on the left side.

Properties of Corundum Corundum is an exceptionally hard and tough material. It is the third-hardest mineral, after diamond and moissanite. It serves as the index mineral for a hardness of nine on the Mohs Hardness Scale. Its hardness, high specific gravity, hexagonal crystals and parting are very good diagnostic properties to use in its identification. A summary of the physical properties of corundum is given in the table below. Physical Properties of Corundum Chemical

Oxide

Classification

Color

Typically gray to brown. Colorless when pure, but trace amounts of various metals produce almost any color. Chromium produces red (ruby) and combinations of iron and titanium produce blue (sapphire).

Streak

Colorless (harder than the streak plate)

Luster

Adamantine to vitreous

Diaphaneity Cleavage

Transparent to translucent None. Corundum does display parting perpendicular to the c-axis.

Mohs Hardness

9

Specific Gravity

3.9 to 4.1 (very high for a nonmetallic mineral)

Diagnostic Properties

Hardness, high specific gravity, hexagonal crystals sometimes tapering to a pyramid, parting, luster, conchoidal fracture

Chemical

Al2O3

Composition Crystal System Uses

Hexagonal Historically used as an abrasive. Specimens with pleasing colors have a long history of gemstone use.

Montana alluvial sapphires: A scatter of small alluvial sapphires found in Montana. These blue stones are untreated and measure about four to five millimeters across.

Geologic Occurrence of Corundum Corundum is found as a primary mineral in igneous rockssuch as syenite, nepheline syenite, and pegmatite. Some of the world's most important ruby and sapphire deposits are found where the gems have weathered from basaltflows and are now found in the downslope soils and sediments. Corundum is also found in metamorphic rocks in locations where aluminous shales or bauxites have been exposed to contact metamorphism. Schist, gneiss, and marbleproduced by regional metamorphism will sometimes contain corundum. Some of the sapphires and rubies of highest quality, color, and clarity are formed in marble along the edges of subsurface magma bodies. Corundum's toughness, high hardness, and chemical resistance enable it to persist in sediments long after other minerals have been destroyed. This is why it is often found concentrated in alluvial deposits. These deposits are the most important source of rubies and sapphires in several parts of the world. Traditional sources of alluvial rubies and sapphires include Burma, Cambodia, Sri Lanka, India, Afghanistan, Montana, and other areas. In the past few decades, several parts of Africa, including Madagascar, Kenya, Tanzania, Nigeria, and Malawi, have become important producers of ruby and sapphire.

Emery wheels: An ad offering emery and corundum wheels, published in 1895 by The Springfield Manufacturing Company of Bridgeport, Connecticut. This was at a time when genuine emery and corundum were used to make the wheels.

Hardness and Use as an Abrasive The extreme hardness of corundum makes it especially useful as an abrasive. Crushed corundum is processed to remove impurities and then screened to produce uniformly sized granules and powders. These are used for grinding media, polishing compounds, sand papers, grinding wheels, and other cutting applications. Some problems with using natural corundum as an abrasive are that the deposits are usually small, irregular in shape, and the corundum is of variable quality. They are not reliable sources of consistent-quality material needed to run a manufacturing process. Synthetic corundum, produced using calcined bauxite, has become a more reliable source with more consistent properties. It has replaced natural corundum in most manufactured products.

Aluminum oxide sandpaper is made by attaching size-graded particles of synthetic corundum (aluminum oxide) to a sheet of paper. It is a sandpaper widely used for woodworking and other manufacturing work. Photo © iStockphoto / Ma-Ke.

Emery rock: A specimen of emery rock that is rich in corundum and spinel from Peekskill, New York. This specimen is approximately six inches (fifteen centimeters) across. Emery has often been crushed, processed, and screened for use as an industrial abrasive.

Emery nail files: "Emery boards" are a manicure and nail-care product that is made by gluing abrasive papers to a thin piece of cardboard. They obtained their name in the 1800s when crushed emery was used as the abrasive. Today's emery boards are not made with emery. Instead, many of them have a coarse side of synthetic corundum (aluminum oxide) and a fine side of garnet abrasive. Photo © iStockphoto / Acerebel.

Emery Emery stone is a granular metamorphic or igneous rock that is rich in corundum. It is a mixture of oxide minerals, typically corundum, magnetite, spinel and/or hematite. It is the most common form of natural corundum that has been used to manufacture abrasives. The use of emery as an abrasive has declined significantly in the last several decades. It has been almost completely replaced by manufactured abrasives such as silicon carbide. Silicon carbide has a Mohs hardness of 9 to 9.5. It is inexpensive and usually performs better than natural abrasives made from corundum or emery.

Corundum as ruby, sapphire, and fancy sapphire: Gem-quality corundum is a highly prized and valuable material. When it is bright red in color it is called "ruby." When it is blue it is called "sapphire." When colorless it is called a "white sapphire." Gemquality corundum of any other color is called "fancy sapphire." In the past, most gem corundum was produced in Asia and Australia. In the 1990s, many gem corundum discoveries were made in Africa. All of the stones in this photo were mined in Africa. Nearly all gem corundums are treated by heating or another process to improve their color.

Use as a Gemstone In the gemstone and jewelry market, almost all of the attention goes to a small group of gems known as "the big four": diamond, ruby, sapphire, and emerald. Two of these, ruby and sapphire, are gem corundums. These most popular gems are highly sought after and have been mined in many parts of the world for thousands of years. Today, millions of rubies and sapphires are required every year to meet the demands of the jewelry market -- from inexpensive commercial stones sold in malls and department stores to spectacular specimens used in designer and custom jewelry. The demand for attractive stones exceeds the abilities of mines to supply. As a result, the prices paid for attractive natural stones have risen to high levels. When a consumer wants a "ruby ring" or a "sapphire pendant," they are generally not interested in substituting a red spinel, blue iolite, or other attractive gem of similar color. They want "ruby" or they want "sapphire." Retail jewelers, especially those selling pieces and sets for under $500, have been increasingly presenting synthetic or "lab-created" gems alongside the natural stones in their display cases.

The synthetic materials are genuine corundums. They have the same aluminum oxide composition and crystal structure as natural rubies and sapphires. Their color is also produced by the same trace elements (chromium for ruby and iron with titanium for sapphire). They have the same optical appeal and a better physical appearance than similar-size natural stones of the same price. As a result, many consumers now gladly purchase synthetic stones because they receive a more attractive product at a price that they can afford. Over the long term, synthetic gems are likely to continue displacing natural stones from the market, especially in the lower and middle price ranges where consumers are very conscious about price.

Corundum watch bearings: Corundum (ruby) bearings in an antique pocket watch with a "jewel" movement. In the early 1900's, synthetic corundum was being used as the jewel bearings in watches. Image © iStockphoto / RobertKacpura.

Corundum bearings: A drawing of jewel bearings and a capstone (red) holding a pivot wheel in a mechanical watch lubricated by oil (yellow). Public domain image by Chris Burks Chetvorno.

Corundum "Jewels" in Watches In the mid-1800s, watch makers in Switzerland needed tiny bearings that were highly resistant to abrasion. They discovered that they could drill a hole into a tiny piece of corundum and use it for a smooth-running, long-life bearing. The corundum was much harder than the metals used to make the moving parts of a watch, and it was able to stand up to the continuous abrasion without failing. This made Swiss watches and their "jewel movements" famous throughout the world for their long life and reliability. In the early 1900s, synthetic corundum bearings replaced natural corundum bearings in most Swiss watches. This use of jewel bearings

created a positive reputation for Swiss watches that continues to this day - even while mechanical watches are being replaced by digital watches.

Synthetic corundum: A boule of synthetic corundum. Because of its red color, it could be called "synthetic ruby." Material like this is used for watch bearings, gemstones, laser gain mediums, and many other purposes.

Ruby laser: Diagram of the first working laser. It employed a thin ruby crystal as its gain medium. Public domain image by Lawrence Livermore National Laboratory.

Ruby Lasers Synthetic corundum is an essential part of many lasers. In fact, the first working laser was a "ruby laser," made by Theodore Maiman at Hughes Research Labs in 1960. It employed a synthetic ruby crystal as the "gain medium." The gain medium is a material in the laser that is the target of an intense burst of light. That light causes electrons in the gain medium to jump up to a higher energy level causing the emission of photons, which strike other atoms in the gain medium, causing them to be excited and emit more photons. This brief chain reaction produces the very intense light of a laser beam. Lasers are named after the material used as a gain medium, such as "ruby laser" or "titanium sapphire laser" or "YAG laser" (yttrium aluminum garnet). In just a few decades, lasers have become common items of our society. Tiny lasers are used in CD and DVD players. Lasers are used to cut metal, stone, and other tough materials. Lasers are used to remove tattoos, perform cosmetic surgery, cataract surgery, and LASIK surgery for vision correction.

Synthetic corundum scanner windows: A self-check-out machine with a barcode scanner window at a retail store in Houston, Texas. The window of the scanner is probably made from synthetic corundum. Public domain image by WhisperToMe.

Other Uses of Corundum Corundum has many other uses. It is chemically inert and resistant to heat. These properties make it a perfect material for making refractory products such as fire brick, kiln liners, and kiln furniture. Today, these products are usually made with synthetic corundum. Pure corundum is colorless, transparent, durable, and scratch resistant. Large crystals of clear synthetic corundum are grown, sawn into thin sheets, and then used as the windows of grocery store scanners, watch crystals, aircraft windows, and protective covers for electronic devices. Contributor: Hobart King

Cuprite Mineral Properties and Uses

What is Cuprite? Cuprite is a supergene copper oxide mineral found in the oxidized zone of copper deposits. It is a minor ore of copper.

Physical Properties of Cuprite Chemical Classification

oxide

Color

various shades of red, sometimes nearly black

Streak

brownish red

Luster

submetallic to adamantine

Diaphaneity Cleavage

subtranslucent none

Mohs Hardness

3.5 to 4

Specific Gravity

5.8 to 6.1

Diagnostic Properties

color, streak, luster

Chemical Composition

copper oxide, Cu2O

Crystal System Uses

isometric ore of copper

Cuprite with chrysocolla from Butte, Montana. This specimen is approximately 3 inches (7.6 centimeters) across.

Diamond The most popular gemstone. The hardest known substance. An amazing number of uses.

Diamond crystal: A gem-quality diamond crystal in the rock in which it was formed. It is a single octahedron with strong growth lines on it surface and an estimated weight of about 1.5 carats. From the Udachnaya Mine, Yakutia, Siberia, Russia. Specimen and photo by Arkenstone / www.iRocks.com.

What is Diamond? Diamond is a rare, naturally-occurring mineral composed of carbon. Each carbon atom in a diamond is surrounded by four other carbon atoms and connected to them by strong covalent bonds. This simple, uniform, tightly-bonded arrangement yields one of the most durable substances known. Diamond is a fascinating mineral. It is chemically resistant and it is the hardest known natural substance. These properties make it suitable for use as a cutting tool and for other uses where durability is required. Diamond also has special optical properties such as a high index of refraction, high dispersion, and high luster. These properties help make diamond the world's most popular gemstone. Diamonds are a bit of a mystery. They are composed of the element carbon, and because of that many people believe that they must have formed from coal. Many teachers still teach this in their classrooms. But that is not true! Physical Properties of Diamond Chemical Classification

Native element

Most diamonds are brown or yellow in color. The jewelry industry has favored colorless diamonds or those that have a Color

color so subtle that it is difficult to notice. Diamonds in vivid hues of red, orange, green, blue, pink, purple, yellow and other hues are very rare and sell for high prices when the color is spectacular. Most industrial grade diamonds are brown, yellow, gray, green and black.

Streak

Diamond is harder than a streak plate. Its streak is known as "none" or "colorless"

Luster

Adamantine - the highest level of luster for a nonmetallic mineral.

Diaphaneity

Transparent, translucent, opaque.

Cleavage

Perfect, octahedral

Mohs Hardness

10 - the hardest mineral

Specific Gravity

3.4 to 3.6

Diagnostic

Hardness, heat conductivity, crystal form, index of refraction, dispersion

Properties Chemical

C (elemental carbon)

Composition Crystal System

Uses

Isometric Gemstones, industrial abrasives, diamond windows, speaker domes, heat sinks, low-friction microbearings, wear-resistant parts, dies for wire manufacturing.

Diamond Consumption in the United States In 2014, consumers in the United States spent about $24.3 billion on gemstones. Of that amount, $22.5 billion was spent on diamonds, and about $1.8 billion was spent on colored stones. These statistics clearly show that diamonds are the most popular gemstones with U.S. consumers by an enormous margin.

How Do Diamonds Form? Diamonds are not native to Earth's surface. Instead they form at high temperatures and pressures that occur in Earth's mantle about 100 miles down.

How do diamonds form? A detailed article that explains the four sources of diamonds found at Earth's surface.

Most of the diamonds that have been discovered were delivered to Earth's surface by deep-source volcanic eruptions. These eruptions begin in the mantle, and on their way up they tear out pieces of mantle rock and deliver them to Earth's surface without melting. These blocks from the mantle are known as xenoliths. They contain diamonds that were formed at the high temperature and pressure conditions of the mantle. People produce diamonds by mining the rock that contains the xenoliths or by mining the soils and sediments that formed as the diamond-bearing rock weathered away.

Some diamonds are thought to form in the high temperature-pressure conditions of subduction zones or asteroid impact sites. Some are delivered to Earth in meteorites. No commercial diamond mines have been developed in deposits with these origins.

Leading diamond producers: This chart shows the estimated annual production of gem-quality diamonds, in millions of carats, for the world's leading diamond-producing nations. Graph by Geology.com. Data from USGS Mineral Commodity Summaries. Learn about the countries that produce diamonds.

Gem Diamonds vs. Industrial Diamonds Gem diamonds are stones with color and clarity that make them suitable for jewelry or investment use. These stones are especially rare and make up a minor portion of worldwide diamond production. Gemstone diamonds are sold for their beauty and quality. Natural diamond crystals have a specific gravity that ranges between approximately 3.4 to 3.6. This range exists because most diamonds contain impurities and have irregularities in their crystal structure. Gem quality diamonds are the most perfect diamonds, with minimal impurities and defects. They have a specific gravity that is very close to 3.52. Industrial diamonds are mostly used in cutting, grinding, drilling, and polishing procedures. Here, hardness and heat conductivity characteristics are the qualities being purchased. Size and other measures of quality relevant to gemstones are not important. Industrial diamonds are often crushed to produce micron-sized abrasive powders. Large amounts of diamonds that are gemstone quality but too small to cut are sold into the industrial diamond trade.

A demonstration of dispersion: White light being separated into its component colors while passing through a prism. Diamonds have a high dispersion. NASA Image.

Diamond as a Gemstone Diamonds are the world's most popular gemstones. Many times more money is spent on diamonds than on all other gemstones combined. Part of the reason for diamond's popularity is a result of its optical properties - or how it reacts with light. Other factors include fashion, custom, and marketing. Diamonds have a very high luster. The high luster is a result of a diamond reflecting a high percentage of the light that strikes its surface and a high percentage of light that passes through the stone and is returned to the eye of the observer. Diamond also has a high dispersion. As white light passes through a diamond, this high dispersion causes that light to separate into its component colors. Dispersion is what enables a prism to separate white light into the colors of the spectrum. This property of dispersion is what gives diamonds their colorful "fire."

Diamond fire: A round brilliant cut diamond showing "fire." © iStockphoto / Greg Stanfield.

Argyle diamonds: Octahedral diamond crystals from the Argyle Mine of Western Australia. The Argyle Mine produces diamonds in a wide range of colors. Most Argyle diamonds are brown, some are near colorless and a very rare number are red, pink, blue or violet. It is the world's only consistent producer of pink diamonds. Image Copyright © 2016 Rio Tinto.

Cubic diamond crystal: A green diamond crystal. The color and cubic crystal shape are natural. Many natural diamond crystals are cubic or octahedral in shape. This diamond is about 4 millimeters across and is suitable for industrial use.

Diamond Gemstone Quality The quality of a diamond gemstone is primarily determined by four factors: color, cut, clarity, and carats. A standardized method of assessing diamond quality was developed in the 1950s by the Gemological Institute of America and is known as "The 4Cs of Diamond Quality" [5]. Color: Most gem-quality diamonds range from colorless to yellow. The most highly regarded stones are those that are completely colorless. These are the ones sold for the highest prices. However, another category of diamond gemstone is increasing in popularity. These are the "fancy" diamonds, which occur in a variety of colors including red, pink, yellow, purple, blue, and green. The value of these stones is based upon their color intensity, rarity, and popularity. Cut: The quality of workmanship in a diamond has a large impact upon its quality. This influences not only the geometric appearance of the stone but also the stone's luster and fire. Ideal stones are perfectly polished to be highly reflective and emit a maximum amount of fire. The faceted faces are equal in size and identical in shape. And, the edges of each faceted face meet perfectly with each of its neighbors. Clarity: The ideal diamond is free from internal flaws and inclusions (particles of foreign material within the stone). These detract from the appearance of the stone and interfere with the passage of light through the stone. When present in large numbers or sizes, they can also reduce the strength of the stone. Carat: Diamonds are sold by the carat (a unit of weight equal to 1/5th of a gram or 1/142nd of an ounce). Small diamonds cost less per carat than larger stones of equal quality. This is because very small stones are very common and large stones are especially rare.

Industrial diamonds: Small diamonds, less then 1 millimeter in size, that are suitable for industrial use as abrasive granules. Industrial diamonds might be polycrystalline, have numerous inclusions, have poor clarity, contain fractures, or have some other characteristic that disqualifies them from gem or technology uses.

Diamond drill bit: A drill bit used in the drilling of oil wells. Each of the cutting tips has small grains of diamond embedded in the metal. These cut their way through the rock as the bit turns. © iStockphoto / mikeuk.

Diamond concrete saw: A concrete saw with a diamond blade of about four feet diameter. US Air Force Image.

Diamonds Used as an Abrasive Because diamonds are very hard (ten on the Mohs scale) they are often used as an abrasive. Most industrial diamonds are used for these purposes. Small particles of diamond are embedded in a saw blade, a drill bit or a grinding wheel for the purpose of cutting, drilling or grinding. They might also be ground into a powder and made into a diamond paste that is used for polishing or for very fine grinding. There is a very large market for industrial diamonds. Demand for them exceeds the supply obtained through mining. Synthetic diamonds are being produced to meet this industrial demand. They can be produced at a low cost per carat and perform well in industrial use.

Other Uses of Diamonds Most industrial diamonds are used as abrasives. However, small amounts of diamond are used in other applications. Diamond windows

are made from thin diamond membranes and are used to cover openings in lasers, x-ray machines, and vacuum chambers. They are transparent, very durable, and resistant to heat and abrasion.

Diamond speaker domes

enhance the performance of high-quality speakers. Diamond is a very stiff material, and when made into a thin dome it can vibrate rapidly without the deformation that would degrade sound quality. Heat sinks

are materials that absorb or transmit excess heat. Diamond has the highest thermal conductivity of any material. It is used to conduct heat away from the heat-sensitive parts of high-performance microelectronics. Low-friction microbearings

are needed in tiny mechanical devices. Just as some watches have jewel bearings in their movements, diamonds are used where extreme abrasion resistance and durability are needed. Wear-resistant parts

can be produced by coating surfaces with a thin coating of diamond. In this process, diamond is converted into a vapor that deposits on the surface of parts prone to wear.

Diamond simulants: The photos above compare strontium titanate, moissanite, and cubic zirconia with diamond. Moissanite and cubic zirconia have dispersions that are competitive with diamond, and the dispersion of strontium titanate is over-the-top. In the photo above, the strontium titanate is a 6-millimeter round. The other stones are 4-millimeter rounds. This difference in size does give strontium titanate an advantage.

Diamond Simulants Diamond simulants are materials that look like diamonds and have many similar physical properties, but they have different chemical compositions. Diamond simulants can be natural materials such as colorless zircon or sapphire. More often they are man-made materials such as cubic zirconia (ZrO2), moissanite (SiC), YAG (yttrium aluminum garnet Y3Al5O12), or strontium titanate (SrTiO3).

Synthetic diamonds of various colors grown by the high-pressure high-temperature technique. Image by Wikipedia contributor Materialscientist. Diamond Information [1] Gemstones: Donald W. Olson, U.S. Geological Survey, Mineral Commodity Summaries, January 2016. [2] Gemstones: Donald W. Olson, U.S. Geological Survey, 2013 Minerals Yearbook, March 2016. [3] Diamond, Industrial: Donald W. Olson, U.S. Geological Survey, 2016 Mineral Commodity Summaries, January 2016. [4] Diamond, Industrial: Donald W. Olson, U.S. Geological Survey, 2013 Minerals Yearbook, December 2015. [5] Diamond Quality Factors: An explanation of how diamonds are evaluated using the 4Cs. Gemological Institute of America. Last accessed July 2016.

Synthetic Diamonds

Synthetic Diamonds by Chemical Vapor Deposition

Diamond is a very valuable material, and for over half a century many people and companies have worked to create them in laboratories and factories. Synthetic diamonds are man-made materials that have the same chemical composition, crystal structure, optical properties and physical behavior as natural diamonds. The first commercially successful synthesis of diamond was accomplished in 1954 by workers at General Electric. Since then, many companies have been successful at producing synthetic diamond suitable for industrial use. Today, most of the industrial diamond consumed is synthetic, with China being the world leader with a production of over 4 billion carats per year.

In the last decade, a few companies have developed technology that enables them to produce gem-quality laboratory-created diamonds up to a few carats in size in several different colors - including colorless. Some companies use high-pressure, high-temperature methods, while others use chemical vapor deposition methods. Their man-made gems are being sold in jewelry stores and on the internet at a significant discount to natural stones of similar quality and size. They have a beautiful appearance and an attractive price tag. Synthetic diamonds are required to be sold with a disclosure that they are "synthetic" or "laboratory-created."

Will Consumers Accept Synthetic Diamonds? Synthetic diamonds have been the dominant type of diamond in industrial applications since the end of the 20th century. Most of the diamonds used to make abrasives and cutting tools are now synthetic. Virtually all diamonds used to make windows, speaker domes, heat sinks, low-friction microbearings, wear-resistant parts, and other technology products are synthetic. Synthetic diamonds are less costly, have more consistent properties, and are becoming available in made-to-order specifications. There are no emotional barriers for synthetic diamonds to replace mined diamonds in these uses. In the jewelry industry there is considerable debate about the willingness of consumers to accept synthetic diamonds. Some believe that jewelry consumers want "real diamonds." Others believe that synthetic diamonds will be favored by people who dislike the human rights and environmental problems associated with mined diamonds. However, the real motivator will likely be price. Currently, synthetic diamonds made for jewelry use have a 15 to 30% price advantage on mined diamonds. This will likely be the greatest motivator for consumers to accept synthetic diamonds. Observation and speculation.... If you walk into almost any mall jewelry store and look into the cases where ruby, sapphire, and emeralds are being sold, you will see that most of the stones being offered are synthetic. A person with very little training can spot them on sight. The synthetic materials have a superior appearance and their prices are small compared to natural gems of similar size and appearance. Consumers get better appearance for a lower price, and the majority of them have accepted that transaction. The battle for emotion and sales dominance in the popular-price ruby, sapphire, and emerald market was won by synthetics a couple decades ago. In the next decade the diamond market might also move in favor of synthetics. It's already starting as synthetic diamonds take a visible position in the market. The price of synthetic diamonds will likely decline as more and more machines to produce them are placed into service, become more efficient, and competition among manufacturers intensifies. Eventually, the price differential between natural and synthetic diamonds will be greater than most customers will be able to ignore, and they will buy the synthetic. If the next world-class advertising campaign promotes synthetic diamonds, the shift in consumer demand might come sooner and with greater intensity. Contributor: Hobart King

Diopside A pyroxene mineral found in igneous and metamorphic rocks. A minor gemstone.

Chromium Diopside: A gemmy green specimen of chromium diopside from the the Outokumpu copper-zinc in Finland. This specimen measures 6.5 x 6.2 x 2.9 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What is Diopside? Diopside is a pyroxene mineral with a chemical composition in igneous and metamorphic rocks at many locations around the world.

of MgCaSi2O6.

It

occurs

Gem-quality crystals of diopside are faceted into attractive gemstones that are occasionally seen in commercial jewelry. Granular diopside can be easily cut and polished. When it has an attractive color, it is sometimes used as an ornamental stone. Perhaps the most important use of diopside is its value as an indicator mineral in the search for diamonds. Trail-to-lode prospecting using diopside and other indicator minerals has found diamond deposits in Canada, the United States, Africa, and other locations. Diopside has potential uses in the glass and ceramics industries, but the mineral usually occurs in accumulations that are too small or impure for effective mining.

Geologic Occurrence of Diopside The most common occurrence of diopside at Earth's surface is as a primary mineral in olivinerich basalts and andesites. In these rocks it can be present in quantities of a few weight percent. Diopside also forms during contact metamorphism of limestones and dolomites. Most of the crystalline diopside used to cut faceted gems and the granular diopside used as ornamental stone occurs in these carbonate deposits.

Diopside is much more abundant in Earth's mantle than at the surface. Evidence for this is diopside as a common mineral in ophiolites, and diopside as a common mineral in kimberlites and peridotites that were formed during deep-source volcanic eruptions. Physical Properties of Diopside Chemical Classification

Silicate.

Color

Grayish white, light blue to purple, light green to vivid green, brown, black.

Streak

White to light green.

Luster

Vitreous, sugary, earthy.

Diaphaneity Cleavage

Opaque, translucent, transparent. Two distinct directions, at 87 and 93 degrees; imperfect; prismatic.

Mohs Hardness

5.5 to 6.5

Specific Gravity

3.2 to 3.5

Diagnostic Properties

Cleavage, monoclinic crystal form.

Chemical Composition

MgCaSi2O6

Crystal System

Monoclinic.

Uses

Gemstone, diamond indicator mineral, potential industrial use in ceramics.

Diopside as a Diamond Indicator Mineral Most diamonds found at or near Earth's surface were delivered from the mantle during deep-source volcanic eruptions. These diamonds occur in vertical igneous structures known as pipes, which are often composed of kimberlite or peridotite. These pipes are difficult to locate. Their surface exposure is usually covered with soil and vegetation, and it might be only a few acres in size. The pipes are often found by searching soils and sediments for mineral grains that are characteristic of the pipe but absent in local surface materials. Small particles of chromiumrich diopside are bright green in color, are often abundant in the pipes, and are easy to recognize in surface materials. Geologists use these green diopside fragments to locate the pipes. They know that the fragments are liberated as the pipe weathers, then scattered by the actions of mass wasting, streams, and glaciers. When diopside fragments are discovered, the geologist knows that they originated up-slope, up-stream, or up-ice from the location in which they were found. A trail of diopside fragments can lead the geologist to the pipe from which they were weathered. This activity, known as "trail-to-lode" prospecting, finds many diamond pipes and an even larger number of pipes without diamonds. Note: It would be almost impossible to locate pipes by looking for diamonds. Diamonds make up such a small fraction of the overall rock in the pipe, and weathering debris from the pipe is then mixed into local rock debris. An exceptional pipe might contain a couple carats of diamond per ton!

Chrome Diopside Gem: A faceted stone cut from chrome diopside mined in Russia. This gem is approximately 1.2 carats in weight and about 7 millimeters by 5 millimeters in size.

Chrome Diopside Beads: Rondelle-shaped beads cut from bright green chrome diopside mined in Russia. The beads range in size between 3 and 5 millimeters in diameter.

Chrome Diopside Some crystals of diopside contain enough chromium to give them a rich green color. These can be cut into beautiful faceted stones, beads, and cabochons. The appearance of these stones is best when they are under two carats because the material is often dark or strongly saturated. Chrome diopside is occasionally seen in commercial jewelry. It has a rich green color that enables it to serve as an alternative gem for emerald at a significantly lower price. Diopside is rarely treated, unlike emerald which is often treated with various materials to seal and hide fractures. One problem with chrome diopside is its durability. It has two directions of perfect cleavage and a Mohs hardness of only 5.5 to 6.5. This gives it a risk of being scratched or broken. The gem is best used in earrings, necklaces, brooches, and other items that will not be subjected to abrasion or impact. Even though chrome diopside is very attractive, there are barriers to it becoming a popular gem that is widely seen in jewelry. First are the durability concerns described above; second is that the jewelry-buying public is

not familiar with diopside; and, third is the fact that a reliable supply of commercial stones in calibrated sizes has not been developed.

Star Diopside: A black star diopside cabochons exhibiting four-ray stars. They are slightly magnetic, indicating that the silk is probably magnetite crystals. These cabochons are approximately 8 millimeters across.

Star Diopside Some diopside crystals are filled with microscopic needle-shaped inclusions that occur in parallel alignment with the crystal structure of the mineral. This network of parallel inclusions is known as a "silk." When this diopside is cut en cabochon, the parallel needles of the silk can reflect light much like how light is reflected from a spool of silk thread. A silk with one direction of needle alignment will produce chatoyance, also known as a cat's eye. Silk with two or three directions of needle alignment will produce asterism. Two directions produces a four-ray star, and three directions produces a six-ray star. For these phenomena to appear, the stone must be cut with the needles oriented parallel to the bottom of the stone, and the top of the cabochon must be symmetrically cut. The mineral needles that form the silk are known in some instances to be magnetite. They are sometimes abundant enough to make the cut gems slightly magnetic. If you approach them slowly with a magnet, the gems will move before the magnet touches them. The needles in some non-magnetic gems are thought to be rutile or ilmenite.

Violane: A rarely-seen variety of diopside is violane. It is usually a blue to purple material that is cut into beads and cabochons. The photo shows a cabochon and a piece of rough from the Khakassia area of Russia. This cabochon is approximately 38 x 28 millimeters in size.

Violane Some diopside formed during the contact metamorphism of dolomite or limestone has a granular texture similar to marble. This material is known as "violane." It is often white, gray, light blue, lilac, or purple in color. Violane accepts a bright polish and is sometimes used to make cabochons, beads, and ornamental items. Violane is a rare material in nature and almost never seen in commerce.

Diopside as an Industrial Mineral Diopside has potential uses in ceramics, glass-making, biomaterials, nuclear waste immobilization, and fuel cell technology. Unfortunately, natural diopside is rarely found in deposits that simultaneously have a size, purity, and location that allows economic mining. This makes synthetic diopside cost-competitive with diopside produced by mining.

Geographic Distribution of Diopside Gem-quality chrome diopside and violane are mined in limited amounts in Siberia, Russia. Most of the chrome diopside used in jewelry today comes from a few locations in Siberia. Small occurrences of chrome diopside are also known in Austria, Brazil, Burma, Canada (Ontario and Quebec), Finland, India, Italy, Madagascar, Pakistan, South Africa, Sri Lanka, and the United States (New York), but none of them produce regularly or in significant quantities.

Dolomite A common rock-forming mineral and the primary constituent of a sedimentary rock known as "dolostone"

Dolomite crystals: Dolomite crystals from Penfield, New York. This specimen is approximately 3 inches (6.7 centimeters) across.

Granular Dolomite: Dolomitic marble from Thornwood, New York. This specimen is approximately 3 inches (6.7 centimeters) across.

What is Dolomite? Dolomite is a common rock-forming mineral. It is a calcium magnesium carbonate with a chemical composition of CaMg(CO3)2. It is the primary component of the sedimentary rock known as dolostone and the metamorphic rock known as dolomitic marble. Limestone that contains some dolomite is known as dolomitic limestone.

Dolomite is rarely found in modern sedimentary environments, but dolostones are very common in the rock record. They can be geographically extensive and hundreds to thousands of feet thick. Most rocks that are rich in dolomite were originally deposited as calcium carbonate muds that were postdepositionally altered by magnesium-rich pore water to form dolomite. Dolomite is also a common mineral in hydrothermal veins. There it is often associated with barite, fluorite, pyrite, chalcopyrite, galena, or sphalerite. In these veins it often occurs as rhombohedral crystals which sometimes have curved faces. Physical Properties of Dolomite Chemical Classification

Carbonate

Color

Colorless, white, pink, green, gray, brown, black

Streak

White

Luster

Vitreous, pearly

Diaphaneity Cleavage

Transparent to translucent Perfect, rhombohedral, three directions

Mohs Hardness

3.5 to 4

Specific Gravity

2.8 to 2.9

Diagnostic Properties Chemical Composition Crystal System

Uses

Rhombohedral cleavage, powdered form effervesces weakly in dilute HCl, hardness

CaMg(CO3)2

Hexagonal Construction aggregate, cement manufacture, dimension stone, calcined to produce lime, sometimes an oil and gas reservoir, a source of magnesia for the chemical industry, agricultural soil treatments, metallurgical flux

Dolostone: Dolostone from Lee, Massachusetts. The "sugary" sparkle displayed by this rock is caused by light reflecting from tiny dolomite cleavage faces. This specimen is approximately 4 inches (10 centimeters) across.

Physical Properties of Dolomite The physical properties of dolomite that are useful for identification are presented in the table on this page. Dolomite has three directions of perfect cleavage. This may not be evident when the dolomite is fine-grained. However, when it is coarsely crystalline the cleavage angles can easily be observed with a hand lens. Dolomite has a Mohs hardness of 3 1/2 to 4 and is sometimes found in rhombohedral crystals with curved faces. Dolomite produces a very weak reaction to cold, dilute hydrochloric acid; however, if the acid is warm or if the dolomite is powdered, a much stronger acid reaction will be observed. (Powdered dolomite can easily be produced by scratching it on a streak plate.) Dolomite is very similar to the mineral calcite. Calcite is composed of calcium carbonate (CaCO3), while dolomite is a calcium magnesium carbonate (CaMg(CO3)2). These two minerals are one of the most common pairs to present a mineral identification challenge in the field or classroom. The best way to tell these minerals apart is to consider their hardness and acid reaction. Calcite has a hardness of 3, while dolomite is slightly harder at 3 1/2 to 4. Calcite is also strongly reactive with cold hydrochloric acid, while dolomite will effervesce weakly with cold hydrochloric acid.

Dolomite aggregate: Dolostone, used for asphalt paving from Penfield, New York. These specimens are approximately 1/2 inch to 1 inch (1.3 centimeters to 2.5 centimeters) across.

Solid Solution and Substitution Dolomite occurs in a solid solution series with ankerite (CaFe(CO3)2). When small amounts of iron are present, the dolomite has a yellowish to brownish color. Dolomite and ankerite are isostructural. Kutnahorite (CaMn(CO3)2) also occurs in solid solution with dolomite. When small amounts of manganese are present, the dolomite will be colored in shades of pink. Kutnahorite and dolomite are isostructural.

Dolomitic marble from Thornwood, New York. This specimen is approximately 4 inches (10 centimeters) across.

Uses of Dolomite Dolomite as a mineral has very few uses. However, dolostone has an enormous number of uses because it occurs in deposits that are large enough to mine. The most common use for dolostone is in the construction industry. It is crushed and sized for use as a road base material, an aggregate in concrete and asphalt, railroad ballast, rip-rap, or fill. It is also calcined in the production of cement and cut into blocks of specific size known as "dimension stone." Dolomite's reaction with acid also makes it useful. It is used for acid neutralization in the chemical industry, in stream restoration projects, and as a soil conditioner. Dolomite is used as a source of magnesia (MgO), a feed additive for livestock, a sintering agent and flux in metal processing, and as an ingredient in the production of glass, bricks, and ceramics. Dolomite serves as the host rock for many lead, zinc, and copper deposits. These deposits form when hot, acidic hydrothermal solutions move upward from depth through a fracture system that encounters a dolomitic rock unit. These solutions react with the dolomite, which causes a drop in pH that triggers the precipitation of metals from solution. Dolomite also serves as an oil and gas reservoir rock. During the conversion of calcite to dolomite, a volume reduction occurs. This can produce pore spaces in the rock that can be filled with oil or natural gas that migrate in as they are released from other rock units. This makes the dolomite a reservoir rock and a target of oil and gas drilling. Contributor: Hobart King

Epidote Epidote is a metamorphic mineral and the name of a silicate mineral group.

Epidote: Epidote from Rockbridge County, Virginia. This specimen is approximately 4 inches (10 centimeters) across.

What is Epidote? Epidote is a name that is used in two different ways in mineralogy: 1) the "Epidote Group" is the name of a group of silicate minerals that share common structural and compositional characteristics; and, 2) "Epidote" is the name of the most common mineral in the Epidote Group. Physical Properties of Epidote Chemical Classification

Silicate

Color

Usually yellowish green to pistachio green, sometimes brownish green to black

Streak

Colorless

Luster

Vitreous to resinous

Diaphaneity Cleavage

Transparent to translucent to nearly opaque Perfect in one direction, imperfect

Mohs Hardness

6 to 7

Specific Gravity

3.3 to 3.5

Diagnostic Properties

Color, cleavage, specific gravity

Chemical Composition

Ca2(Al2,Fe)(SiO4)(Si2O7)O(OH)

Crystal System Uses

Monoclinic Semiprecious gem

What is Epidote (the mineral)? Epidote is a silicate mineral that is commonly found in regionally metamorphosed rocks of low-to-moderate grade. In these rocks, epidote is often associated with amphiboles, feldspars, quartz, and chlorite. It occurs as replacements of mineral grains that have been altered by metamorphism. It is frequently found in veins that cutgranite. It occurs as monoclinic crystals in pegmatites. It is also found in massive form and as monoclinic crystals in marbles and schists that were formed or altered through contact metamorphism. Epidote usually ranges between yellowish green to pistachio green in color. Less often it is brownish green to black. In massive form it is usually translucent with a vitreous luster. Well-formed crystals from marble and pegmatite are often transparent. Epidote has a chemical composition of Ca2(Al2,Fe)(SiO4)(Si2O7)O(OH). It is an end member of a solid solution series with clinozoisite. In that series, the iron of epidote is gradually replaced by aluminum to the end member clinozoisite composition of Ca2Al3(SiO4)(Si2O7)O(OH). Clinozoisite is usually lighter in color than epidote because iron is what produces epidote's greenish to brownish color. Mineral

Allanite

Askagenite

Clinozoisite

Dissakisite

Dollaseite

Epidote

Ferriallanite

Chemical Composition (CaX)(Al2Fe)(Si2O7)(SiO4)O(OH) X is one of these rare earths: Ce, La, Nd, Y. (MnNd)(Al2Fe)(Si2O7)(SiO4)O2 Ca2Al3(SiO4)(Si2O7)O(OH) Sr sometimes substitutes for one Ca. (CaX)(Al2Mg)(Si2O7)(SiO4)O(OH) X can be Ce or La. (CaCe)(Mg2Al)(Si2O7)(SiO4)(OH)F Ca2(Al2Fe)(SiO4)(Si2O7)O(OH) Pb or Sr can substitute for one of the calcium. (CaX)(Fe2Al)(SiO4)(Si2O7)O(OH) X can be Ce or La.

Hancockite

(CaPb)(Al2(Fe,Mn)(SiO4)(Si2O7)O(OH)

Khristovite

(CaCe)(MgAlMn)(SiO4)(Si2O7)(OH)F

Manganipiemontite

Mukhinite

Piemontite

(XY)(Mn2Al)(SiO4)(Si2O7)O(OH) (XY) can be (MnLa) or (CaSr). (Ca2)(Al2V)(SiO4)(Si2O7)O(OH) (X)(Al2Mn)(SiO4)(Si2O7)O(OH) (X) can be (Ca2), (CaPb) or (CaSr).

Uedaite

(MnCe)(Al2Fe)(SiO4)(Si2O7)O(OH)

Zoisite

Ca2Al3(SiO4)(Si2O7)O(OH)

What is Epidote (the mineral group)? Members of the epidote mineral group have a crystal structure that consists of isolated and paired silica tetrahedrons. They share a generalized chemical composition of A2M3(Si2O7)(SiO4)O(OH). "A" is a pairing of calcium, manganese, strontium, lead, or sometimes a rare earth element. "M" is usually aluminum pairing with iron, magnesium, manganese, or vanadium. Some of the member minerals of the epidote group are listed in the table with their chemical compositions.

Epidote crystals: Epidote from Rockbridge County, Virginia. This specimen is approximately 4 inches (10 centimeters) across.

Epidote in unakite: Tumbled stones made from unakite, an igneous rock composed mainly of green epidote, pink orthoclase feldspar, and quartz. This unakite was mined in South Africa.

Epidote in Rocks Epidote is a rock-forming mineral. Many regionally-metamorphosed rocks contain small amounts of epidote. Two rock types that contain significant amounts of epidote are epidosite and unakite. Locations where these rocks can be found are rare, but at those locations significant amounts of these rocks can be present. Epidosite is a metamorphic rock composed mainly of epidote with small amounts of quartz. It forms whenbasalts in sheeted dikes and ophiolites are transformed by hydrothermal activity or metasomatism.

Unakite is a rock that forms from the metamorphism of granite. Less-resistant minerals in the granite are altered to epidote or replaced by epidote, with the orthoclase and quartz remaining. It is an interesting pink and green colored rock that was first discovered in the Unakas Mountains of North Carolina, from which its name was derived.

Unakite cabochons: Two cabochons cut from unakite. The cab on the left is about 30 x 19 millimeters in size and is cut from material with a very coarse grain size. The cab on the right is about 39 x 30 millimeters in size and is cut from material with a fine grain size.

Uses of Epidote Epidote has no significant use as an industrial mineral and has only minor use as a gemstone. High-quality transparent crystals are sometimes cut into faceted stones. These have never attracted much interest in the commercial jewelry market, probably because their colors are not customer favorites. Most of the faceted stones produced are purchased by gem and mineral collectors. Unakite is a popular rock used by lapidaries to make beads, ornamental objects, and cut into cabochons. It is considered to be a semiprecious stone. The bright pink and pistachio green colors are very unusual and attract attention. Unakite is popular as a tumbled stone. A small amount of epidosite is also cut into cabochons. Contributor: Hobart King

Fluorite (also known as Fluorspar) An important industrial mineral used in many chemical, ceramic, and metallurgical processes.

Fluorite: This photo shows several beautiful blue cubic crystals of fluorite with occasional pyrite crystals on their faces. Fluorite is commonly found as cubic crystals, but blue crystals are unusual. The blue color can be caused by trace amounts of yttrium substituting for calcium in the fluorite crystal structure. Photo by Giovanni Dall'Orto, used here under a Creative Commons license.

Fluorite cleavage: Fluorite is the only common mineral with four directions of perfect cleavage. This perfect cleavage combined with the mineral's isometric crystal structure frequently cause it to cleave into perfect octahedrons as shown here. These specimens also show the purple and yellow colors that are typical of fluorite. Photo by Hannes Grobe, used here under a Creative Commons license.

What is Fluorite? Fluorite is an important industrial mineral composed of calcium and fluorine (CaF2). It is used in a wide variety of chemical, metallurgical, and ceramic processes. Specimens with exceptional diaphaneity and color are cut into gems or used to make ornamental objects. Fluorite is deposited in veins by hydrothermal processes. In these rocks it often occurs as a gangue mineral associated with metallic ores. Fluorite is also found in the fractures and cavities of some limestones and dolomites. It is a very common rock-forming mineral found in many parts of the world. In the mining industry, fluorite is often called "fluorspar." Physical Properties of Fluorite Chemical

Halide

Classification Color

Typically purple, green, and yellow. Also colorless, blue, red, and black.

Streak

White

Luster

Vitreous

Diaphaneity

Transparent to translucent

Cleavage

Four directions of perfect cleavage

Mohs Hardness

4

Specific Gravity

3.2

Diagnostic

Cleavage, hardness, specific gravity, color

Properties Chemical

CaF2

Composition Crystal System

Uses

Isometric Numerous uses in the metallurgical, ceramics, and chemical industries. A source of fluorine, hydrofluoric acid, metallurgical flux. High-clarity pieces are used to make lenses for microscopes, telescopes, and cameras.

Physical Properties of Fluorite Fluorite is very easy to identify if you consider cleavage, hardness, and specific gravity. It is the only common mineral that has four directions of perfect cleavage, often breaking into pieces with the shape of an octahedron. It is also the mineral used for a hardness of four in the Mohs Hardness Scale. Finally, it has a specific gravity of 3.2, which is detectably higher than most other minerals.

Although color is not a reliable property for mineral identification, the characteristic purple, green, and yellow translucent-to-transparent appearance of fluorite is an immediate visual clue for the mineral.

Fluorescent fluorite: Tumble-polished specimens of fluorite in normal light (top) and under short-wave ultraviolet light (bottom). The fluorescence appears to be related to the color and banding structure of the minerals in plain light.

Fluorescence In 1852, George Gabriel Stokes discovered the ability of specimens of fluorite to produce a blue glow when illuminated with light, which in his words was "beyond the violet end of the spectrum." He called this phenomenon "fluorescence" after the mineral fluorite. The name gained wide acceptance in mineralogy, gemology, biology, optics, commercial lighting, and many other fields. (See photo pair for an example of fluorite fluorescence in tumbled stones.) Fluorite typically glows a blue-violet color under short-wave ultraviolet and long-wave ultraviolet light. Some specimens are known to glow a cream or white color. Many specimens do not fluoresce. Fluorescence in fluorite is thought to be caused when trace amounts of yttrium, europium, samarium, or other elements substitute for calcium in the fluorite mineral structure.

Fluorite crystal mass: An impressive cluster of fluorite crystals from the Berbes Mine, Ribadesella, Asturias, Spain. Specimen and photo by Arkenstone / www.iRocks.com.

Fluorite Occurrence Most fluorite occurs as vein fillings in rocks that have been subjected to hydrothermal activity. These veins often contain metallic ores which can include sulfides of tin, silver, lead, zinc, copper, and other metals. Fluorite is also found in the fractures and vugs of some limestones and dolomites. Fluorite can be massive, granular, or euhedral as octahedral or cubic crystals. Fluorite is a common mineral in hydrothermal and carbonate rocks worldwide.

Fluorite unit cell: Illustration showing the relative size and position of fluorine and calcium ions in the isometric unit cell of fluorite. Public domain image by Benjah-bmm27.

Fluoride products: Most people are familiar with fluoride products used in the prevention of tooth decay. Fluoride is added to drinking water as a systemic fluoride therapy and added to toothpastes, mouthwashes and dental rinse as a topical fluoride therapy. These uses of fluoride have been controversial.

Fluorite gemstone: Fluorite can be a beautiful gemstone when faceted. It is mainly a gemstone for collectors because it has a hardness of 4 on the Mohs scale and because it cleaves easily in four directions.

Uses of Fluorite Fluorite has a wide variety of uses. The primary uses are in the metallurgical, ceramics, and chemical industries; however, optical, lapidary, and other uses are also important. Fluorspar, the name used for fluorite when it is sold as a bulk material or in processed form, is sold in three different grades (acid, ceramic, and metallurgical). Acid Grade Fluorspar

Acid grade fluorspar is a high-purity material used by the chemical industry. It contains over 97% CaF2. Most of the fluorspar consumed in the United States is acid grade even if it is used in lower grade applications. It is used mainly in the chemical industry to manufacture hydrofluoric acid (HF). The HF is then used to manufacture a variety of products which include: fluorocarbon chemicals, foam blowing agents, refrigerants, and a variety of fluoride chemicals.

Ceramic Grade Fluorspar

Ceramic grade fluorspar contains between 85% and 96% CaF2. Much of this material is used in the manufacture of specialty glass, ceramics, and enamelware. Fluorspar is used to make glazes and surface treatments that produce hard glossy surfaces, opalescent surfaces, and a number of other appearances that make consumer glass objects more attractive or more durable. The non-stick cooking surface known as Teflon is made using fluorine derived from fluorite. Metallurgical Grade Fluorspar

Metallurgical grade fluorspar contains between 60 and 85% CaF2. Much of this material is used in the production of iron, steel, and other metals. Fluorspar can serve as a flux that removes impurities such as sulfur and phosphorous from molten metal and improves the fluidity of slag. Between 20 and 60 pounds of fluorspar is used for every ton of metal produced. In the United States many metal producers use fluorspar that exceeds metallurgical grade. Optical Grade Fluorite

Specimens of fluorite with exceptional optical clarity have been used as lenses. Fluorite has a very low refractive index and a very low dispersion. These two characteristics enable the lens to produce extremely sharp images. Today, instead of using natural fluorite crystals to manufacture these lenses, high-purity fluorite is melted and combined with other materials to produce synthetic "fluorite" lenses of even higher quality. These lenses are used in optical equipment such as microscopes, telescopes, and cameras. Lapidary Grade Fluorite

Specimens of fluorite with exceptional color and clarity are often used by lapidaries to cut gemstones and make ornamental objects. High-quality specimens of fluorite make beautiful faceted stones; however, the mineral is so soft and cleaves so easily that these stones are either sold as collector's specimens or used in jewelry that will not be subjected to impact or abrasion. Fluorite is also cut and carved into ornamental objects such as small figurines and vases. These are often treated with a coating or impregnation to enhance their stability and protect them from scratches.

Banded fluorite cabochon: Colorful pieces of fluorite can be cut into beautiful cabochons and other ornamental objects. However, because of its low hardness and perfect cleavage, it is not suitable for many purposes.

Fluorite Production in the United States Deposits of minable fluorite exist in the United States; however, nearly all of the fluorite consumed in the United States is imported. The primary countries that supplied fluorite to the United States in 2011 were China,

Mexico, Mongolia, and South Africa. All of this fluorite is imported because production costs in the United States are so high that the material can be produced in these other countries and shipped directly to customers in the United States at a lower cost. In 2011 several companies were producing and selling synthetic fluorite as a byproduct of their phosphoric acid production, petroleum processing, or uranium processing activities. A limestone producer in Illinois was also recovering and selling small amounts of fluorite from their quarry. That company is developing an underground mine to exploit a large vein of fluorite which they hope will be in production in 2013. Contributor: Hobart King

Fuchsite and Ruby in Fuchsite Many people use the word “fuschite” for this material. That is a misspelling.

Fuchsite: Photograph of a verdite specimen consisting almost exclusively of fuchsite platelets with a foliated texture. Specimen is approximately 2 inches across. Mineral Muscovite Fuchsite

Composition KAl2(Si3AlO10)(OH)2 K(Al,Cr)2(Si3AlO10)(OH)2

What is Fuchsite? Fuchsite is a green variety of muscovite mica. It differs from most other muscovite by having a variable amount of trivalent chromium substituting for aluminum within the mineral. Chromium is the source of fuchsite’s green color. Muscovite begins to take on a very light green color with the substitution of a small amount of chromium for aluminum. As the amount of chromium increases, the green color becomes stronger and ranges to a rich emerald green when abundant chromium is present. The chemical formulae of muscovite and fuchsite are shown in the table. Fuchsite is found in phyllites and schists in metamorphic rocks of the greenschist facies. In most instances it occurs as tiny grains scattered through the rock mass, but occasionally rocks composed almost entirely of fuchsite are found. These green fuchsite-rich rocks are known as “verdite.” Physical Properties of Fuchsite Chemical Classification

Silicate

Color

Light green to emerald green depending upon chromium content

Streak

White, often sheds tiny green flakes

Luster Diaphaneity

Pearly to vitreous Transparent to translucent

Cleavage

Perfect

Mohs Hardness

2 to 3

Specific Gravity

2.8 to 2.9

Diagnostic Properties Chemical Composition

Cleavage, color, transparency

K(Al,Cr)2(Si3AlO10)(OH)2

Crystal System

Uses

Monoclinic Fuchsite of good purity is not abundant enough to support manufactured products. The primary use is as a gem material, especially ruby in fuchsite for cabochons, spheres, and small utility objects that will not be subjected to impact or wear.

Spelling and Pronunciation Problems Fuchsite is one of the most commonly misspelled minerals - especially in the lapidary market. It is often spelled (and pronounced) “fuschite” with a long “u” and a long “i”. The material is named after Johann Nepomuk von Fuchs, a German chemist and mineralogist. His name is pronounced “fooks” - similar to the way you pronounce “books” and “looks.” You can hear a pronunciation here.

Ruby in Fuchsite: Photo of a ruby-in-fuchsite cabochon with a rim of blue kyanite around the red ruby crystal. This blue kyanite rim is diagnostic of ruby in fuchsite and can be used to avoid misidentification as ruby in zoisite. Cabochon is about 1 inch in height.

Fuchsite and Verdite as Gem Materials Verdite is usually soft and fragile; however, some competent specimens can be cut into cabochons and polished to a very high luster. Some people who cut verdite stabilize it for cutting by gluing it to a backing. Thin slices of black obsidian, basalt, or another black material are often used as backing.

Verdite is typically a foliated rock, with the mica grains oriented with their flat faces perpendicular to the direction of compression. The most attractive orientation of verdite when cutting cabochons is with the mica flakes aligned parallel to the bottom of the cab. Then, when the dome on the cabochon is cut and polished, the mica flakes reflect light and produce a green aventurescence.

Green Aventurine: Photo of green aventurine as a tumble-polished stone. In this photo you can see the green mica flakes suspended in the quartz.

Green Aventurine Sometimes small platelets of fuchsite are suspended in quartz to produce the gem known as green aventurine. It has become a very popular and typically inexpensive gem material that is cut into cabochons, beads, and small sculptures. It is also very popular as tumbled stones. Green aventurine is much more frequently seen as a gem material than verdite, fuchsite, and ruby in fuchsite combined. See tumbled stone photo.

Carved Ruby in Fuchsite: A pendant carved from ruby in fuchsite in which the artist took advantage of the red rubies to produce flowers. Blue kyanite alteration rims can be seen around the rubies.

What is “Ruby in Fuchsite”?

Occasionally, corundum crystals are found in fuchsite. When these corundum crystals are of a bright red color, the material is known as ruby in fuchsite. This material attracts a lot of attention at rock, mineral, gem, and lapidary shows because of the contrasting colors of the fuchsite and ruby, and because the corundum crystals often exhibit spectacular hexagonal shapes when cut in slabs, cabochons, spheres, and other objects.

Ruby-in-Zoisite Cabochons: Two ruby-in-zoisite cabochons. Note that they do not show blue kyanite alteration rims around the ruby. The material also has the characteristic scatter of black hornblende crystals.

Identification Problems? If spelling and pronunciation problems were not enough, ruby in fuchsite is one of the most commonly misidentified gem materials. If you visit lapidary shows and online auctions, you will probably see ruby in fuchsite more often presented incorrectly as “ruby in zoisite” than as "ruby in fuchsite." This identification problem can easily be solved if a person learns the three facts below and uses them for identification. 1) Fuchsite has a hardness of 2 to 3, while zoisite has a hardness of at least 6. 2) Rubies have blue kyanite alteration rims in fuchsite but no alteration rims in zoisite. See the cabochon photos. 3) Ruby in zoisite is usually marked with a scattering of black hornblende crystals. The next time you are at a rock, gem, or mineral show, watch for green and red cabochons or carvings. If you see blue alteration rims, it is ruby in fuchsite. Contributor: Hobart King

Galena The primary ore of lead that is sometimes mined for its silver content

Galena: Photograph of a nice cubic galena crystal with adjacent calcite crystals. The galena crystal is about two inches on a side. Collected from the Sweetwater Mine, Reynolds County, Missouri. Specimen and photo by Arkenstone / www.iRocks.com.

What is Galena? Galena is a lead sulfide mineral with a chemical composition of PbS. It is the world's primary ore of leadand is mined from a large number of deposits in many countries. It is found in igneous and metamorphic rocksin medium- to low-temperature hydrothermal veins. In sedimentary rocks it occurs as veins, breccia cements, isolated grains, and as replacements of limestone and dolostone. Galena is very easy to identify. Freshly broken pieces exhibit perfect cleavage in three directions that intersect at 90 degrees. It has a distinct silver color and a bright metallic luster. Galena tarnishes to a dull gray. Because lead is a primary element in galena, the mineral has a high specific gravity (7.4 to 7.6) that is immediately noticed when picking up even small pieces. Galena is soft with a Mohs hardness of 2.5+ and produces a gray to black streak. Crystals are common and they usually are cubes, octahedrons, or modifications.

Structure of galena: Galena has a chemical composition of PbS. That means it contains an equal number of lead and sulfide ions. The ions are arranged in a cubic pattern that repeats in all directions. This structure is what causes crystals of galena to have a cubic habit and causes galena to break in three directions at right angles.

Physical Properties of Galena Chemical Classification

Sulfide

Color

Fresh surfaces are bright silver in color with a bright metallic luster, tarnishes to a dull lead gray

Streak

Lead gray to black

Luster

Metallic on fresh surfaces, tarnishes dull

Diaphaneity Cleavage

Opaque Perfect, cubic, three directions at right angles

Mohs Hardness

2.5+

Specific Gravity

7.4 to 7.6

Diagnostic Properties

Color, luster, specific gravity, streak, cleavage, cubic or octahedral crystals.

Chemical Composition

Lead sulfide, PbS

Crystal System Uses

Isometric An ore of lead

Argentiferous galena: Argentiferous galena from Coeur d'Alene, Idaho. Specimen is approximately 2-1/2 inches (6.4 centimeters) across. Argentiferous galena has a silver content that is often high enough for the galena to be mined as an ore of silver. Some galena mines receive more revenue from their silver than from their lead production.

Argentiferous Galena - The Silver Ore The typical specimen of galena is about 86.6% lead and 13.4% sulfur by weight. However, some specimens of galena contain up to a few percent silver by weight. They are called "argentiferous galena" because of their silver content. In these specimens, silver can substitute for lead in the atomic structure of the galena, or it can occur in tiny grains of silver minerals included in the galena. Silver within the galena disrupts the crystal structure, which often causes the galena to have curved cleavage faces. This tiny bit of knowledge can be a powerful prospecting tool. In addition to silver, galena can contain minor amounts of antimony, arsenic, bismuth, cadmium, copper, and zinc. Sometimes selenium substitutes for sulfur in galena.

Cleavage fragments of galena: One of the most diagnostic properties of galena is its ability to break by cleavage in three directions that intersect at right angles. This forms cleavage fragments that are cubic and rectangular in shape. This photo shows pieces of crushed galena that clearly exhibit the right angle cleavage. This characteristic cleavage is caused by the mineral's cubic internal structure as shown above. Photo © iStockphoto / Tyler Boyes.

Galena value: Some mines produce more revenue from the silver content of their galena than from the lead content. Assume that we have a mine that produces argentiferous galena with an average composition of 86% lead, 13% sulfur and just 1% silver (as shown in the diagram on the left). If the silver price is $25 per troy ounce and the lead price is $1 per avoirdupois pound, the value of the lead in one ton of ore will be $1720, while the value of the silver in that same ton of ore will be $7292 (as shown in the diagram on the right). The small amount of silver has a huge impact on revenue because at the prices assumed, silver is 364 times more valuable than an equal weight of lead. It is easy to understand why mining companies get excited by argentiferous galena! Even though galena is the ore being removed and lead makes up the bulk of the product, these mines are often called "silver mines."

Smelting metals: Galena is one of the easiest ores to smelt. It can simply be placed in a fire and then lead can be recovered from under the ashes when the fire goes out. Archaeologists have found evidence that lead was smelted as early as 6500 BC in what is now Turkey [1]. Small amounts of silver were refined from lead by the Romans about 2000 years ago [2]. Public domain image by Georgius Agricola.

Smelting Galena

Galena is very easy to smelt. If rocks that contain galena are placed in a fire, lead can be collected from below the ashes after the fire burns out. People have taken advantage of this simple smelting for thousands of years. Archaeologists have found lead beads and statues in Turkey that date back to about 6500 BC [1]. Lead is probably the first metal to have been processed from an ore. The ancient Romans made lead pipe and used it as indoor plumbing. (Plumbum is the Latin word for lead. The word "plumbing" and our use of "Pb" as the chemical symbol for lead come from the ancient Romans.) The ancient Greeks and Romans were able to separate silver from lead about 2000 years ago [2]. Many of the Roman lead ingots were inscribed "Ex Arg" or "Ex Argent" to signify that the silver had been removed from the lead. The Greeks were able to desilver lead to a 0.02 percent silver content and the Romans to a 0.01 percent silver content [3]. It is surprising that they were able to realize that the lead contained silver and amazing that they were able to develop such an efficient method of refining! Galena Information [1] Lead Fact Sheet: General Information and History, Stanford University, General Health and Safety Program, last accessed July 2016. [2] On the Nature of Metals (De Re Metallica): Georgius Agricola, 1556. Translated by Herbert Clark Hoover and Lou Henry Hoover, republished by Farlang.com. [3] Pliny the Elder on Science and Technology: John F. Healy, Oxford University Press, page 324, 1999. [4] 'Heavy Metal' Snow: Carolyn Jones Otten, press release of Washington University of St. Louis, February 2004.

Alteration of Galena Galena weathers easily. Fresh surfaces of galena tarnish rapidly from a silver metallic luster to a dull gray to dull black color. When exposed to the elements or buried in soil, galena quickly weathers to anglesite, cerussite, pyromorphite, or another lead mineral. These minerals are often used in prospecting. When they are found at the surface, they often reveal that galena is present below.

Does It Really "Snow" Galena on Venus? The planet Venus has an inhospitable environment where volcanoes vent superheated gases into the atmosphere. Sulfur and lead are among the gases erupted from the volcanoes on Venus. They remain in the gaseous phase until they are high enough in the atmosphere to condense. In 2004, researchers at Washington University in St. Louis provided plausible evidence that "heavy metal snow" - which is most likely a combination of lead sulfide (galena) and bismuth sulfide - falls on the higher elevations of Venus [4].

Galena crystal radio: One of the most interesting uses of galena was in early crystal radios. The operation of these radios required alternating current to be converted into a pulsing direct current. For that to occur, a semiconductor material was used to limit the flow of electricity to one direction. The alternating current flowed through a wire, known as a cat's whisker, into a semiconductor crystal, which was usually a crystal of galena, which only allowed flow in a single direction. Image © iStockphoto / Greg_H.

Uses of Galena Galena is a very important mineral because it serves as an ore for most of the world's lead production. It is also a significant ore of silver. Galena has very few uses beyond its service as an ore, but that should not diminish its importance to society. The number one use of lead today is in the lead-acid batteries that are used to start automobiles. The typical auto battery contains about twenty pounds of lead and must be replaced every four or five years. There are billions of these batteries in the United States alone. Lead-acid batteries are also used as standby power supplies for computer networks, communication facilities, and other critical systems. Lead is also one of the metals used in energy storage systems associated with power generation and hybrid vehicles.

Related: The Many Uses of Lead

Lead Safety Many uses of lead and lead compounds have been discontinued or significantly reduced over the past few decades in response to health concerns. Some of these uses include lead in residential paints, motor vehicle fuels, solder, ammunition, fishing weights, ceramic glazes, pesticides, cosmetics, glass, plastics, alloys and many other products. For this reason, many schools have removed galena from student mineral kits and have replaced it with a mineral with a lower level of concern. Contributor: Hobart King

Uses of Lead Lead has been used by humans for a variety of purposes for more than 5,000 years. Republished from a 2011 Fact Sheet by S.J. Kropschot and Jeff L. Doebrich of the United States Geological Survey.

The Changing Uses of Lead

Scientific research demonstrating how accumulated ingested lead is toxic to human health and how accumulations of lead in the soil, air, and water are toxic to ecosystems is changing both how lead is used and how it is disposed of after use.

What is Lead? Lead is a corrosion-resistant dense metal that is easily molded and shaped. The chemical symbol for lead, Pb, is an abbreviation of the Latin word plumbum, meaning soft metal. Archeological research indicates that lead has been used by humans for a variety of purposes for more than 5,000 years. Lead is rarely found in native form in nature but it combines with other elements to form a variety of interesting and beautiful minerals. Galena, which is the dominant lead ore mineral, is blue-white in color when first uncovered but tarnishes to dull gray when exposed to air.

Typical lead-acid ignition batteries in automobiles contain about 10 kilograms of lead and need to be replaced every 4 to 5 years. Lead-acid batteries also supply standby power for computer networks and telecommunications systems and energy storage for wind and solar energy systems and hybrid-electric vehicles. Image © iStockphoto and Hywit Dimyadi.

Ancient Uses of Lead Water pipes that date back to the Roman Empire, glazes on prehistoric ceramics, and the cosmetic kohl, used by ancient Egyptians to darken their eyelids, are a few examples of ancient uses of lead. Today, lead, which has been mined on all continents except Antarctica, is one of the most important metalsto industrialized economies.

Did You Know? The English words plumbing, plumber, plumb, and plumb-bob derive from the Latin word for lead.

Modern Uses of Lead Prior to the early 1900s, lead was used in the United States primarily in ammunition, burial vault liners, ceramic glazes, leaded glass and crystal, paints or other protective coatings, pewter, and water lines and pipes. Following World War I, the demand for lead increased because of growth in the production of motorized vehicles, many of which use lead-acid batteries to start their engines. The use of lead as radiation shielding in medical analysis and video display equipment and as an additive in gasoline also contributed to an increase in the demand for lead. By the mid-1980s, a significant shift in the uses of lead had taken place in the United States as a result of compliance with environmental regulations and the substitution of other materials for lead in nonbattery products, such as gasoline, paints, solders, and water systems. By the early 2000s, 88 percent of apparent U.S. lead consumption was in lead-acid batteries, which was a substantial increase from 1960 when only 30 percent of global lead consumption was in lead-acid batteries. Today, the other significant uses of lead are in ammunition, oxides in glass and ceramics, casting metals, and sheet lead.

Galena, a lead sulfide mineral (PbS), is the primary ore of lead. It is mined at many locations worldwide.

Did You Know? Lead levels in ambient air are currently 92 percent lower than they were in 1980 owing to changes resulting from the Clean Air Act of 1970.

Lead in the Environment According to the U.S. Agency for Toxic Substances and Disease Registry, environmental levels of lead have increased more than 1,000-fold over the past three centuries as a result of human activity. The greatest increase took place between 1950 and 2000 and reflected the increased use of leaded gasoline worldwide. During this period, the U.S. Government established Federal regulations and made recommendations to limit lead emissions to protect public health in the United States.

Types of Lead Deposits Research to better understand the geologic processes that form mineral deposits, including those containing lead, is an important component of the USGS Mineral Resources Program. Lead commonly occurs in mineral deposits along with other base metals, such as copper and zinc. Lead deposits are broadly classified on the basis of how they are formed. Lead is produced mainly from three types of deposits: sedimentary

The Viburnum Trend in southeastern Missouri contains the largest concentration of lead in North America. The Buick mine is one of six underground mines that currently produce lead in the Viburnum Trend ore district. Image by USGS.

exhalative (Sedex), Mississippi Valley type (MVT), and volcanogenic massive sulfide (VMS).

Sedimentary Exhalative Deposits Sedex deposits account for more than 50 percent of the world's lead resources. They are formed when metal-rich hot liquids are released into a water-filled basin (usually an ocean) or in basin sediments, which results in the precipitation of ore-bearing material within basin-floor sediments.

Mississippi Valley Deposits MVT deposits are found throughout the world and get their name from deposits that occur in the Mississippi Valley region of the United States. The deposits are characterized by ore mineral replacement of the carbonate host rock; they are often confined to a single stratigraphic layer and extend over hundreds of square kilometers. MVT deposits were a major source of lead in the United States from the 19th century through the mid-20th century.

Volcanogenic Massive Sulfide Deposits In contrast to Sedex and MVT deposits, VMS deposits have a clear association with submarine volcanic processes. They also can contain significant amounts of copper, gold, and silver, in addition to lead and zinc. The "black smoker" sea vents discovered during deep ocean expeditions are examples of VMS deposits being formed on the sea floor today.

Worldwide Supply of and Demand for Lead Currently, approximately 240 mines in more than 40 countries produce lead. World mine production was estimated to be 4.1 million metric tons in 2010, and the leading producers were China, Australia, the United States, and Peru, in descending order of output. In recent years, lead was mined domestically in Alaska, Idaho, Missouri, Montana, and Washington. In addition, secondary (recycled) lead is a significant portion of the global lead supply. World consumption of refined lead was 9.35 million metric tons in 2010. The leading refined lead consuming countries were China, the United States, and Germany. Demand for lead worldwide is expected to grow largely because of increased consumption in China, which is being driven by growth in the automobile and electric bicycle markets.

Ensuring Future Supplies of Lead

Lead pipes, such as these discovered in Bath, England, were used for plumbing by ancient Romans. Image by USGS.

Lead Information

-- On production and consumption of lead: http://minerals.usgs.gov/minerals/pubs/commodity/lead/ -- On the assessment of undiscovered deposits of gold, silver, copper, lead, and zinc in the United States: http://pubs.usgs.gov/of/2002/of02-198/ -- On Mississippi Valley-type lead-zinc deposit model: http://pubs.usgs.gov/sir/2010/5070/a/ -- On sedimentary exhalative zinc-lead-gold deposit model: http://pubs.usgs.gov/of/2009/1209/ -- On volcanogenic massive sulfide deposit model: http://pubs.usgs.gov/of/2009/1235/

To help predict where future lead supplies might be located, USGS scientists study how and where identified lead resources are concentrated in the Earth's crust and use that knowledge to assess the likelihood that undiscovered lead resources exist. Techniques to assess mineral resource potentials have been developed and refined by the USGS to support the stewardship of Federal lands and to better evaluate mineral resource availability in a global context.

United States Lead Resources Lead Production and Reserves

Country

Production Reserves

USA

400

7,000

Australia

620

27,000

Bolivia

90

1,600

Canada

65

650

China

1,600

13,000

India

95

2,600

Ireland

45

600

Mexico

185

5,600

Peru

280

6,000

Poland

35

1,500

Russia

90

9,200

South Africa

50

300

Sweden

65

1,100

Other

330

4,000

Total

4,100

80,000

Data is in thousand metric tons. Data from USGS Mineral Commodity Summary, January 2011. In the 1990s, the USGS conducted an assessment of U.S. lead resources and concluded that about as much lead remained to be found as had already been discovered. Specifically, the USGS found that 92 million metric tons of lead had been discovered and estimated that about 85 million metric tons of lead remained undiscovered in the United States.

Mineral resource assessments are dynamic. Because they provide a snapshot that reflects our best understanding of how and where resources are located, the assessments must be updated from time to time as better data become available and new concepts are developed. Current research by the USGS involves updating mineral deposit models and mineral environmental models for lead and other important nonfuel commodities and improving the techniques used to assess for concealed mineral resource potential. The results of this research will provide new information and decrease the amount of uncertainty in future mineral resource assessments.

Did You Know? In 2009, with a 96 percent recycling rate, the standard lead-acid car battery was the most recycled product in the United States.

Garnet Best known as a red gemstone. It occurs in many colors and has numerous industrial uses.

Gem garnets: Most people think that garnet is a red gemstone. However, garnet occurs in a wide variety of colors. Clockwise from the top left: red almandine (Madagascar), green tsavorite (Tanzania), yellow mali (Mali), orange spessartite (Mozambique), pink malaya (Tanzania), green merelani mint (Tanzania), red pyrope (Ivory Coast), green demantoid (Namibia), purple rhodolite (Mozambique), and orange hessonite (Sri Lanka). Seven out of eight of the garnets above are from Africa, the relatively new source of spectacular garnets.

What is Garnet? Garnet is the name used for a large group of rock-forming minerals. These minerals share a common crystal structure and a generalized chemical composition of X3Y2(SiO4)3. In that composition, "X" can be Ca, Mg, Fe2+or Mn2+, and "Y" can be Al, Fe3+, Mn3+, V3+ or Cr3+. These minerals are found throughout the world in metamorphic, igneous, and sedimentary rocks. Most garnet found near Earth's surface forms when a sedimentary rock with a high aluminum content, such as shale, is subjected to heat and pressure intense enough to produce schist or gneiss. Garnet is also found in the rocks of contact metamorphism, subsurface magma chambers, lava flows, deep-source volcanic eruptions, and the soils and sediments formed when garnet-bearing rocks are weathered and eroded. Most people associate the word "garnet" with a red gemstone; however, they are often surprised to learn that garnet occurs in many other colors and has many other uses. In the United States, the major industrial uses of garnet in 2012 were waterjet cutting (35%), abrasive blasting media (30%), water filtration granules (20%), and abrasive powders (10%).

The Garnet Group: This chart summarizes the members of the garnet group that are most important as gemstones. The aluminum garnets are normally red in color with a higher specific gravity and hardness. The calcium members are usually green in color and have a lower hardness.

Physical Properties of Garnet Chemical Classification

Silicate

Color

Typically red, but can be orange, green, yellow, purple, black, or brown. Blue garnets are extremely rare.

Streak

Colorless

Luster

Vitreous

Diaphaneity

Transparent to translucent

Cleavage

None

Mohs Hardness

6.5 to 7.5

Specific Gravity

3.5 to 4.3

Diagnostic Properties

Hardness, specific gravity, isometric crystal habit, lack of cleavage

Chemical Composition

General formula: X3Y2(SiO4)3

Crystal System Uses

Isometric Waterjet cutting granules, abrasive blasting granules, filtration granules, abrasive grits and powders, gemstones

Garnet Physical and Chemical Properties The most commonly encountered minerals in the garnet group include almandine, pyrope, spessartine, andradite, grossular, and uvarovite. They all have a vitreous luster, a transparent-to-translucent diaphaneity, a brittle tenacity, and a lack of cleavage. They can be found as individual crystals, stream-worn pebbles, granular aggregates, and massive occurrences. Their chemical composition, specific gravity, hardness, and colors are listed below. Garnet Minerals Mineral

Composition

Specific Gravity

Hardness

Colors

Almandine

Fe3Al2(SiO4)3

4.20

7 - 7.5

red, brown

Pyrope

Mg3Al2(SiO4)3

3.56

7 - 7.5

red to purple

Spessartine

Mn3Al2(SiO4)3

4.18

6.5 - 7.5

orange to red to brown

Andradite

Ca3Fe2(SiO4)3

3.90

6.5 - 7

green, yellow, black

Grossular

Ca3Al2(SiO4)3

3.57

6.5 - 7.5

green, yellow, red, pink, clear

Uvarovite

Ca3Cr2(SiO4)3

3.85

6.5 - 7

green

The compositions listed above are for end members of several solid solution series. There are a number of other garnet minerals that are less frequently encountered and not as important in industrial use. They include goldmanite, kimzeyite, morimotoite, schorlomite, hydrogrossular, hibschite, katoite, knorringite, majorite, and calderite.

As seen above, there are a variety of different types of garnet, and each has a different chemical composition. There are also solid solution series between most of the garnet minerals. This wide variation in chemistry determines many of their physical properties. As an example, the calcium garnets generally have a lower specific gravity, a lower hardness and are typically green in color. In contrast, the iron and manganese garnets have a higher specific gravity, a greater hardness and are typically red in color.

Almandine garnet: Excellent cubic crystals of almandine garnet in a fine-grained mica schist from Granatenkogel Mountain, Austria. Specimen and photo by Arkenstone / www.iRocks.com.

Andradite garnet: Green andradite garnet of the demantoid variety on a matrix of marble. This specimen is about 8.9 x 6.5 x 4.8 centimeters in size and was collected in Antsiranana Province, Madagascar. Garnets formed within marble often have excellent crystal form and are of very high quality. Specimen and photo by Arkenstone / www.iRocks.com.

Garnet gneiss: A coarse-grained gneiss composed mainly of hornblende (black), plagioclase (white) and garnet (red) from Norway. Public domain photo by Woudloper.

Alluvial garnet crystals: These almandine-spessartine garnets are from an alluvial deposit in Idaho. They have been transported a short distance from their source rock, and some still retain evidence of their dodecahedral crystal form. They are about four to five millimeters in size and weigh about 0.6 to 0.8 carats each.

How Does Garnet Form?

Garnet in Metamorphic Rocks

Most garnet forms at convergent plate boundaries where shale is being acted upon by regional metamorphism. The heat and pressure of metamorphism breaks chemical bonds and causes minerals to recrystallize into structures that are stable under the new temperature-pressure environment. The aluminum garnet, almandine, generally forms in this environment. As these rocks are metamorphosed, the garnets start as tiny grains and enlarge slowly over time as metamorphism progresses. As they grow, they displace, replace, and include the surrounding rock materials. The photo below shows a microscopic view of a garnet grain that has grown within a schist matrix. It included a number of the host rock's mineral grains as it grew. This explains why so many garnets formed by regional metamorphism are highly included.

Garnet mica schist in thin section: This is a microscopic view of a garnet grain that has grown in schist. The large black grain is the garnet, the red elongate grains are mica flakes. The black, gray, and white grains are mostly silt or smaller size grains of quartz and feldspar. The garnet has grown by replacing, displacing, and including the mineral grains of the surrounding rock. You can see many of these grains as inclusions within the garnet. From this photo it is easy to understand why clean, gem-quality garnets with no inclusions are very hard to find. It is also hard to understand how garnet can grow into nice euhedral crystals under these conditions. Photo by Jackdann88, used here under a Creative Commons license.

The calcium garnets typically form when argillaceous limestone is altered into marble by contact metamorphism along the edges of igneous intrusions. These are andradite, grossular, and uvarovite, the slightly softer, typically green garnets with a lower specific gravity. Two calcium garnets are highly regarded in the gem trade; they are tsavorite (a bright green grossular) and demantoid (a golden-green andradite).

Garnet in Igneous Rocks

Garnet often occurs as an accessory mineral in igneous rocks such as granite. Many people are familiar with almandine garnet because it is sometimes seen as dark red crystals in the igneous rocks used as granite countertops. Spessartine is an orange garnet found as crystals in granite pegmatites. Pyrope is a red garnet that is brought to Earth's surface in pieces of peridotite that were torn from the mantle during deep-source volcanic eruptions. Garnet is also found in basaltic lava flows.

Garnet in Sedimentary Rocks and Sediments

Garnets are relatively durable minerals. They are often found concentrated in the soils and sediments that form when garnet-bearing rocks are weathered and eroded. These alluvial garnets are often the target of mining operations because they are easy to mine and remove from the sediment/soil by mechanical processing.

Uses of Garnet Garnet has been used as a gemstone for thousands of years. In the past 150 years, it has seen many additional uses as an industrial mineral. The chart below shows recent industrial uses of garnet in the United States. Garnet is also used as an indicator mineral during mineral exploration and geologic assessments.

Uses of Garnet: This chart shows the most common industrial uses of garnet minerals. Almandine is the variety of garnet that is most often used in industry.

Garnet abrasive: This photo shows garnet granules that have been crushed and size-graded for use as abrasive, cutting, and filter media. They are used in waterjet cutting, "sand" blasting, sandpaper, water filtration, and a number of other uses. Almandine is the hardest garnet and also the most abundant. It is the garnet of choice for most abrasive applications. Photo by the United States Geological Survey.

Garnet crystal: Almandine, a variety of garnet from River Valley, Ontario, Canada. This specimen is a nice euhedral crystal approximately 2 inches (5 centimeters) across. These types of crystals are often weathered out of a garnet-bearing mica schist and are transported by streams.

Almandine garnet: Almandine, a variety of garnet from Lount Township, Ontario, Canada. This is a granular specimen approximately 11.4 centimeters across.

Garnet as an Industrial Mineral Garnet Abrasives

The first industrial use of garnet was as an abrasive. Garnet is a relatively hard mineral with a hardness that ranges between 6.5 and 7.5 on the Mohs Scale. That allows it to be used as an effective abrasive in many types of manufacturing. When crushed, it breaks into angular pieces that provide sharp edges for cutting and sanding. Small granules of uniform size are bonded to paper to produce a reddish color sandpaper that is widely used in woodworking shops. Garnet is also crushed, screened to specific sizes, and sold as abrasive granules and powders. In the United States, New York and Idaho have been important sources of industrial garnet for abrasives.

Waterjet Cutting

The largest industrial use of garnet in the United States is in waterjet cutting. A machine known as a waterjet cutter produces a high-pressure jet of water with entrained abrasive granules. When these are directed at a piece of metal, ceramic, or stone, a cutting action can occur that produces very little dust and cuts at a low temperature. Waterjet cutters are used in manufacturing and mining.

Abrasive Blasting

Garnet granules are also used in abrasive blasting (commonly known as "sand blasting"). In these processes, a tool propels a stream of abrasive granules (also known as "media") against a surface using a highly pressurized fluid (usually air or water) as a propellant. Abrasive blasting is done in order to smooth, clean, or remove oxidation products from metals, brick, stone, and other materials. It is usually much faster than sanding by hand or with a sanding machine. It can clean small and intricate surfaces that other cleaning methods would miss. Abrasives of various hardnesses can be used to clean a surface of greater hardness, without damaging the surface.

Filtration

Garnet granules are often used as a filter media. Small garnet particles are used to fill a container through which a liquid flows. The pore spaces of the garnet are small enough to allow passage of the liquid but are too small to allow passage of some contaminant particles, which are filtered from the flow. Garnet is suited for this use because it is relatively inert and has a relatively high specific gravity. Garnet granules, crushed and graded to about 0.3 millimeters in size, can be used to filter out contaminant particles as small as a few microns in diameter. Garnet's high specific gravity and high hardness reduce bed expansion and particle abrasion during backflushing.

Garnet peridotite: Garnet peridotite from Alpe Arami, near Bellinzona, Switzerland. The material in this rock originated within Earth's mantle and was delivered to the surface through a volcanic pipe during a deep-source volcanic eruption. The garnets are the reddish purple grains within the rock. Garnets weathered from such pipes often serve as indicator minerals when exploring for volcanic pipes that might contain diamond.Public domain photo by Woudloper.

Garnet as a Geological Indicator Mineral Although most of the garnets found at Earth's surface have formed within the crust, some garnets are brought up from the mantle during deep-source volcanic eruptions. These eruptions entrain pieces of mantle rock known as "xenoliths" and deliver them to the surface in a structure known as a "pipe." These xenoliths are the source of most diamonds found at or near Earth's surface.

Diamond pipe: Simplified cross-section of a diamond pipe and residual soil deposit showing the relationships of xenoliths and diamonds with the pipe and residual soil.

Although xenoliths contain diamonds, they often contain a tremendous number of garnets for every diamond, and those garnets are generally larger in size. These deep-source garnets are very different from the garnets that form in the crust at shallow depth. So, a good way to prospect for diamonds is to look for these unique garnets. The garnets serve as "indicator minerals" for geologists exploring for diamond deposits. As the xenoliths weather, their garnets are liberated in large numbers. These unusual garnets then move downslope in soils and streams. Geologists who find them can follow the garnet trail to the source deposit. Some of the diamond pipes in Canadawere found by following a garnet trail produced by moving ice.

African garnets: African garnets of various colors: orange spessartine (Mozambique), yellow mali (Mali), red almandine (Madagascar), green tsavorite (Tanzania), and purple rhodolite (Mozambique). In the past two decades, Africa has become a major source of excellent beautiful garnets with great color and clarity.

Melanite garnet: Melanite is an opaque black garnet that is unusual to find in jewelry today. Along with jet, black chalcedony, and other black gems, melanite was often used in jewelry during the Victorian Era. These two rose-cut melanite rounds are about 9 millimeters across.

Garnets as Gemstones Garnet has been used as a gemstone for over 5000 years. It has been found in the jewelry of many Egyptian burials and was the most popular gemstone of Ancient Rome. It is a beautiful gem that is usually sold without treatment of any kind. It is also durable and common enough that it can be used in jewelry at a relatively low cost. Garnet continues as a popular gemstone today. It serves as a birthstone for the month of January and is a traditional gem given on a second anniversary. Most people will think of a red gemstone when they hear the name "garnet" because they are not aware that garnet occurs in a variety of colors. However, gem-quality garnets occur in every color - with red being the most common and blue garnets being especially rare. Red almandine is the red garnet most often found in jewelry because it is abundant and inexpensive. Pyrope and spessartine are reddish garnets that are commonly encountered in jewelry for the same reason. In recent decades, green demantoid garnet has become popular. It has a dispersion of 0.057 that gives it a "fire" which exceeds that of diamond's at 0.044. Green tsavorite has a bright, rich color that is very much like emerald. It is commonly used as an alternative stone to emerald. Both of these green garnets are becoming more popular, but their price is much higher than almandine.

Rhodolite garnet: Rhodolite, a variety of garnet, in mica schist from Jackson County, North Carolina. Specimen is approximately 8.9 centimeters across.

Andradite Garnet: Andradite, a variety of garnet, massive with wollastonite from Willsboro, New York. Specimen is approximately 6.4 centimeters across.

Glauconite Mineral Properties and Uses Physical Properties of Glauconite Chemical Classification

silicate

Color

green, blue green

Streak

dull green

Luster

earthy to dull

Diaphaneity Cleavage

transparent to translucent perfect

Mohs Hardness

2

Specific Gravity

2.4 to 3.0

Diagnostic Properties

color

Chemical Composition

(K,Na)(Fe3+,Al,Mg)2((Si,Al)4O10)(OH)2

Crystal System Uses

monoclinic fertilizer, soil amendment

This is glauconite in sandstone from Afton, Minnesota. Specimen is approximately 3-1/2 inches (8.9 centimeters) across.

Glauconite, or greensand, from Birmingham, New Jersey. These specimens are small particles 1 to 2 millimeters across.

Gold Mineral Properties and Uses

Gold nuggets from Colorado. These specimens range between three and eight millimeters across.

What is Gold? Native gold is an element and a mineral. It is highly prized by people because of its attractive color, resistance to tarnish, and its many special properties - some of which are unique to gold. Its rarity, usefulness, and desirability make it command a high price. Trace amounts of gold are found almost everywhere, but large deposits are found in only a few locations. Although there are about twenty different gold minerals, all of them are quite rare. Therefore, most gold found in nature is in the form of the native metal. Gold occurs in hydrothermal veins deposited by ascending solutions, as disseminated particles through some sulfide deposits, and in placer deposits. Physical Properties of Gold Chemical Classification

Native element

Color

Gold yellow

Streak

Gold yellow

Luster

Metallic

Diaphaneity

Opaque

Cleavage Mohs Hardness

None 2.5 to 3

Specific Gravity Diagnostic Properties Chemical Composition Crystal System

Uses

19.3

Color, hardness, streak, specific gravity

Gold, Au

Isometric Numerous uses in jewelry; coinage; bullion; currency backing; an electrical conductor used in computers, circuits, appliances, cell phones, etc.; dental work; gilding.

Vein gold: White "vein quartz" with gold from Colorado. This specimen is approximately one inch (2.5 centimeters) across.

Uses of Gold Most of the gold that is newly consumed or recycled each year is used in the production of jewelry. About 10% is used in coinage or in the financial stores of governments. The remaining 12% is consumed in a wide range of other uses which include electronics, medicine, dentistry, computers, awards, pigments, gilding, and optics. More information on the uses of gold.

Vein gold: Vein quartz with gold attached to basalt from California. This specimen is approximately 1 inch (2.4 centimeters) across.

Graphite Graphite and diamond have the same composition but completely different properties.

Graphite: Graphite crystals in a piece of marble from the Saint-Jovite Skarn Zone, Mont-Tremblant, Les Laurentides RCM, Quebec, Canada. This specimen is approximately three inches (7.6 cm) in length.

What is Graphite? Graphite is a naturally-occurring form of crystalline carbon. It is a native element mineral found in metamorphic and igneous rocks. Graphite is a mineral of extremes. It is extremely soft, cleaves with very light pressure, and has a very low specific gravity. In contrast, it is extremely resistant to heat and nearly inert in contact with almost any other material. These extreme properties give it a wide range of uses in metallurgy and manufacturing.

Flake graphite: Flake graphite produced in Madagascar.

Graphite chunk: Lump graphite from Kropfmuhl, Austria. Specimen is about one and one half inches (3.8 cm) across.

Graphite with garnet: A specimen of graphite-mica schist with two red almandine / pyrope garnets from the Red Embers Mine, Erving, Massachusetts. This specimen is about two inches (5.08 cm) across.

Geologic Occurrence Graphite is a mineral that forms when carbon is subjected to heat and pressure in Earth's crust and in the upper mantle. Pressures in the range of 75,000 pounds per square inch and temperatures in the range of 750 degrees Celsius are needed to produce graphite. These correspond to the granulite metamorphic facies. Graphite From Regional Metamorphism (Flake Graphite) Most of the graphite seen at Earth's surface today was formed at convergent plate boundaries where organicrich shales and limestones were subjected to the heat and pressure of regional metamorphism. This produces marble, schist, and gneiss that contain tiny crystals and flakes of graphite. When graphite is in high enough concentrations, these rocks can be mined, crushed to a particle size that liberates the graphite flakes, and processed by specific gravity separation or froth flotation to remove the low-density graphite. The product produced is known as "flake graphite."

Graphite From Coal Seam Metamorphism ("Amorphous" Graphite) Some graphite forms from the metamorphism of coalseams. The organic material in coal is composed mainly of carbon, oxygen, hydrogen, nitrogen, and sulfur. The heat of metamorphism destroys the organic molecules of coal, volatilizing the oxygen, hydrogen, nitrogen, and sulfur. What remains is a nearly pure carbon material that crystallizes into mineral graphite. This graphite occurs in "seams" that correspond to the original layer of coal. When mined, the material is known as "amorphous graphite." The word "amorphous" is actually incorrect in this usage, as it does have a crystalline structure. From the mine, this material has an appearance similar to lumps of coal without the bright and dull banding. Graphite from Hydrothermal Metamorphism A small amount of graphite forms by the reaction of carbon compounds in the rock during hydrothermal metamorphism. This carbon can be mobilized and deposited in veins in association with hydrothermal minerals. Because it is precipitated, it has a high degree of crystallinity, and that makes it a preferred material for many electrical uses. Graphite in Igneous Rocks and Meteorites Small amounts of graphite are known to occur as a primary mineral in igneous rocks. It is known as tiny particles in basalt flows and syenite. It is also known to form in pegmatite. Some iron meteorites contain small amounts of graphite. These forms of graphite are occurrences without economic importance. Physical Properties of Graphite Chemical

Native element

Classification Color

Steel gray to black

Streak

Black

Luster

Metallic, sometimes earthy

Diaphaneity

Opaque

Cleavage

Perfect in one direction

Mohs Hardness

1 to 2

Specific Gravity

2.1 to 2.3

Diagnostic Properties

Color, streak, slippery feel, specific gravity

Chemical

C

Composition Crystal System

Uses

Hexagonal Used to manufacture heat and chemical resistant containers and other objects. Battery anodes. A dry lubricant. The

Graphite and Diamond

"lead" in pencils.

Graphite and diamond are the two mineral forms of carbon. Diamond forms in the mantle under extreme heat and pressure. Most graphite found near Earth's surface was formed within the crust at lower temperatures and pressures. Graphite and diamond share the same composition but have very different structures. The carbon atoms in graphite are linked in a hexagonal network which forms sheets that are one atom thick. These sheets are poorly connected and easily cleave or slide over one another if subjected to a small amount of force. This gives graphite its very low hardness, its perfect cleavage, and its slippery feel. In contrast, the carbon atoms in diamond are linked into a frameworks structure. Every carbon atom is linked into a three-dimensional network with four other carbon atoms with strong covalent bonds. This arrangement holds the atoms firmly in place and makes diamond an exceptionally hard material.

Graphite consumption: United States graphite consumption by use during 2012. Data from the USGS Mineral Commodity Summary.

Synthetic Graphite "Synthetic graphite" is made by heating high-carbon materials like petroleum coke and coal-tar pitch to temperatures in the range of 2500 to 3000 degrees Celsius. At these high temperatures, all volatile materials and many metals in the feedstock are destroyed or driven off. The graphite that remains links into a sheetlike crystalline structure. Synthetic graphite can have a purity of over 99% carbon, and it is used in manufactured products where an extremely pure material is required.

Graphite in schist from Essex County, New York. Specimen is approximately 5 inches (12.7 centimeters) across.

Graphite in schist from Essex County, New York. Specimen is approximately 5 inches (12.7 centimeters) across.

Gypsum An important construction material that has been used for thousands of years

Gypsum: Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 4 inches (10 centimeters) across.

What is Gypsum? Gypsum is an evaporite mineral most commonly found in layered sedimentary deposits in association with halite, anhydrite, sulfur, calcite, and dolomite. Gypsum (CaSO4.2H2O) is very similar to Anhydrite (CaSO4). The chemical difference is that gypsum contains two waters and anhydrite is without water. Gypsum is the most common sulfate mineral.

Gypsum wallboard and plaster: Wallboard and construction plaster are the primary industrial uses of gypsum in the United States. Photo © iStockphoto / George Peters.

Uses of Gypsum Gypsum uses include: manufacture of wallboard, cement, plaster of Paris, soil conditioning, a hardening retarder in portland cement. Varieties of gypsum known as "satin spar" and "alabaster" are used for a variety of ornamental purposes; however, their low hardness limits their durability.

Physical Properties of Gypsum Chemical Classification

Sulfate

Color

Clear, colorless, white, gray, yellow, red, brown

Streak

White

Luster

Vitreous, silky, sugary

Diaphaneity Cleavage

Transparent to translucent Perfect

Mohs Hardness

2

Specific Gravity

2.3

Diagnostic Properties

Cleavage, specific gravity, low hardness

Chemical Composition

Hydrous calcium sulfate, CaSO4.2H2O

Crystal System Uses

Monoclinic Used to manufacture dry wall, plaster, joint compound. An agricultural soil treatment.

Gypsum from Michigan: Gypsum from Grand Rapids, Michigan. Specimen is approximately 4 inches (10 centimeters) across.

Alabaster Gypsum: Alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Alabaster gypsum jar: Jar made of beautiful translucent alabaster gypsum by David MacFarlane, photo © iStockphoto / David MacFarlane.

Gypsum translucency: The translucent characteristic of alabaster, a variety of gypsum, from Pomaia, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Selenite Gypsum: Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.

Gypsum from Virginia: Gypsum from North Holston, Virginia. Specimen is approximately 1-1/2 inches (3.8 centimeters) across.

Satin spar Gypsum: Satin spar, a fibrous variety of gypsum from Derbyshire, England. Specimen is approximately 3 inches (7.6 centimeters) across.

Gypsum from New York: Selenite, a variety of gypsum from Penfield, New York. Specimen is approximately 2-1/2 inches (6.4 centimeters) across.

Halite The mineral that everyone knows as "salt"

Halite: Halite from Retsof, New York. Specimen is approximately 3 inches (7.6 centimeters) across.

What is Halite? Halite is the mineral name for the substance that everyone knows as "salt." Its chemical name is sodium chloride, and a rock composed primarily of halite is known as "rock salt."

Salton Sea Halite: Halite from the Salton Sea, California. Specimen is approximately 4 inches (10 centimeters) across.

How Does Halite Form? Halite is mainly a sedimentary mineral that usually forms in arid climates where ocean water evaporates. However, many inland lakes such as the Great Salt Lake of North America and the Dead Sea between Jordan and Israel are also locations where halite is forming today. Over geologic time, several enormous salt

deposits have been formed when repeated episodes of seawater evaporation occurred in restricted basins. Some of these deposits are thousands of feet thick. When buried deeply they can erupt to form salt domes.

How is Halite Used? Salt has many uses. Most of the salt produced is crushed and used in the winter on roads to control the accumulation of snow and ice. Significant amounts of salt are also used by the chemical industry. Salt is an essential nutrient for humans and most animals, and it is also a favorite seasoning for many types of food. Salt is a mineral that everyone knows. Physical Properties of Halite Chemical Classification

Halide

Color

Colorless or white when pure; impurities produce any color but usually yellow, gray, black, brown, red

Streak

White

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent Perfect, cubic, three directions at right angles

Mohs Hardness

2.5

Specific Gravity

2

Diagnostic Properties

Chemical Composition Crystal System Uses

Cleavage, solubility, salty taste (The taste test is discouraged. Some minerals are toxic or contaminated by other people tasting them.)

NaCl

Isometric Winter road treatment, a source of sodium and chlorine for chemical processes, food preservation, seasoning

Halite structure: This diagram shows the arrangement of sodium and chloride ions in a crystal of halite.

Hematite Properties, uses, and occurrence of the most important ore of iron.

Oolitic Hematite: A specimen of oolitic hematite iron ore. Oolites are tiny round spheres of chemically precipitated hematite. The specimen in the photo is about four inches (ten centimeters) across, and the largest oolites are a few millimeters in diameter.

What is Hematite? Hematite is one of the most abundant minerals on Earth's surface and in the shallow crust. It is an iron oxide with a chemical composition of Fe2O3. It is a common rock-forming mineral found in sedimentary, metamorphic, and igneous rocks at locations throughout the world. Hematite is the most important ore of iron. Although it was once mined at thousands of locations around the world, today almost all of the production comes from a few dozen large deposits where significant equipment investments allow companies to efficiently mine and process the ore. Most ore is now produced in China, Australia, Brazil, India, Russia, Ukraine, South Africa, Canada, Venezuela, and the United States. Hematite has a wide variety of other uses, but their economic significance is very small compared to the importance of iron ore. The mineral is used to produce pigments, preparations for heavy media separation, radiation shielding, ballast, and many other products.

Hematite's Streak: All specimens of hematite will produce a reddish streak. The streak of a mineral is its color in powdered form when scraped across a streak plate (a small piece of unglazed porcelain used to produce a small amount of mineral powder). Some specimens of hematite will produce a brilliant red streak, others will produce a reddish brown streak. Care is needed when testing a specimen of hematite with a metallic luster. These specimens are often brittle and leave a trail of debris along with the streak. That debris is not a powder - it is a trail of fragments. So, to assess the streak, the loose particles must be gently shaken free from the streak plate or very lightly brushed off. This leaves behind the powder that is embedded within the textured surface of the streak plate. In the photo above, the streak on the left has been cleaned of fragments, and you can see that it is a reddish brown. The streak on the right still has a trail of glittery fragments that must be gently removed for proper evaluation.

Physical Properties of Hematite Chemical Classification

Oxide

Color

Black to steel-gray to silver; red to reddish brown to black

Streak

Red to reddish brown

Luster

Metallic, submetallic, earthy

Diaphaneity Cleavage

Opaque None

Mohs Hardness

5 to 6.5

Specific Gravity

5.0 to 5.3

Diagnostic Properties Chemical Composition Crystal System

Uses

Red streak, specific gravity

Fe2O3

Trigonal The most important ore of iron. Pigment, heavy media separation, radiation shielding, ballast, polishing compounds, a minor gemstone

Physical Properties of Hematite Hematite has an extremely variable appearance. Its luster can range from earthy to submetallic to metallic. Its color ranges include red to brown and black to gray to silver. It occurs in many forms that include micaceous, massive, crystalline, botryoidal, fibrous, oolitic, and others. Even though hematite has a highly variable appearance, it always produces a reddish streak. Students in introductory geology courses are usually surprised to see a silver-colored mineral produce a reddish streak. They quickly learn that the reddish streak is the most important clue for identifying hematite. Hematite is not magnetic and should not respond to a common magnet. However, many specimens of hematite contain enough magnetite that they are attracted to a common magnet. This can lead to an incorrect assumption that the specimen is magnetite or the weakly magnetic pyrrhotite. The investigator must check other properties to make a proper identification. If the investigator checks the streak, a reddish streak will rule out identification as magnetite or pyrrhotite. Instead, if the specimen is magnetic and has a reddish streak, it is most likely a combination of hematite and magnetite.

Specular Hematite: Specular hematite, sometimes called "micaceous hematite," has a metallic luster and appears to be a rock composed of shiny mica flakes. Instead those flakes are hematite. Even though this hematite has a silver color, it still produces a reddish streak - which is a key to hematite's identification. Hardness testing on specular hematite is difficult because the specimens tend to crumble. This specimen is about four inches across (ten centimeters) and was collected near Republic, Michigan.

Banded Iron Formation: Close-up of a banded iron formation. In this specimen, bands of hematite (silver) alternate with bands of jasper (red). The rock produced where these formations are mined is often called "taconite." This photo spans an area of rock about one foot (30 centimeters) wide. Photo taken by André Karwath, GNU Free Documentation License.

Composition of Hematite Pure hematite has a composition of about 70% iron and 30% oxygen by weight. Like most natural materials, it is rarely found with that pure composition. This is particularly true of the sedimentary deposits where hematite forms by inorganic or biological precipitation in a body of water. Minor clastic sedimentation can add clay minerals to the iron oxide. Episodic sedimentation can cause the deposit to have alternating bands of iron oxide and shale. Silica in the form of jasper, chert, or chalcedony can be added by chemical, clastic, or biological processes in small amounts or in significant episodes. These layered deposits of hematite and shale or hematite and silica have become known as the "banded iron formations" (see image).

Massive Hematite: A specimen of massive hematite about four inches across (ten centimeters) collected near Antwerp, New York.

Kidney Ore Hematite: Some hematite precipitates in cavities and has the opportunity to form an unrestricted habit. A habit known as "kidney ore" often develops in cavities and is named for its similar visual appearance to an internal organ. This type of chemically precipitated hematite is often relatively uncontaminated with sedimentary clay or host rock inclusions and has a higher purity. The high purity makes this the hematite of choice for making pigments. This specimen is about four inches across (ten centimeters) and was collected near Cumberland, England.

Geologic Occurrence Hematite is found as a primary mineral and as an alteration product in igneous, metamorphic, and sedimentary rocks. It can crystallize during the differentiation of a magma or precipitate from hydrothermal fluids moving through a rock mass. It can also form during contact metamorphism when hot magmas react with adjacent rocks. The most important hematite deposits formed in sedimentary environments. About 2.4 billion years ago, Earth’s oceans were rich in dissolved iron, but very little free oxygen was present in the water. Then a group of cyanobacteria became capable of photosynthesis. The bacteria used sunlight as an energy source to convert carbon dioxide and water into carbohydrates, oxygen, and water. This reaction released the first free oxygen into the ocean environment. The new oxygen immediately combined with the iron to form hematite, which sank to the bottom of the seafloor and became the rock units that we know today as the banded iron formations. Soon, photosynthesis was occurring in many parts of Earth’s oceans, and extensive hematite deposits were accumulating on the seafloor. This deposition continued for hundreds of millions of years - from about 2.4 to 1.8 million years ago. This allowed the formation of iron deposits hundreds to several thousand feet thick that are laterally persistent over hundreds to thousands of square miles. They comprise some of the largest rock formations in Earth’s rock record. Many of the sedimentary iron deposits contain both hematite and magnetite as well as other iron minerals. These are often in intimate association, and the ore is mined, crushed, and processed to recover both minerals. Historically, much of the hematite was not recovered and was sent to tailings piles. More efficient processing today allows more hematite to be recovered from the ore. The tailings can also be reprocessed to recover additional iron and reduce tailings volume.

Martian "Blueberries": In 2004, NASA's Mars Exploration Rover Opportunity discovered that soil near its landing site contained millions of tiny spheres that researchers nicknamed "blueberries." Upon analysis, they were determined to be composed of iron oxide, mostly in the form of hematite. The iron content of Martian rocks and soil contribute to its red appearance from Earth and helped it earn the name "The Red Planet." Image by NASA.

Hematite on Mars? NASA has discovered that hematite is one of the most abundant minerals in the rocks and soils on the surface of Mars. An abundance of hematite in Martian rocks and surface materials gives the landscape a reddish brown color and is why the planet appears red in the night sky. It is the origin of Mars' "Red Planet" nickname.

Taconite Pellets: These taconite pellets consist of finely crushed taconite rock that has been processed to improve the iron content and mixed with a small amount of clay to improve pelletization. This is one of the standard ways of shipping iron ore from a mine to a steel mill. The round particles are about 1/2 inch in diameter (1 1/4 centimeter) and are very easy to handle during shipping and at the mill. Image by Harvey Henkelmann. GNU Free Documentation License.

Uses of Hematite (Iron Ore)

Hematite is the world’s most important ore of iron. Although magnetite contains a higher percentage of iron and is easier to process, hematite is the leading ore because it is more abundant and present in deposits in many parts of the world. Hematite is mined in some of the largest mines in the world. These mines require investments of billions of dollars, and some will remove over 100 million tons of ore per year. These open-pit mines can be hundreds to thousands of feet deep and several miles across by the time they have been worked to completion. China, Australia, Brazil, India, Russia, Ukraine, South Africa, and the United States are the world’s leading producers of iron ore (includes hematite, magnetite, and other ores). Iron ore production in the United States occurs in Michigan and Minnesota.

Hematite Pigment: Hematite was one of the first pigment minerals used by people. At least 40,000 years ago, people obtained hematite, crushed it into a fine powder, and used it to make paints. Shown above are commercial hematite pigments that are available today. From top left, going clockwise, they are: Blue Ridge Hematite, Blue Ridge Violet Hematite, Venetian Red, and Pozzuoli Red. Since the Renaissance, pigments have often been named after the locations where they were produced. The color variations are a result of the type of hematite used and the impurities, such as clay and other iron oxides, that are commingled with it.

Hematite gems: Hematite and taconite are often made into tumbled stones or cut into cabochons and beads. These are popular as inexpensive jewelry items. Tumble-polished hematite is also popular as a "healing stone." Some people believe that carrying it will help relieve certain medical problems. This use has no scientific merit and can actually be harmful because it diverts people who need medical attention from seeing a doctor.

Uses of Hematite (Pigment) The name hematite is from the Greek word "haimatitis" which means "blood-red." That name stems from the color of hematite when it has been crushed to a fine powder. Primitive people discovered that hematite could be crushed and mixed with a liquid for use as a paint or cosmetic. Cave paintings, known as "pictographs," dating back to 40,000 years ago were created with hematite pigments. Hematite continues to be one of the most important pigment minerals. It has been mined at many locations around the world and has been traded extensively as a red pigment. During the Renaissance when many painters began using oils and canvas, hematite was one of the most important pigments. Hematite color was opaque and permanent. It could be mixed with a white pigment to produce a variety of pink colors that were used to paint flesh.

Uses of Hematite (Gem Material) Hematite is a minor gem material used to produce cabochons, beads, small sculptures, tumbled stones, and other items. The material used to manufacture these products is a silver-colored hematite with a solid, uniform texture. The bright silver color of hematite and its "weighty feel" make it a very popular tumbled stone.

Hematite Novelties: Products called "magnetic hematite" and "iridescent hematite" are often offered for sale in gift, tourist, novelty, and science shops and their websites. Most of the time these materials are not hematite but are man-made materials that do not even have the same chemical composition as hematite. Buy them if you like them but not because you think that you are getting a unique mineral specimen.

Uses of Hematite (Healing Stone) Some people believe that carrying pieces of tumble-polished hematite, known as "healing stones," will bring relief from certain medical problems. There is no scientific proof that this use of hematite has any positive effect beyond being a placebo. Using hematite as a "healing stone" or a "healing crystal" can actually be harmful because it diverts people from seeing a doctor who can provide proper care. Then when the person with the problem finally decides to see a doctor, their situation is more severe.

Iron Furnace: In the 1700s and 1800s, small mines in the eastern United States produced hematite which served as the primary iron ore of the region. The ore was processed by heating it by burning charcoal in simple stone furnaces. The iron ore deposits were small and difficult to exploit. When the large iron ore deposits of the Great Lakes region were discovered, iron ore was no longer mined in the eastern United States. Shown is the Vesuvius Iron Furnace of southern Ohio. USGS photo.

Other Uses of Hematite Hematite is used for a number of other purposes. It is a very dense and inexpensive material that is effective at stopping x-rays. For that reason it is used for radiation shielding around medical and scientific equipment. The low cost and high density of hematite and other iron ores also makes them useful as ballast for ships. Hematite can also be ground to a fine powder that when mixed with water will make a liquid with a very high specific gravity. These liquids are used in the "float-sink" processing of coal and other mineral material. The crushed coal, which has a very low specific gravity, is placed on the heavy liquid and the light clean coal floats, while high-specific-gravity impurities such as pyrite sink. Finally, hematite is the material used to make polishing compounds known as "red rouge" and "jeweler's rouge." Red rouge is a hematite powder used to polish brass and other soft metals. It can be added to crushed corn cob media or crushed walnut shell media for tumble-polishing brass shell casings. Jeweler's rouge is a paste used on a soft cloth to polish gold and silver jewelry.

Hornblende A common rock-forming mineral found in igneous and metamorphic rocks

Hornblende: Hornblende with a typical black granular to fibrous appearance from Faraday Township, Ontario, Canada. This specimen is approximately 3 inches (7.6 centimeters) across.

What is Hornblende? Hornblende is a field and classroom name used for a group of dark-colored amphibole minerals found in many types of igneous and metamorphic rocks. These minerals vary in chemical composition but are all double-chain inosilicates with very similar physical properties. A generalized composition for the hornblende group is shown below. (Ca,Na)2-3(Mg,Fe,Al)5(Si,Al)8O22(OH,F)2

Note that calcium, sodium, magnesium, iron, aluminum, silicon, fluorine and hydroxyl can all vary in abundance. This creates a huge number of compositional variants. Chromium, titanium, nickel, manganese, and potassium can also be part of the complex composition and further indicates the generalization of the formula given above.

Biotite hornblende granite: Hornblende is an important constituent in many igneous rocks. This piece of biotite hornblende granite is an example. Image by NASA.

Hornblende Minerals As noted above, hornblende is a name used for a number of dark-colored amphibole minerals that are compositional variants with similar physical properties. These minerals cannot be distinguished from one another without laboratory analysis. A small list of the hornblende minerals is given below with their chemical compositions.

Mineral

Chemical Composition

Edenite

Ca2NaMg5(AlSi7)O22(OH)2

Ferro-actinolite

Ca2(Fe,Mg5)(Si8O22(OH)2

Ferro-edenite

Ca2NaFe5(AlSi7)O22(OH)2

Ferro-pargasite

Ca2NaFe4Al(Al2Si6)O22(OH)2

Ferro-tschermakite

Ca2Fe3Al2(Al2Si6)O22(OH)2

Glaucophane

Na2Mg3Al2Si8O22(OH)2

Kaersutite

Ca2Na(Fe,Mg)4Ti(Al2Si6O22(OH)2

Pargasite

Ca2NaMg4Al(Al2Si6)O22(OH)2

Tremolite

Ca2(Mg,Fe5)(Si8O22(OH)2

Tschermakite

Ca2Mg3Al2(Al2Si6)O22(OH)2

Hornblende andesite: Hornblende is an important constituent in many igneous rocks. In extrusive rocks, hornblende sometimes crystallizes below the ground, in the magma, before eruption. That can produce large phenocrysts of hornblende in a fine-grained rock. This piece of hornblende andesite is an example. Image by NASA.

Hornblende as a Rock-Forming Mineral Hornblende is an important constituent in acidic and intermediate igneous rocks such as granite, diorite, syenite, andesite, and rhyolite. It is also found in metamorphic rocks such as gneiss and schist. A few rocks consist almost entirely of hornblende. Amphiboliteis the name given to metamorphic rocks that are mainly composed of amphibole minerals. Lamprophyre is an igneous rock that is mainly composed of amphibole and biotite with a feldspar ground mass.

Identification of Hornblende Hornblende minerals as a group are relatively easy to identify. The diagnostic properties are their dark color (usually black) and two directions of excellent cleavage that intersect at 124 and 56 degrees. The angle between the cleavage planes and hornblende's elongate habit can be used to distinguish it from augite and other pyroxene minerals that have a short blocky habit and cleavage angles intersecting at about 90 degrees. The presence of cleavage can be used to distinguish it from black tourmaline that often occurs in the same rocks. Identifying the individual members of the hornblende group is difficult to impossible unless a person has the skills and equipment to do optical mineralogy, x-ray diffraction, or elemental analysis. The introductory student or the beginning mineral collector can be satisfied to assign the name of "hornblende" to a specimen. Physical Properties of Hornblende Chemical Classification

Silicate

Color

Usually black, dark green, dark brown

Streak

White, colorless - (brittle, often leaves cleavage debris behind instead of a streak)

Luster

Vitreous

Diaphaneity Cleavage

Translucent to nearly opaque Two directions intersecting at 124 and 56 degrees

Mohs Hardness

5 to 6

Specific Gravity

2.9 to 3.5 (varies depending upon composition)

Diagnostic Properties

Cleavage, color, elongate habit

Chemical Composition

(Ca,Na)2–3(Mg,Fe,Al)5(Al,Si)8O22(OH,F)2

Crystal System Uses

Monoclinic Very little industrial use

Uses of Hornblende The mineral hornblende has very few uses. Its primary use might be as a mineral specimen. However, hornblende is the most abundant mineral in a rock known as amphibolitewhich has a large number of uses. It is crushed and used for highway construction and as railroad ballast. It is cut for use as dimension stone. The highest quality pieces are cut, polished, and sold under the name "black granite" for use as building facing, floor tiles, countertops, and other architectural uses. Hornblende has been used to estimate the depth of crystallization of plutonic rocks. Those with low aluminum content are associated with shallow depths of crystallization, while those with higher aluminum content are associated with greater depths of crystallization. This information is useful in understanding the crystallization of magma and also useful for mineral exploration.

Ilmenite A black iron titanium oxide mineral. The primary ore of titanium, source of titanium dioxide.

Ilmenite: A specimen of massive ilmenite from Saint-Urbain, Quebec, Canada. Massive ilmenite can be formed as a vein-filling material or during magmatic segregation. This specimen is approximately 4 inches (10 centimeters) across.

What is Ilmenite? Ilmenite is a common accessory mineral in igneous rocks, sediments, and sedimentary rocks in many parts of the world. Apollo astronauts found abundant ilmenite in lunar rocks and the lunar regolith. Ilmenite is a black iron-titanium oxide with a chemical composition of FeTiO3. Ilmenite is the primary ore of titanium, a metal needed to make a variety of high-performance alloys. Most of the ilmenite mined worldwide is used to manufacture titanium dioxide, TiO2, an important pigment, whiting, and polishing abrasive.

Heavy Mineral Sand: Shallow digging at Folly Beach, South Carolina, exposes thin layers of heavy-mineral sands. Most of the ilmenite mined today is from sands with a heavy mineral concentration. Photograph by Carleton Bern, United States Geological Survey.

Mining Heavy Minerals: Excavators remove heavy mineral sands at the Concord Mine in south-central Virginia. Weakly consolidated sands containing about 4% heavy minerals are excavated and processed to remove ilmenite, leucoxene, rutile, and zircon. The sands were weathered and eroded from an anorthocite exposure a short distance away. Photo by the United States Geological Survey.

Geologic Occurrence Most ilmenite forms during the slow cooling of magma chambers and is concentrated through the process of magmatic segregation. A large underground magma chamber can take centuries to cool. As it cools, crystals of ilmenite will begin forming at a specific temperature. These crystals are heavier than the surrounding melt and sink to the bottom of the magma chamber. This causes ilmenite and similar-temperature minerals, such as magnetite, to accumulate in a layer at the bottom of the magma chamber. These ilmenite-bearing rocks are often gabbro, norite, or anorthosite. Ilmenite also crystallizes in veins and cavities and sometimes occurs as well-formed crystals in pegmatites. Ilmenite has a high resistance to weathering. When rocks containing ilmenite weather, grains of ilmenite disperse with the sediment. The high specific gravity of these grains causes them to segregate during stream transport and accumulate as "heavy mineral sands." These sands are black in color and easily recognized by geologists. "Black sand prospecting" has long been a method of finding heavy mineral placer deposits. Most commercially produced ilmenite is recovered by excavating or dredging these sands, which are then processed to remove the heavy mineral grains such as ilmenite, leucoxene, rutile, and zircon.

Ilmenite: A specimen of massive ilmenite from Normanville, South Australia. Specimen is approximately 3 inches (7.6 centimeters) across.

Chemical Composition of Ilmenite Ilmenite's ideal chemical composition is FeTiO3. However, it often departs from that composition by containing variable amounts of magnesium or manganese. These elements substitute for iron in complete solid solution. A solid solution series exists between ilmenite (FeTiO3) and geikielite (MgTiO3). In this series, variable amounts of magnesium substitutes for iron in the mineral's crystal structure. A second solid solution series exists between ilmenite and pyrophanite (MnTiO3), with manganese substituting for iron. At high temperatures, a third solid solution series exists between ilmenite and hematite (Fe2O3).

Ilmenite: A specimen of massive ilmenite from Kragero, Norway. Specimen is approximately 4 inches (10 centimeters) across.

Black Sand Ilmenite: Ilmenite sand from Melbourne, Florida. Specimens are sand-size grains.

Physical Properties of Ilmenite Ilmenite is a black mineral with a submetallic to metallic luster. With just a glance it can easily be confused with hematite and magnetite. The differentiation is easy. Hematite has a red streak, while ilmenite has a black streak. Magnetite is strongly magnetic, while ilmenite is not magnetic. Occasionally ilmenite is weakly magnetic, possibly from small amounts of included magnetite. Physical Properties of Ilmenite Chemical Classification

Oxide

Color

Black

Streak

Black

Luster

Metallic, submetallic

Diaphaneity Cleavage

Opaque None

Mohs Hardness

5.5 to 6

Specific Gravity

4.7 to 4.8

Diagnostic Properties

Chemical Composition

Crystal System Uses

Streak; sometimes weakly magnetic. Iron titanium oxide - FeTiO3. Sometimes has significant amounts of magnesium and manganese in solid solution with the iron to yield a composition of (Fe,Mg,Mn)TiO3 Hexagonal The primary ore of titanium. A minor source of iron. Used to make titanium dioxide.

Ilmenite is usually more durable than the other minerals in the igneous rocks in which it is abundant. For that reason, the weathering debris produced during the weathering of these rocks is especially rich in

ilmenite. Its relatively high specific gravity causes it to become concentrated in placer deposits like gold, gems, and other heavy minerals.

Pigments and Polishing Compounds: Titanium dioxide powder is carefully processed to remove impurities and classified by particle size. It is then sold for use as whiting, pigments, and polishing compounds. The image is a rock tumbler barrel just opened with a thick white froth of metal oxide polish.

Lunar Ilmenite Basalt: Apollo astronauts found ilmenite-rich basalts at multiple locations on the Moon. The reference block at lower right is one cubic centimeter. Image by NASA.

Uses of Ilmenite Ilmenite is the primary ore of titanium metal. Small amounts of titanium combined with certain metals will produce durable, high-strength, lightweight alloys. These alloys are used to manufacture a wide variety high-performance parts and tools. Examples include: aircraft parts, artificial joints for humans, and sporting equipment such as bicycle frames. About 5% of the ilmenite mined is used to produce titanium metal. Some ilmenite is also used to produce synthetic rutile, a form of titanium dioxide used to produce white, highly reflective pigments. Most of the remaining ilmenite is used to make titanium dioxide, an inert, white, highly reflective material. The most important use of titanium dioxide is as a whiting. Whitings are white, highly reflective materials

that are ground to a powder and used as pigments. These pigments produce a white color and brightness in paint, paper, adhesives, plastics, toothpaste, and even food. Titanium dioxide is also used to make powders with a tightly controlled particle size range. These powders are used as inexpensive polishing abrasives in a variety of lapidary work that includes rock tumbling, lapping, cabbing, sphere making, and faceting. Titanium oxide abrasives are used in many other industries.

Lunar Ilmenite Regolith: Apollo astronauts found deposits of lunar regolith composed mostly of silt- to sand-size ilmenite (black) and mafic volcanic glass (orange). Image by NASA.

Ilmenite on the Moon Apollo astronauts found ilmenite-rich basalts at multiple locations on the Moon. Most of these basalts were extremely old, forming at least 3 billion years ago. These rocks often contained over 10% titanium dioxide (TiO2). Minerals present in these rocks were mostly feldspars and pyroxenes, with ilmenite next in abundance. Some samples of lunar regolith contained significant amounts of ilmenite. It occurred in particles ranging from fine silt to coarse sand. The ilmenite was thought to have been liberated from lunar basalts during impact events. Samples of lunar regolith collected at Shorty Crater contained a mix of volcanic glass spheres and ilmenite grains. The deposit was stratified with a bottom layer composed mostly of ilmenite and other black opaque materials. This graded upwards to an upper layer, known as "orange soil," that was composed mostly of spherical-shaped beads of orange volcanic glass with minor amounts of ilmenite. The grains were mostly less than 1/2 millimeter in size. This regolith was thought to have been produced by fountaining volcanic eruptions during the early lunar history

What is Jade? Jadeite and nephrite are materials that have both been called "jade" for thousands of years.

Green Jadeite Buttons: Hand-made, antique Chinese jadeite buttons showing the typical color of quality green jadeite. The jadeite in these buttons was most likely mined in Burma (the Union of Myanmar today). This photo was taken by Gregory Phillips and is distributed under a GNU Free Documentation License.

What is Jade? "Jade" is a cultural term used for a very durable, and often beautiful, material that has been fashioned into tools, sculptures, jewelry, gemstones, and other objects for over 5,000 years. It was first used to manufacture ax heads, weapons, and tools for scraping and hammering because of its toughness. Then, because some specimens had a beautiful color and could be polished to a brilliant luster, people started to use jade for gemstones, talismans, and ornamental objects. Although most people who think of jade imagine a beautiful green gemstone, the material occurs in a wide variety of colors that include green, white, lavender, yellow, blue, black, red, orange, and gray.

Jade Ax: A reproduction of a Mayan or Aztec ax head. Photo © iStockphoto and Stacy Brogan.

Are All Jades the Same? Originally, all jade objects were thought to be made from the same material. However, in 1863 a Frenchman, Alexis Damour, discovered that the material known as "jade" could be divided into two different minerals: jadeite and nephrite. Because these two materials can be difficult to distinguish, and because the word "jade" is so entrenched in common language, the name "jade" is still widely used across many societies, industries, and academic disciplines. In this article, the word "jade" will be used for undifferentiated materials. "Jadeite" or "nephrite" will be used when the identity of the material is known. The word "nephrite" is also an imprecise term. It is used for materials composed of the minerals actinolite and tremolite. Physical Properties: Jadeite and Nephrite Jadeite

Nephrite

Silicate - pyroxene.

Silicate - amphibole.

Usually various shades of white to dark green, sometimes gray, pink,

Usually ranges in color between white, cream, and

lilac, red, blue, yellow, orange, black, colored by impurities.

dark green.

Streak

Colorless.

Colorless.

Luster

Vitreous to sugary.

Vitreous to silky, waxy.

Translucent to opaque.

Translucent to opaque.

Chemistry

Color

Diaphaneity

Cleavage

Usually not seen because of a small grain size and splintery fracture.

Prismatic but usually not seen because of a small grain size and splintery fracture.

Mohs Hardness

6.5 to 7

5 to 6

Specific Gravity

3.3 to 3.5

3.0 to 3.3

Diagnostic Properties Chemical Formula Crystal System Uses

Color, toughness, hardness, specific gravity, grain size and habit.

Color, toughness, hardness, specific gravity, grain size and habit.

NaAlSi2O6 or Na(Al,Fe3+)Si2O6

Ca2(Mg,Fe)5Si8O22(OH)2

Monoclinic.

Monoclinic.

Jewelry, ornaments, tools, weapons, gemstones.

Jewelry, ornaments, tools, weapons, gemstones.

Jadeite, Nephrite, and Science Jadeite and nephrite have distinctly different mineral compositions. Jadeite is an aluminum-rich pyroxene, while nephrite is a magnesium-rich amphibole. However, the two minerals have very similar physical properties in the eye of the average person. Only trained observers with significant experience are able to reliably differentiate them without mineral testing equipment. This is why jadeite and nephrite were not properly distinguished by scientists until 1863.

Jade Dragon: Hand-made jade dragon from the Western Han Dynasty (202 BC - 9 AD). This photo was taken by Snowyowls and is distributed under a Creative Commons license.

Mayan jadeite: Hand-made Mayan jadeite pectoral from the Mayan Classic period. This photo was taken by John Hill and is distributed under a GNU Free Documentation License.

Jadeite, Nephrite, and Artisans China has been the leading producer of jade objects for over 5,000 years. A few hundred years ago, master Chinese craftsmen who worked with jade daily recognized that some of the jade obtained from Burma (now the Union of Myanmar) was different. It was harder, denser, worked easier, and produced a higher luster upon polishing. It gradually became the form of jade preferred by Chinese artisans and the jade most highly prized by the Chinese people. They realized this long before scientists differentiated jadeite and nephrite in 1863. Unknowingly, Chinese craftsmen had distinguished jadeite from nephrite and appreciated it enough to pay premium prices for jadeite. However, they didn't have the knowledge and equipment of chemistry and crystallography to distinguish them in a formal way. Rarely, the Chinese craftsmen encountered fine-grained jadeite with a bright translucence and a rich, uniform green color. This beautiful material was given the name "Imperial Jade" and regarded as the stone of highest quality. At that time in China, ownership of Imperial Jade was reserved only for the Emperor. Now, anyone who can afford it can own Imperial Jade. The best specimens can cost more per carat than high-quality diamonds.

Pendant of Green Nephrite: A pendant made from a green nephrite known in New Zealand as "Maori Green Stone" or "Maori Jade." Photo © iStockphoto and Steve Patterson.

Jade Treatments Waxes, dyes, bleaches, polymer impregnation, heat treatments, and other procedures are sometimes used to improve the color and luster of jadeite and nephrite to give them the appearance of the finest jade. These treatments can usually be detected in a careful examination by an experienced person using a microscope, hand lens, and ultraviolet light. An untrained person is unable to recognize most of these treatments. Sellers have an ethical obligation to accurately identify the material that they are selling and reveal any treatment that has been applied. The caution to buyers is this: If you are spending serious money for a jade object, be sure that you are buying from a knowledgeable and trusted dealer. If you don't know what you are buying, then you should pay no more for jade than you would pay for the same object made from a material with no intrinsic value.

New Zealand Greenstone: These boulders, harvested from glacial outwash on the South Island of New Zealand, were originally called "Pounamu" by the local Maori people, then "greenstone" (a literal translation) by European explorers. They are actually nephrite jade. They were used by the Maori for making tools and weapons. Pieces with an attractive color or pattern were used to make ornaments and fashioned into pendants. Public domain image by Sarang.

Early Use of Jade in Tools

People have used jade for at least 100,000 years. The earliest objects made from jade were tools. Jade is a very hard material and is used as a tool because it is extremely tough and breaks to form sharp edges. Most jade does not have a color and translucence that is expected in a gemstone. However, when early people found these special pieces of jade, they were often inspired into crafting them into a special object. "Toughness" is the ability of a material to resist fracturing when subjected to stress. "Hardness" is the ability of a material to resist abrasion. Early toolmakers took advantage of these properties of jade and formed it into cutting tools and weapons. It was used to make axes, projectile points, knives, scrapers, and other sharp objects for cutting.

Translucent Green Jade: A translucent green jade cabochon with beautiful color, set into a gold ring and surrounded by small diamonds. Photo © iStockphoto and Biggereye.

Use of Jade as a Gemstone Jade is a durable, colorful material that can be worked into shapes and given a high polish. These properties make it a very desirable gemstone. Jade has been used to make a variety of jewelry items such as pendants, necklaces, rings, bracelets, earrings, beads, cabochons, tumbled stones, and other items. These jewelry items are often made of solid jade, combined with other gems, or placed in settings made from gold, silver, or other precious metals. In addition to jewelry, jade is used to make small sculptures, ornaments, religious art, and small functional objects.

Commonly Confused With Jade: Pictured here are four gems commonly confused with jade. From top left and going clockwise they are chrysoprase, maw sit sit, serpentine, and hydrogrossular garnet. They have a color, luster, and translucence that is known to occur in jade.

Maw Sit Sit is a rock composed of jadeite, albite, and kosmochlor (a mineral related to jadeite). It has a bright chrome-green color and accepts a bright polish. For those reasons it is used as a gemstone. Maw sit sit was first properly identified in 1963 near the village of Maw Sit Sit in northwestern Burma in the foothills of the Himalayas. This is the only location where it has been discovered to date. It is used to cut cabochons and produce small sculptures. Because of its scarcity and low production, it is rarely seen in jewelry.

Other Materials Confused With Jade A number of other minerals and materials that are commonly cut and polished are easily confused with jade. All of these materials can have a color, luster, and translucence that is very similar to jade - so similar that the average person is unable to recognize them. These materials are often used to manufacture cabochons, beads, and other objects in the same style as jade. They sometimes enter the jade market without distinction. Chalcedony is a translucent variety of microcrystalline quartz that occurs in a range of colors similar to jade.Chrysoprase is a bright green chalcedony colored by chromium that, when cut into cabochons, beads, and small sculptures, will look very similar to jade. Chalcedony occurs in a variety of other translucent colors such as black, lavender, yellow, and orange that can look like the color varieties of jade. Chalcedony

can be a very close gemstone look-alike with jade. It can be differentiated from jade using is lower specific gravity and by a variety of instrumental methods. Serpentine occurs in a variety of wonderful translucent to nearly transparent green and yellowish green colors that look very much like jade. It is a metamorphic mineral that is often found in the same geographic areas and same types of rocks as jade. Serpentine is significantly softer than jade and also has a much lower specific gravity. Vesuvianite, also known as idocrase, is another jade look-alike that is very difficult to distinguish from jade without laboratory testing. It has similar hardness, specific gravity, and physical appearance. It is not nearly as tough as jade and will break easier - but that requires destruction of the specimen. Maw Sit Sit is a rock with a bright chrome-green color mined in Myanmar. It has a very similar appearance to jade. Maw sit sit is composed of jadeite, albite, and kosmochlor (a mineral related to jadeite). It is used to cut cabochons, beads, and make small sculptures, and is easily confused with jade. Hydrogrossular Garnet is a green massive variety of garnet that is usually green in color with black markings. It looks so much like jade that in South Africa, where it is common, it is known as "Transvaal Jade." It is frequently cut into beads, cabochons, and small sculptures. Aventurine is a trade name used for a green quartz that is often colored by fuchsite inclusions. These typically color the quartz a light to dark green color and produce some aventurescent sparkle. Aventurine is sometimes confused with jade. All of the above natural minerals and rocks can be confused with jade. Many people like them, enjoy them, and knowingly purchase them for that reason. It is important to know that these jade look-alikes, along with plastic and glass made into objects in the same style as jade, are abundant in the market place. Know what you are buying or purchase from a dealer you can trust if you are shopping for these items and desire jade instead of an alternative. Errors and deception are common.

British Columbia jade cabochons: A pair of translucent cabochons cut from bright green British Columbia nephrite. Approximately 10 x 12 millimeters in size. Today, tons of jade are mined in British Columbia and shipped to China, where both demand and prices are higher than in western Canada.

Geography of Jade Most people immediately think of China as the source of jade and jade objects. China has always been an important producer of jade, a leading jade cutting center, jade consumer, and jade market. The only time

dominance in any of these activities moved outside of China was between World War II and the early 1980s. At that time the Chinese government suppressed jade commerce, and Hong Kong temporarily became the center of jade commerce. Jade jewelry and jade artwork are extremely important in China. Jade is more important in China than the importance of diamonds in the United States. Per-carat prices for the best imperial jade in China rival the per-carat price paid for diamonds in the United States. Since prehistoric times, jade has been used to make tools, weapons, and important ornamental objects in Asia, Europe, Australia, the Americas, and numerous Pacific islands. The toughness of jade made it an excellent material for making tools and weapons. Because of its beauty, people held jade in highest esteem and used it to make religious art and ornaments for their rulers. None of these ancient cultures had contact with one another, yet they all independently used jade for many of their most sacred and important objects. Such is the appeal of jade.

Green Jadeite Boulder: An alluvial boulder of green jadeite with a brown weathering rind from northern Burma. This boulder is about 6 centimeters across. Photo by James St. John, used here under a Creative Commons license.

Lavender Jadeite Boulder: An alluvial boulder of lavender jadeite with a brown weathering rind from northern Burma. This boulder is about 18 centimeters across. Photo by James St. John, used here under a Creative Commons license.

Geologic Occurrence and Jade Prospecting Jadeite and nephrite are minerals that form through metamorphism. They are mostly found in metamorphic rocks associated with subduction zones. This places most jadeite and nephrite deposits along the margins of current or geologically ancient convergent plate boundaries involving oceanic lithosphere. Jadeite is typically found in rocks that have a higher pressure origin than nephrite. This normally causes a geographic separation of jadeite and nephrite deposits. From ancient times, much of the prospecting for jade has been done in the steeper parts of drainage basins, where pebble- to boulder-size pieces of rocks are found in stream valleys. Boulders and pebbles of jade normally have a brown weathering rind that hides their inner beauty and potential value. Prospectors search these valleys looking for jade boulders. Small windows are often cut into the boulders in the field to assess the material's quality and to determine if it is worth the labor of transport. Jade boulders can be very difficult to transport without damage. Human and animal labor was the only way to transport them historically. Today in some areas that is still the only way to move the boulders to market. Where economics allow, a helicopter with a basket or sling on a cable will fly in to difficult areas. Workers on the ground will load jade boulders, and the helicopter will lift them out. Although helicopters are very expensive to use for this type of work, one nice boulder can be worth many thousands of dollars or more in rough form. Some jade is also mined from hard rock deposits. Boulders are sometimes mined from ancient conglomerates, but ophiolite exposures are the most important type of hard rock deposit. Ophiolites are the metamorphosed rocks of ancient subduction zones, now exposed at the surface by faulting or uplift, followed by exhumation by weathering. Jade is mined from ophiolites by both surface and underground methods. Geographically, much of the world's jade is found around the rim of the Pacific Ocean, where subduction transports large slabs of oceanic lithosphere beneath continents and volcanic island arcs. This accounts for much of the jade found in South America, Central America, the United States, Canada, eastern Asia, and New Zealand. Perhaps the most attractive and valuable jade found in the United States is from the area around Jeffrey City and Crooks Gap in Wyoming. There, nephrite jade is found by prospecting alluvial sediments, looking for jade in stream-rounded pebble- to boulder-size pieces.

Wyoming Jade: A nice oval cabochon cut from Wyoming jade. This cabochon was cut from a thin slab of jade only a few millimeters thick to conserve material and produce a translucent stone.

Social Importance of Jade In the United States and Europe, diamonds, rubies, sapphires, emeralds, opals, garnets, and a few other gems are much more popular than jade. Jade is not thought to be as precious in these regions as it is in China. The Chinese have a much higher regard for jade than any other people. For thousands of years, jade has been the most popular gemstone in China. Chinese emperors desired excellent specimens of jade, and they traded or waged war with distant people to acquire them. In China, gifts made from jade are given at almost every important station in life, such as birthdays, anniversaries, marriages, and other celebrations. It is also a commonly used material for producing religious art. China is the country where the importance of jade is the highest. Contributor: Hobart King

Kyanite A mineral used to make porcelain, as an abrasive, and occasionally as a gemstone

Blue kyanite crystals: A very common habit of kyanite is blue bladed crystals. Image by Aelwyn, Creative Commons license.

What is Kyanite? Kyanite is a mineral found mainly in metamorphic rocks. It most often forms from the high-pressure alteration of clay minerals during the metamorphism of sedimentary rocks. It is found in the schists and gneisses of regionally metamorphosed areas and less often in quartzite or eclogite. Kyanite's typical habit is a bladed crystal, although it sometimes occurs as radiating masses of crystals. Kyanite is often associated with other metamorphic minerals such as garnet, staurolite, and corundum.

Radiating kyanite: Sometimes kyanite occurs as radiating masses of crystals such as this specimen from Petaca, New Mexico. Specimen is about 4 inches (ten centimeters) across.

Kyanite's Unusual Hardness Kyanite specimens have a variable hardness. The long crystals have a Mohs hardness of about 4.5 to 5 if tested parallel to the length of a crystal, and a hardness of 6.5 to 7 if tested across the short dimension of a crystal. The mineral was once commonly called "disthene" which means "two strengths." Physical Properties of Kyanite Chemical

Silicate

Classification Color

Blue, white, gray, green, colorless

Streak

White, colorless

Luster

Vitreous, pearly

Diaphaneity

Transparent to translucent

Cleavage

Mohs Hardness

Perfect in two directions, faces sometimes striated Kyanite often occurs in long, bladed crystals. These have a hardness of 4.5 to 5 along the length of the crystals and 6.5

Specific Gravity Diagnostic Properties Chemical Composition Crystal System Uses

to 7 across the width of the crystals. 3.5 to 3.7

Color, cleavage, bladed crystals

Al2SiO5

Triclinic Ceramics, gemstones

Polymorphs of Al2SiO5 Three minerals have a chemical composition of Al2SiO5. These are kyanite, andalusite, and sillimanite. Kyanite is the high-pressure polymorph, sillimanite forms at high temperature, and andalusite is the lowpressure polymorph.

Kyanite porcelain sink: Kyanite is used in the porcelain of sanitary fixtures. © iStockphoto / Carl Kelliher.

Many Industrial Uses of Kyanite Kyanite is used to manufacture a wide range of products. An important use is in the manufacture of refractory products such as the bricks, mortars, and kiln furniture used in high-temperature furnaces. For foundries, the molds that are used for casting high-temperature metals are often made with kyanite. Kyanite is also in products used in the automotive and railroad industries where heat resistance is important. Mullite, a form of calcined kyanite, is used to make brake shoes and clutch facings.

Kyanite spark plug: The porcelain insulator on this spark plug was made with kyanite. © iStockphoto / Juergen Barry.

Use in High-Refractory-Strength Porcelain Kyanite has properties that make it exceptionally well suited for the manufacture of a high-refractorystrength porcelain - a porcelain that holds its strength at very high temperatures. A familiar use of this type of porcelain is the white porcelain insulator on a spark plug. Kyanite is also used in some of the more common forms of porcelain, such as those used to make dentures, sinks, and bathroom fixtures.

Kyanite cutting wheel: Kyanite is used as a heat-resistant binding medium in cutting tools and grinding wheels. © iStockphoto / Ron Sumners.

Use in Abrasive Products Kyanite's heat resistance and hardness make it an excellent material for use in the manufacture of grinding wheels and cutting wheels. It is not used as the primary abrasive; instead, it is used as part of the binding agent that holds the abrasive particles together in the shape of a wheel.

Expansion of Kyanite When Heated Kyanite, unlike most other minerals, can expand significantly when heated. Depending upon particle size, temperatures, and heating conditions, kyanite can expand to up to twice its original volume when heated. This expansion is predictable. In the manufacture of certain refractory products, specific amounts of kyanite are added to the raw material (which shrinks during heating) to maintain volume in the finished product.

Kyanite cabochons: Kyanite is often cut "en cabochon" or as a faceted gemstone. Shown above are kyanite cabochons ranging in color from clear to blue to green and black.

Kyanite Use as a Gemstone Kyanite is a gemstone that you will rarely encounter in the typical jewelry store. Most people have not heard of kyanite, as it is infrequently used in jewelry. It is an "exotic" gem. Perhaps that is what makes it so interesting? If you are interested in kyanite as a gemstone or in jewelry, the best place to find it is in artisan jewelry stores or in jewelry stores that are associated with a mineral dealer. The people who own these businesses are likely to be interested in kyanite and incorporate it into their product line. High-quality and nicely colored kyanite can be cut into attractive and desirable cabochons and faceted stones. These are often used in rings, earrings, pendants, and other jewelry. Kyanite is also used to make beads. These beads often have a flat geometry because the mineral typically occurs in thin blades.

Faceted kyanite: A faceted kyanite gemstone with a beautiful deep blue color.

Kyanite Gemstones are Challenging to Cut Kyanite is a challenging mineral to cut because it has two distinctly different hardnesses. Kyanite crystals are typically long, narrow blades. They have a hardness of about 4.5 parallel to their length but a hardness of 6.5 to 7.0 across the width of the blade. Skilled cutters are needed to work these stones.

Green kyanite crystals: Green kyanite blades in quartzite from Avery County, North Carolina. Specimen is about four inches (ten centimeters) across.

Blue Kyanite - Green Kyanite Most gemstone-quality kyanite is blue in color. However, kyanite can be clear, green, black, and rarely purple. Some kyanite gemstones are pleochroic (appear to be different colors when viewed from different directions). Blue kyanite stones can be found in a continuous color range between clear and dark blue. The most popular kyanite gemstones are transparent with a deep sapphire-blue color. Some deep blue stones are shown in the photos on this page. Transparent blue kyanite with a lower color intensity might look like blue topaz or blue aquamarine. Contributor: Hobart King

Green kyanite crystals: Green bladed kyanite (same specimen as above) - looking down the long axis of the blades. Specimen is about 4 inches (10 centimeters) across.

Limonite An amorphous iron oxide used as an ore of iron and as a pigment for thousands of years.

Limonite: A specimen of iridescent, botryoidal limonite from Guangxi, China. Specimen is approximately 15 x 9 x 5 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Limonite pseudomorph: A pseudomorph of limonite after pyrite that preserves the original cubic habit of the pyrite with its striations. Limonite often replaces pyrite crystals and other materials. This specimen is approximately 4.2 x 3.5 x 3.3 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

What is Limonite? Before modern mineral analysis, the name "limonite" was given to many of the yellowish to yellowish brown iron oxides produced during the weathering of iron-bearing rocks or deposited as bog, lake, and shallow marine sediments.

Researchers who studied "limonite" discovered that it is amorphous and has a variable composition. It often contains significant amounts of iron oxide minerals such as goethite and hematite. This research revealed that the material called "limonite" does not meet the definition of a mineral. Instead, limonite is a mineraloid composed mainly of hydrous iron oxides that are often found in intimate associations with iron minerals. Today the word "limonite" is used as a field and classroom term for these materials because they cannot be identified in hand specimens and their identity is unknown without laboratory testing. The time and expense required to do this testing is generally not needed, unless the material is going to be used in industry or it is the subject of a detailed study. Thus the name "limonite" is not obsolete; it is still meaningful and useful.

Geologic Occurrence Limonite usually occurs as a secondary material, formed from the weathering of hematite, magnetite, pyrite, and other iron-bearing materials. Limonite is often stalactitic, reniform, botryoidal, or mammillary in habit rather than crystalline. It also occurs as pseudomorphs and coatings on the walls of fractures and cavities. Some limonite is found in stratified deposits where hydrous iron oxides form as precipitated sediment on the floor of shallow swamps, lakes, and marine environments. These can be of inorganic or biogenic origin. Limonite often forms as a precipitate at springs and mine openings where acidic, iron-laden waters emerge from the subsurface. Most subsurface waters contain very little oxygen, and when they discharge to the surface, they often encounter oxygenated waters. Dissolved metals in the groundwater rapidly combine with the dissolved oxygen of the surface water to form a precipitate that falls onto the bed of the stream. This precipitate is a characteristic sign of acid mine drainage. Limonite is very resistant to weathering and often accumulates as a residual deposit. It is often the main form of iron and colorant in lateritic soils.

Limonite staining laterite soil: A profile of laterite soil heavily stained by limonite from Parque Nacional la Mensura, Cuba. USGS photo by Paul Golightly.

Physical Properties of Limonite Chemical Classification Color

Amorphous, mineraloid Yellowish brown to brown to black

Streak

Yellowish brown

Luster

Dull to earthy

Diaphaneity Cleavage

Opaque Does not cleave because it has an amorphous structure.

Mohs Hardness

1 to 5 (weathered material can be deceptively soft)

Specific Gravity

2.7 to 4.3 (varies due to impurities)

Diagnostic Properties

Variable - can be yellow-brown, brown, reddish brown

Chemical Composition

A hydrated iron oxide of variable composition

Crystal System Uses

Amorphous to cryptocrystalline Ocher pigments, a minor ore of iron

Limonite: Limonite from Newport, New York. This specimen is approximately 6.4 centimeters across.

Limonite with goethite: Massive limonite with goethite from Ironton, Minnesota. This specimen is approximately 6.4 centimeters across.

Uses of Limonite Limonite has been used by people since prehistoric times. Their first use of limonite was probably as a pigment. It is found in many Neolithic pictographs, and throughout history it has been one of the most important pigments for creating paints in the yellow to brown color range known as ocher. Its use as a pigment continues today. It can sometimes be used directly from the deposit with minimal processing, but it is often heat treated to drive off water, simplify the production of a powder, and improve color.

Limonite pigment: Several colors of limonite pigment. They are clockwise from top left: lemon ocher, yellow ocher, orange ocher, and brown ocher. These pigments were prepared by grinding limonite to a fine powder. They are mixed with oil to produce a pigment of the desired consistency and mixed with one another, or other pigments, to produce an infinite number of other hues.

Limonite has been used as a low-quality iron ore for thousands of years. Commercial mining of limonite as a source of iron is no longer done in areas where reasonable deposits of hematite and magnetite are present or readily imported. Limonite deposits are usually too small and too impure for use in modern metallurgy. Names such as "brown iron," "brown hematite," "bog iron," and "brown ocher" have been used by miners to relate limonite with its potential uses. Their use has declined significantly, and the name "limonite" is now used for these various materials. Contributor: Hobart King

Magnesite Mineral Properties and Uses Physical Properties of Magnesite Chemical Classification

carbonate

Color

white, grayish, yellowish, brownish, colorless

Streak

white

Luster

vitreous

Diaphaneity Cleavage

transparent to translucent perfect

Mohs Hardness

3.5 to 5.0

Specific Gravity

3.0 to 3.2

Diagnostic Properties Chemical Composition Crystal System Uses

Magnesite from Chewelah, Washington. Specimen is approximately 3-1/2 inches (8.9 centimeters) across.

dissolves with warm HCl in the powdered form MgCO3 hexagonal refractory bricks, cement

Magnesite from Chewelah, Washington. Specimen is approximately 21/2 inches (6.4 centimeters) across.

Magnesite from Riverside County, California. Specimen is approximately 4 inches (10 centimeters) across.

Magnetite and Lodestone The primary ore of iron, a mineral used in heavy media separation, and a recorder of Earth magnetism

Magnetite: A typical magnetite specimen exhibiting a gray metallic luster. This specimen is approximately 10 centimeters across.

What is Magnetite? Magnetite is a very common iron oxide (Fe3O4) mineralthat is found in igneous, metamorphic, and sedimentaryrocks. It is the most commonly mined ore of iron. It is also the mineral with the highest iron content (72.4%).

Identification of Magnetite Magnetite is very easy to identify. It is one of just a few minerals that are attracted to a common magnet. It is a black, opaque, submetallic to metallic mineral with a Mohs hardness between 5 and 6.5. It is often found in the form of isometric crystals. It is the most strongly magnetic mineral found in nature.

Lodestone: A specimen of lodestone that has attracted numerous tiny particles of iron. This specimen is approximately 10 centimeters across.

Magnetite Crystals: Octahedral crystals are a common habit of magnetite. They are often seen in igneous and metamorphic rocks and sometimes seen in sediments near the magnetite source area. The magnetite crystals in this photo are about eight to twelve millimeters in maximum dimension.

Magnetite as "Lodestone" Normal magnetite is attracted to a magnet, but some specimens are automagnetized and have the ability to attract small pieces of iron, small pieces of magnetite, and other magnetic objects. This form of magnetite, known as "lodestone," was man's first encounter with the property of magnetism. Lodestone is easily identified because it is usually covered with small particles of magnetite and other magnetic minerals (see photo). Pieces of lodestone suspended on a string served as the first magnetic compasses and were used in China as early as 300 BC. When freely suspended on a string, a small piece of lodestone will align itself with Earth's magnetic field. Physical Properties of Magnetite Chemical Classification

Oxide

Color

Black to silvery gray

Streak

Black

Luster

Metallic to submetallic

Diaphaneity Cleavage

Opaque None

Mohs Hardness

5 to 6.5

Specific Gravity

5.2

Diagnostic Properties

Strongly magnetic, color, streak, octahedral crystal habit.

Chemical Composition

Fe3O4

Crystal System Uses

Isometric The most important ore of iron. Heavy media separation. Studies of Earth's magnetic field.

Taconite pellets: These red spheres are taconite pellets that are ready to ship to a steel mill. The pellets are approximately 10 millimeters in diameter. Creative Commons photo by Harvey Henkelmann.

Use of Magnetite as an Ore of Iron Most of the iron ore mined today is a banded sedimentary rock known as taconite that contains a mixture of magnetite, hematite, and chert. Once considered a waste material, taconite became an important ore after higher grade deposits were depleted. Today's commercial taconites contain 25 to 30% iron by weight. At the mine site, the taconite ore is ground to a fine powder, and strong magnets are used to separate magnetically susceptible particles containing magnetite and hematite from the chert. The concentrate is then mixed with small amounts of limestone and clay, then rolled into small round pellets. These pellets are easy to handle and transport by ship, rail, or truck. They can be directly loaded into a blast furnace at a mill and be used to produce iron or steel.

Use of Magnetite as a Heavy Media Powdered magnetite is often mixed with a liquid to produce a thick, high-density slurry that is used for specific gravity separations. Much of the high-sulfur coal that is mined in the eastern United States is floated across a slurry of magnetite. Clean coal particles have a low specific gravity and float on the slurry. Particles contaminated with pyrite (a sulfide mineral with a high specific gravity) sink into the high-density slurry.

Magnetite sand: Some beach and river sands contain high concentrations of magnetite. Magnetite-rich "black sands" are commonly encountered by people panning for gold. Although magnetite sands and other heavy mineral accumulations are common, they are infrequently developed as mineral deposits because their size or grade is inadequate. The pile in the photo is approximately four inches (10 centimeters) across.

Use of Magnetite as an Abrasive The abrasive known as "emery" is a natural mixture of magnetite and corundum. Some synthetic emery is produced by mixing magnetite with aluminum oxide particles. The production of synthetic emery gives the manufacturer control over the particle size and the relative abundance of aluminum oxide and magnetite in the product. Some finely ground magnetite is also used as an abrasive in waterjet cutting. In the past few decades, synthetic abrasives have filled many of the applications where magnetite was previously used.

Other Uses of Magnetite Small amounts of magnetite are also used as a toner in electrophotography, as a micronutrient in fertilizers, as a pigment in paints, and as an aggregate in high-density concrete.

Magnetite and Earth's Magnetic Field Tiny crystals of magnetite are present in many rocks. In the crystallization of an igneous rock, tiny crystals of magnetite form in the melt, and because they are magnetic, they orient themselves with the direction and polarity of Earth's magnetic field. This preserves the orientation of Earth's magnetic field within the rock at the moment of crystallization. Today, geologists can study the magnetic properties of rocks of various age and reconstruct the history of change in Earth's magnetic field. This information is available for multiple locations on multiple continents. It can also be used to learn about the movement of continents over time. A similar orientation of tiny magnetite grains occurs in the settling of sediment particles, locking clues to Earth's magnetic history into some sedimentary rocks. Contributor: Hobart King

Malachite Used as an ore of copper, a pigment, a gemstone, and a sculptural material for thousands of years.

Malachite Gemstones: A malachite cabochon (30x40 millimeter) and a malachite puffed heart, both cut from rough mined in the Democratic Republic of the Congo. This oval cabochon shows the agate-like banding in various shades of green that is typical of malachite. The puffed heart shows concentric structures.

What is Malachite? Malachite is a green copper carbonate hydroxide mineralwith a chemical composition of Cu2(CO3)(OH)2. It was one of the first ores used to produce copper metal. It is of minor importance today as an ore of copper because it is usually found in small quantities and can be sold for higher prices for other types of use. Malachite has been used as a gemstone and sculptural material for thousands of years and is still popular today. Today it is most often cut into cabochons or beads for jewelry use. Malachite has a green color that does not fade over time or when exposed to light. Those properties, along with its ability to be easily ground to a powder, made malachite a preferred pigment and coloring agent for thousands of years.

Botryoidal Malachite: Close-up of botryoidal malachite in a seafoam green color from Bisbee, Arizona. This view spans an area of the specimen about 5 millimeters wide and high. Specimen and photo by Arkenstone / www.iRocks.com.

Where Does Malachite Form? Malachite is a mineral that forms at shallow depths within the Earth, in the oxidizing zone above copper deposits. It precipitates from descending solutions in fractures, caverns, cavities, and the intergranular spaces of porous rock. It often forms within limestone where a subsurface chemical environment favorable for the formation of carbonate minerals can occur. Associated minerals include azurite, bornite, calcite, chalcopyrite, copper, cuprite, and a variety of iron oxides. Some of the first malachite deposits to be exploited were located in Egypt and Israel. Over 4000 years ago, they were mined and used to produce copper. Material from these deposits was also used to produce gemstones, sculptures, and pigments. Several large deposits in the Ural Mountains of Russia were aggressively mined, and they supplied abundant gem and sculptural material in the 1800s. Very little is produced from these deposits today. Much of the malachite entering the lapidary market today is from deposits in the Democratic Republic of the Congo. Smaller amounts are produced in Australia, France, and Arizona.

Stalactitic malachite: A specimen of stalactitic malachite from the Kasompi Mine, Democratic Republic of the Congo. The specimen is approximately 21 x 16 x 12 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Malachite Chemical

Carbonate

Classification Color

Green

Streak

Green

Luster

Diaphaneity Cleavage

Rare crystals are vitreous to adamantine. Fibrous specimens are silky. Massive specimens are dull to earthy. Polishes to a very bright luster. Most specimens are opaque. Crystals are translucent. Perfect in one direction. Fair in a second direction.

Mohs Hardness

3.5 to 4.0

Specific Gravity

3.6 to 4.0

Diagnostic Properties

Green color, soft, effervesces with dilute HCl to produce a green liquid.

Chemical

Cu2(CO3)(OH)2

Composition Crystal System Uses

Monoclinic A minor ore of copper. Gemstones, small sculptures, pigment.

Physical Properties of Malachite Malachite's most striking physical property is its green color. All specimens of the mineral are green and range from a pastel green, to a bright green, to an extremely dark green that is almost black. It is typically found as stalactites and botryoidal coatings on the surfaces of underground cavities - similar to the deposits

of calcite found in caves. When these materials are cut into slabs and pieces, the sawn surfaces often exhibit banding and eyes that are similar to agate. Malachite is rarely found as a crystal, but when found, the crystals are usually acicular to tabular in shape. The crystals are bright green in color, translucent, with a vitreous to adamantine luster. Non-crystalline specimens are opaque, usually with a dull to earthy luster. Malachite is a copper mineral, and that gives malachite a high specific gravity that ranges from 3.6 to 4.0. This property is so striking for a green mineral that malachite is easy to identify. Malachite is one of a small number of green minerals that produces effervescence in contact with cold, dilute hydrochloric acid. It is also a soft mineral with a Mohs hardness of 3.5 to 4.0.

Painting with Malachite: Pietro Perugino (c. 1446-1523) used malachite pigment when painting the green garment colors in his Nativity (c. 1503). He used "Verona green earth" pigment for the grass. The deeper green color of malachite gave the garments a contrasting and more vivid appearance.

Malachite Pigment: A photograph looking down into a jar of malachite pigment. This pigment was produced from malachite mined near the city of Nizhniy Tagil, in the Ural Mountains of Russia. It has a particle size of 20 microns. We obtained this pigment from NaturalPigments.com.

Malachite as a Pigment Malachite has been used as a pigment for thousands of years. It was one of the oldest known green pigments to be used in paintings. The mineral malachite is an excellent material for producing a powdered pigment because it can easily be ground into a fine powder, it mixes easily with vehicles, and it retains its color well when exposed to light over time. Alternative names for malachite pigment include copper green, Bremen green, Olympian green, green verditer, green bice, Hungarian green, mountain green, and iris green. Malachite pigment is found in the paintings of Egyptian tombs and in paintings produced throughout Europe during the 15th and 16th centuries. Its use declined significantly in the 17th century as alternative green colors were developed. Today, malachite pigment is sold by a few manufacturers who specialize in providing materials to painters who practice historically accurate techniques.

Azurmalachite: Cabochons of azurmalachite showing nice patterns of azurite (blue) and malachite (green). They were cut from material produced at the Morenci Mine in Arizona. These cabs were cut from thin vein material and have a natural wall-rock backing. Both cabs are about 25 millimeters tall.

Banded Malachite: Two views of a specimen of botryoidal malachite - one external and one internal polished surface. This photo pair shows how agate-like bands and eyes of malachite occur beneath a botryoidal structure. This specimen was collected near Katanga, Democratic Republic of the Congo. Photograph by Didier Descouens. Used here under a Creative Commons License.

Malachite as a Gem Material The vivid green color, bright polished luster, banding and eyes of malachite make it very popular as a gemstone. It is cut into cabochons, used to produce beads, sliced into inlay material, sculpted into ornamental objects, and used to manufacture tumbled stones. Small boxes made from slices of malachite are attractive and popular. Some of the most spectacular gem-quality malachite involves intergrowths, inclusions, and admixtures of malachite with other copper minerals such as azurite(azurmalachite), chrysocolla, turquoise, and pseudomalachite (eilat stone). Malachite's use as gem and ornamental stone is limited by its properties. It has perfect cleavage and a Mohs hardness of 3.5 to 4. These limit its use to items that will not suffer abrasion and impact. It is also sensitive to heat and reacts with weak acids. These properties further limit its use and require care during cleaning, repair, and maintenance. Malachite is sometimes treated with wax to fill small voids and improve its luster. Synthetic malachite has been produced and used to make jewelry and small sculptures. Poorly done synthetics are often recognized by their unnatural color. The better synthetics can usually be recognized because their banding and eyes do not have a natural geometry. An experienced person can identify most of the synthetic and imitation materials on sight.

Molybdenite Mineral Properties and Uses What is Molybdenite? Molybdenite is a molybdenum sulfide mineral and the leading ore of molybdenum. It occurs as an accessory mineral in some granites and pegmatites. It is also found in some copper porphyry deposits, contact metamorphic rocks and high temperature vein deposits. It is a soft gray mineral that is easily confused with graphite.

Physical Properties of Molybdenite Chemical Classification Color

sulfide lead gray

Streak

bluish gray grayish black

Luster

metallic

Diaphaneity

opaque

Cleavage

perfect

Mohs Hardness

1 to 2

Specific Gravity

4.6 to 4.8

Diagnostic Properties Chemical Composition Crystal System Uses

Molybdenite from Whitehall, Montana. Specimens are approximately 1/2 inch to 1 inch (1.3 centimeters to 2.5 centimeters) across.

greasy feel, color, streak, specific gravity molybdenum sulfide, MoS2 hexagonal primary ore of molybdenum, specialty libricant Molybdenite in quartz from Climax, Colorado. Specimen is approximately 4 inches (10 centimeters) across.

Molybdenite in quartz from Calaveras County, California. Specimen is approximately 4 inches (10 centimeters) across

Monazite A rare phosphate mineral mined from placer deposits for its rare earth and thorium content.

Monazite sand: Monazite sand with a resinous luster from Malaysia. Monazite is produced from heavy-mineral concentrates and then separated out by specific gravity, magnetic processes, and other processes. Specimens are sand-grain size particles.

What is Monazite? Monazite is a rare phosphate mineral with a chemical composition of (Ce,La,Nd,Th)(PO4,SiO4). It usually occurs in small isolated grains, as an accessory mineral in igneous and metamorphic rocks such as granite,pegmatite, schist, and gneiss. These grains are resistant to weathering and become concentrated in soils and sediments downslope from the host rock. When in high enough concentrations, they are mined for their rare earthand thorium content.

Monazite crystal: An exceptionally large monazite crystal, approximately two inches across, collected in Brazil. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Monazite Chemical

Phosphate

Classification Color

Yellowish to reddish brown, greenish

Streak

White

Luster

Resinous, waxy, vitreous

Diaphaneity

Translucent

Cleavage

Good to poor

Mohs Hardness

5 to 5.5

Specific Gravity

4.6 to 5.4 (varies greatly depending upon rare earth type and concentration)

Diagnostic Properties

Specific gravity

Chemical

(Ce,La,Nd,Th)PO4

Composition Crystal System

Uses

Monoclinic An important source of thorium, cerium, and other rare elements. Often mined as a byproduct from heavy mineral deposits.

A Mineral or a Mineral Group? The generic chemical formula for monazite, (Ce,La,Nd,Th)(PO4,SiO4), reveals that cerium, lanthanum, neodymium, and thorium can substitute for one another in the mineral's structure; and, substitution of silica for phosphate also occurs. Monazite is part of several solid-solution series with other minerals. "Monazite" is also the name of a group of monoclinic phosphate and arsenate minerals that share traits of composition and crystal structure. A list of minerals in the monazite group is provided below. Note that several varieties of monazite are included.

Monazite Mineral Group Mineral

Chemical Composition

Brabantite

CaTh(PO4)2

Cheralite

(Ca,Ce,Th)(P,Si)O4

Gasparite-(Ce)

(Ce,La,Nd)AsO4

Monazite-(Ce)

CePO4

Monazite-(La)

LaPO4

Monazite-(Nd)

NdPO4

Monazite-(Sm)

SmPO4

Rooseveltite

BiAsO4

Physical Properties of Monazite Monazite is a yellowish brown to reddish brown or greenish brown mineral with a resinous to vitreous luster. It is translucent and rarely seen in large grains or as well-formed crystals. Granular masses are sometimes seen where monazite is locally abundant. It breaks with good to distinct cleavage. Its hardness ranges from 5 to 5.5. It has an unusually high specific gravity that ranges from 4.6 to 5.4 depending upon its composition.

Monazite and quartz crystals: Orange-pink twinned crystals of monazite-(Ce), about 5 millimeters in length, with quartz from Bolivia. Specimen and photo by Arkenstone / www.iRocks.com.

Geologic Occurrence of Monazite Monazite is known more for where it accumulates instead of where it forms. It forms during the crystallization of igneous rocks and during the metamorphism of clastic sedimentary rocks. When these rocks weather, monazite is one of the more resistant minerals and becomes concentrated in the weathering debris. The soils and sediments found near a weathering outcrop can have a higher concentration of monazite than the source rock. The liberated grains of monazite then begin a journey downslope. Eventually they are brought to a stream or a dry wash. There, the actions of gravity and running water help the heavy grains of monazite and other heavy minerals segregate from lighter minerals. They accumulate behind boulders, on the inside bends of stream channels and work their way down into the lower portions of the sediment deposit. Some are washed to the sea where they accumulate in deltaic, beach, or shallow water sediments.

Monazite sands at Frasier Island, Australia: Australia was once the world's largest producer of monazite and is thought to have the world's largest monazite resource. However, Australia has not produced significant quantities of monazite since public objection shut down mining at Frasier Island, Queensland.

Monazite Mining All monazite mining is focused on placer deposits because they are easier to mine and the monazite is often present in higher concentrations than in hard rock deposits. Other heavy minerals that accumulate with monazite include gold, platinum, magnetite, ilmenite, rutile, zircon, and a variety of gemstones. The heavy sands recovered are processed to separate these heavy minerals, and the light fraction is returned to the deposit. Stream sediments, alluvial terraces, beach sediments, beach terraces, and shallow water sediments have all been dredged for heavy minerals. Today, most of the world's monazite is produced in the offshore waters of India, Malaysia, Vietnam, and Brazil. Southern India and Sri Lanka have the most extensive offshore monazite resources known. Australia was once the world's largest producer of monazite and is thought to have the world's largest monazite resource. It has not been a significant producer since the 1990s, after public objection shut down mining on Frasier Island. Monazite is not currently mined in the United States. In the past it was mined from stream placer deposits in Idaho. These deposits formed from weathering of the Idaho batholith. Monazite has also been mined as a byproduct from offshore deposits along the southeast coast of the United States, from North Carolina to Florida. Inland and offshore deposits are known to exist in many states, but they are small, low-grade deposits when compared to what is currently mined in other countries.

Muscovite The most abundant mica is used in a variety of construction materials and manufactured products.

Muscovite: Bladed muscovite from the Nuristan Province of Afghanistan with a crystal of pink morganite beryl. Specimen is approximately 2 1/4 x 2 x 1 1/2 inches (5.9 x 4.8 x 3.4 centimeters). Specimen and photo by Arkenstone / www.iRocks.com.

What is Muscovite? Muscovite is the most common mineral of the mica family. It is an important rock-forming mineral present in igneous,metamorphic, and sedimentary rocks. Like other micas it readily cleaves into thin transparent sheets. Muscovite sheets have a pearly to vitreous luster on their surface. If they are held up to the light, they are transparent and nearly colorless, but most have a slight brown, yellow, green, or rose-color tint. The ability of muscovite to split into thin transparent sheets - sometimes up to several feet across - gave it an early use as window panes. In the 1700s it was mined for this use from pegmatites in the area around Moscow, Russia. These panes were called "muscovy glass" and that term is thought to have inspired the mineral name "muscovite." Sheet muscovite is an excellent insulator, and that makes it suitable for manufacturing specialized parts for electrical equipment. Scrap, flake, and ground muscovite are used as fillers and extenders in a variety of paints, surface treatments, and manufactured products. The pearlescent luster of muscovite makes it an important ingredient that adds "glitter" to paints, ceramic glazes, and cosmetics. Physical Properties of Muscovite Chemical Classification

Color

Streak

Silicate

Thick specimens often appear to be black, brown, or silver in color; however, when split into thin sheets muscovite is colorless, sometimes with a tint of brown, yellow, green, or rose White, often sheds tiny flakes

Luster

Pearly to vitreous

Diaphaneity

Transparent to translucent

Cleavage

Perfect

Mohs Hardness

2.5 to 3

Specific Gravity

2.8 to 2.9

Diagnostic

Cleavage, color, transparency

Properties Chemical

KAl2(Si3AlO10)(OH)2

Composition Crystal System Uses

Monoclinic Used in the manufacturing of paint, joint compound, plastics rubber, asphalt roofing, cosmetics, drilling mud.

Physical Properties Muscovite is easily identified because its perfect cleavage allows it to be split into thin, flexible, elastic, colorless, transparent sheets with a pearly to vitreous luster. It is the only common mineral with these properties.

Muscovite: Muscovite from Stoneham, Maine. Specimen is approximately 4 inches (10 centimeters) across. Hand specimens of this size and thickness often appear to have a black, brown, or silver color; however, when they are split into thin sheets, the clear transparent nature of muscovite is revealed. Thin sheets often have a slight tint of brown, green, yellow, or rose.

An Important Rock-Forming Mineral Muscovite is found in igneous, metamorphic, and sedimentary rocks. In igneous rocks, it is a primary mineral that is especially common in granitic rocks. In granite pegmatites, muscovite is often found in large crystals with a pseudohexagonal outline. These crystals are called "books" because they can be split into

paper-thin sheets. Muscovite rarely occurs in igneous rocks of intermediate, mafic, and ultramafic composition. Muscovite can form during the regional metamorphism of argillaceous rocks. The heat and pressure of metamorphism transforms clay minerals into tiny grains of mica which enlarge as metamorphism progresses. Muscovite can occur as isolated grains in schist and gneiss, or it can be abundant enough that the rocks are called "mica schist" or "micaceous gneiss." Muscovite is not especially resistant to chemical weathering. It is quickly transformed into clay minerals. Tiny flakes of muscovite sometimes survive long enough to be incorporated into sediments and immature sedimentary rocks. It is evidence that these sediments and rocks have not been subjected to severe weathering.

Muscovite schist: A specimen of muscovite schist. Muscovite is formed during the metamorphism of argillaceous rocks. Specimen shown is about two inches (five centimeters) across.

Chemical Composition Muscovite is a potassium-rich mica with the following generalized composition... KAl2(AlSi3O10)(OH)2

In this formula potassium is sometimes replaced by other ions with a single positive charge such as sodium, rubidium, or cesium. Aluminum is sometimes replaced by magnesium, iron, lithium, chromium, or vanadium. When chromium substitutes for aluminum in muscovite the material takes on a green color and is known as "fuchsite." Fuchsite is often found disseminated through metamorphic rocks of the greenschist facies. Occasionally it will be abundant enough to give the rock a distinct green color, and for those rocks the name "verdite" is used.

Ground muscovite: Photograph of ground muscovite from Mt. Turner, Australia. USGS image.

Muscovite: Muscovite from Mitchell County, North Carolina. Specimen is approximately 3 inches (7.6 centimeters) across.

Muscovite: Muscovite from Mitchell County, North Carolina. Specimen is approximately 3 inches (7.6 centimeters) across. The transparent nature of muscovite is clearly seen in this photo.

Uses of Ground Mica Ground mica, mostly muscovite, is used in the United States to manufacture a variety of products [1]. JOINT COMPOUND The primary use of ground mica is in joint compound used to finish seams and blemishes in gypsum wallboard. The mica serves as a filler, improves the workability of the compound, and reduces cracking in the finished product. In 2011 about 69% of the dry-ground mica consumed in the United States was used in joint compound. PAINT Ground mica is used as a pigment extender in paint. It helps keep pigment in suspension; reduces chalking, shrinking, and shearing of the finished surface; reduces water penetration and weathering, and brightens the tone of colored pigments. In some automotive paints tiny flakes of mica are used to produce a pearlescent luster. DRILLING MUD Ground mica is an additive to drilling mud that helps to seal porous sections of the drill hole to reduce circulation loss. In 2011, about 17% of the dry-ground mica consumed in the United States was used in drilling muds. PLASTICS The auto industry in the United States uses ground mica to improve the performance of plastic parts. In plastics, particles of ground mica serve as an agent to absorb sound and vibration. It can also improve mechanical properties by increasing stability, stiffness, and strength. RUBBER Ground mica is used as an inert filler and mold release agent in the manufacture of molded rubber products such as tires and roofing. The platy grains of mica act as an antisticking agent. ASPHALT ROOFING Dry-ground mica is used as a surface coating on asphalt shingles and rolled roofing. The flat mica particles coat the surface and act as an antistick agent. The mica does not absorb the asphalt and stands up well to weathering. COSMETICS Some of the highest quality ground mica is used in the cosmetics industry. The pearly luster of ground mica makes it an important ingredient in blushes, eyeliner, eye shadow, foundation, hair and body glitter, lipstick, lip gloss, mascara, and nail polish.

Mica windows: Mica is heat resistant and is often used as a "window" for wood stoves, ovens, and furnaces. These mica windows are for a wood stove and are about the same thickness as a piece of paper. Sheet size is 3 inches x 4 inches. They can be trimmed with scissors to fit the size of the window.

Mica with inclusions: Sheets of mica with inclusions are often sold as low-quality windows for woodstoves, ovens, and furnaces at a reduced price. Common inclusions are magnetite, rutile, and hematite. Sheet size is 6 inches x 6 inches.

Uses of Sheet Mica While ground mica might sell for $300 per metric ton, sheet mica for specialty uses can sell for prices up to $2000 per kilogram. Mica has several properties that make it suitable for very special uses: 1) it can be split into thin sheets 2) the sheets are chemically inert, dielectric, elastic, flexible, hydrophilic, insulating, lightweight, reflective, refractive and resilient 3) it is stable when exposed to electricity, light, moisture and extreme temperatures Most sheet mica is used to make electronic devices. In these uses the sheets are cut, punched, stamped and machined to precision dimensions. Uses include: diaphragms for oxygen breathing equipment, marker dials

for navigation compasses, optical filters, pyrometers, retardation plates in helium-neon lasers, missile systems components, medical electronics, optical instrumentation, radar systems, radiation detector windows, and calibrated capacitors. The quality of sheet mica is influenced by the presence of inclusions. These can impair splitting, decrease transparency, and reduce dielectric strength. Tiny crystals of staurolite, zircon, garnet, tourmaline, magnetite, hematite and other minerals can form between the sheets and orient parallel to the mica's crystal structure. Inclusions decrease the mica's value and its ability to be used in most applications. (See image.)

Mica Outlook Information Sources [1] Mica: Jason Christopher Willett, 2011 Minerals Yearbook, United States Geological Survey, September 2012. [2] Mica (Natural): Jason Christopher Willett, 2013 Mineral Commodity Summary, United States Geological Survey, January 2013.

The use of ground mica is mainly determined by activity levels of the construction and auto industries. An increase in domestic oil and gas drilling should sustain the demand for mica additives for drilling mud. Producers in the United States should be able to supply domestic demand, with some mica being imported for specialty use or where transportation from domestic producers to the consumer is more costly than imported mica. About 50,000 tons were produced in the United States in 2011, with about 25,000 tons being imported. China at 700,000 tons, is the largest producer and largest consumer. Although the demand for sheet mica is growing with the advance of technology, the prices are so high that the invention of substitute materials is growing. Some of these involve making mica sheets from ground mica composites or the creation of synthetic micas in laboratories. Acrylic, fiberglass, nylatron, nylon, polyester, styrene, vinyl-PVC, and vulcanized fibers are all finding use as sheet mica substitutes [2]. Contributor: Hobart King

Nepheline Mineral Properties and Uses Physical Properties of Nepheline Chemical Classification

silicate

Color

colorless, white, yellowish, brownish red, greenish

Streak

white

Luster Diaphaneity Cleavage

greasy to vitreous transparent to translucent poor

Mohs Hardness

5.5 to 6

Specific Gravity

2.6 to 2.7

Diagnostic Properties

greasy luster, poor cleavage

Chemical Composition

(Na,K)AlSiO4

Crystal System Uses

Nepheline from Dungannon Township, Ontario, Canada. Specimen is approximately 3 inches (7.6 centimeters) across.

hexagonal

Nepheline from Dungannon Township, Ontario, Canada. Specimen is approximately 3 inches (7.6 centimeters) across.

industrial use, glass industry.

Nepheline from Bancroft, Ontario, Canada. Specimen is approximately 4 inches (10 centimeters) across.

Nepheline cancrinite syenite from Litchfield, Maine. Specimen is approximately 4 inches (10 centimeters) across.

Olivine A group of rock-forming minerals found in Earth's crust. An abundant mineral in Earth's mantle. A constituent of many meteorites.

Olivine in basalt: Lherzolite (a variety of peridotite) nodules in a xenolith collected from a basalt flow at Peridot Mesa, Arizona. These xenoliths often contain crystals of olivine with a color and clarity that is suitable for use as a peridot gemstone. This specimen is approximately 3 inches (7.6 centimeters) across.

What is Olivine? Olivine is the name of a group of rock-forming mineralsthat are typically found in mafic and ultramafic igneous rocks such as basalt, gabbro, dunite, diabase, and peridotite. They are usually green in color and have compositions that typically range between Mg2SiO4 and Fe2SiO4. Many people are familiar with olivine because it is the mineral of a very popular green gemstone known as peridot.

Olivine gemstone: The gemstone known as peridot is a variety of olivine. These two faceted stones are nice specimens of yellowish green peridot. The gem on the left is a 1.83 carat cushion cut peridot of about 8 x 6 millimeters from Myanmar. The gem on the right is a 1.96 carat cushion checkerboard cut peridot of about 10 x 8 millimeters from China. Photo © geology.com.

Geological Occurrence of Olivine Most olivine found at Earth's surface is in dark-colored igneous rocks. It usually crystallizes in the presence of plagioclase and pyroxene to form gabbro or basalt. These types of rocks are most common at divergent plate boundaries and at hot spots within the centers of tectonic plates. Olivine has a very high crystallization temperature compared to other minerals. That makes it one of the first minerals to crystallize from a magma. During the slow cooling of a magma, crystals of olivine may form and then settle to the bottom of the magma chamber because of their relatively high density. This concentrated accumulation of olivine can result in the formation of olivine-rich rocks such as dunite in the lower parts of a magma chamber. Crystals of olivine are sometimes formed during the metamorphism of a dolomitic limestone or dolomite. The dolomite contributes magnesium, and silica is obtained from quartz and other impurities in the limestone. When olivine is metamorphosed, it is transformed into serpentine. Olivine is one of the first minerals to be altered by weathering. Because it is so easily altered by weathering, olivine is not a common mineral in sedimentary rocks and is only an abundant constituent of sand or sediment when the deposit is very close to the source.

Olivine sand: Green olivine sand from Papakolea Beach, Hawaii. The white grains are coral fragments, and the gray-black grains are pieces of basalt. If you think the grains have a "gemmy" appearance, olivine is the mineral name of a gemstone known as "peridot." The width of this view is about 10 millimeters. Photograph by Siim Sepp, used here under a Creative Commons License.

Composition of Olivine Olivine is the name given to a group of silicate mineralsthat have a generalized chemical composition of A2SiO4. In that generalized composition, "A" is usually Mg or Fe, but in unusual situations can be Ca, Mn, or Ni. The chemical composition of most olivine falls somewhere between pure forsterite (Mg2SiO4) and pure fayalite (Fe2SiO4). In that series, Mg and Fe can substitute freely for one another in the mineral's atomic structure - in any ratio. This type of continuous compositional variation is known as a "solid solution" and is represented in a chemical formula as (Mg,Fe)2SiO4. The name "olivine" is used instead of "forsterite" or "fayalite" because a chemical analysis or other detailed testing is needed to determine which one is dominant - if either is dominant. The name "olivine" serves as a

quick, convenient, and inexpensive way to put a name on the material. A list of the more common olivine minerals and their composition is given in the table below. Olivine Minerals Mineral

Chemical Composition

Forsterite

Mg2SiO4

Fayalite

Fe2SiO4

Monticellite

CaMgSiO4

Kirschsteinite

CaFeSiO4

Tephroite

Mn2SiO4

Olivine receives its name from its usual olive-green color. Many geology students remember the color of olivine by using a rhyme: "olivine is green." That rhyme is true with most classroom specimens; however, there are rare iron-rich olivines (fayalites) that are brownish in color.

Olivine: Olivine from Mitchell County, North Carolina. Specimen is approximately 3 inches (7.6 centimeters) across.

Olivine in Earth's Mantle Olivine is thought to be an important mineral in Earth's mantle. Its presence as a mantle mineral has been inferred by a change in the behavior of seismic waves as they cross the Moho - the boundary between Earth's crust and mantle. The presence of olivine in Earth's interior is also confirmed by the presence of olivine in xenoliths, which are thought to be pieces of the upper mantle delivered to Earth's surface in the magmas of deepsource volcanic eruptions. Olivine is also an abundant mineral in the lower portion of many ophiolites. These are slabs of oceanic crust (with part of the upper mantle attached) that have been thrust up onto an island or a continent.

Physical Properties of Olivine Chemical Classification

Silicate

Color

Usually olive green, but can be yellow-green to bright green; iron-rich specimens are brownish green to brown

Streak

Colorless

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent Poor cleavage, brittle with conchoidal fracture

Mohs Hardness

6.5 to 7

Specific Gravity

3.2 to 4.4

Diagnostic Properties

Green color, vitreous luster, conchoidal fracture, granular texture

Chemical Composition

Typically (Mg, Fe)2SiO4. Ca, Mn, and Ni rarely occupy the Mg and Fe positions.

Crystal System Uses

Orthorhombic Gemstones, a declining use in bricks and refractory sand

Olivine in pallasite: A part slice of the Esquel pallasite from Chubut, Argentina. The large, colorful, oblong olivine crystals are typical of this meteorite. Note the way in which crystals near the rough (natural) edge have turned orange and yellow due to terrestrial weathering, while the crystals nearer to the center of the original mass have retained their true olive green color. Photograph by Geoffrey Notkin © Aerolite Meteorites, used with permission.

Physical Properties of Olivine Olivine is usually green in color but can also be yellow-green, greenish yellow, or brown. It is transparent to translucent with a glassy luster and a hardness between 6.5 and 7.0. It is the only common igneous mineral with these properties. The properties of olivine are summarized in the table.

Pallasite peridot: This is one of the most incredible gemstones. It is a piece of gem-quality olivine (peridot) from a pallasite meteorite, and it has been faceted into a wonderful little gemstone. This may be the most scarce gem material on Earth - but it actually originated in space. This stone is 2.85 millimeters in diameter and weighs about ten points. Photo by TheGemTrader.com.

Extraterrestrial Olivine Olivine has been identified in a large number of stony and stony-iron meteorites. These meteorites are thought to have originated from the mantle of a rocky planet that used to occupy an orbit between Mars and Jupiter - or they might be from an asteroid that was large enough to have developed a differentiated internal structure consisting of a rock mantle and a metallic core. Pallasites are thought to represent the part of an asteroid or planet that was near the mantle-core boundary where rocky materials of the mantle were mixed with the metallic materials of the core. Pallasites typically have distinct crystals of olivine (usually fayalite) surrounded by a nickel-iron matrix. A photograph of a slice from a pallasite meteorite is shown on this page.

Olivine rain: An artist's concept of crystalline olivine rain on a developing star, inspired by the Spitzer Space Telescope. Image by NASA/JPL Caltech/University of Toledo.

Olivine Rain on a Developing Star In 2011, NASA's Spitzer Space Telescope observed what is believed to have been tiny crystals of olivine falling like rain through the dusty cloud of gas of a developing star. This "olivine rain" was thought to have occurred as strong air currents lifted newly crystallized particles of olivine from the surface of the forming star, high into its atmosphere, and then dropped them when the currents lost their momentum. The result was a rain of glittering green olivine crystals.

Uses of Olivine Olivine is a mineral that is not often used in industry. Most olivine is used in metallurgical processes as a slag conditioner. High-magnesium olivine (forsterite) is added to blast furnaces to remove impurities from steel and to form a slag. Olivine has also been used as a refractory material. It is used to make refractory brick and used as a casting sand. Both of these uses are in decline as alternative materials are less expensive and easier to obtain.

Olivine peridot rough: These three specimens are peridot, a gem variety of olivine, from a deposit in Arizona. At this deposit the olivine occurs in xenoliths that were erupted with a basalt flow. These specimens are approximately 12 millimeters across.

Olivine and the Gemstone Peridot Olivine is also the mineral of the gemstone known as "peridot." It is a yellow-green to green gemstone that is very popular in jewelry. Peridot serves as a birthstone for the month of August. The most valued colors are dark olive green and a bright lime green. These specimens are of the mineral forsterite because the ironrich fayalite is usually a brownish, less desirable color. Much of the world's peridot used in mass-production jewelry is mined at the San Carlos Reservation in Arizona. There, a few basalt flows containing nodules of granular olivine are the source of the peridot. Most of the stones produced there are a few carats or less in size and often contain visible crystals of chromite or other minerals. They are cut in Asia and returned to the United States in commercial jewelry. Higher quality and larger peridot crystals are mined in Pakistan and Myanmar. There, crystals of olivine are found in metamorphic rocks. These are usually found in association with serpentine or dolomitic marble. Contributor: Hobart King

Realgar and Orpiment Arsenic Sulfide Minerals

Realgar: Realgar crystals from the Royal Reward Mine in King County, Washington. Specimen measures about 2.2 x 1.1 x 0.8 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Toxic Arsenic Sulfide Minerals Realgar and orpiment are very similar minerals. They are both arsenic sulfides and members of the monoclinic crystal system. They form in the same geological environments and can be closely associated in the same deposits. They have similar physical properties and similar histories of use by man. Because of these similarities, we decided to describe realgar and orpiment in a single article. Realgar and orpiment are both toxic minerals, and contact with them should be avoided. They are not suitable for classroom specimens. Physical Properties: Realgar / Orpiment

Chemical Classification Color

Realgar

Orpiment

Sulfide

Sulfide

Red to orange, gray

Yellow, golden, yellow-brown

Streak

Red-orange to red

Light yellow

Luster

Resinous to pearly on cleavage

Resinous to pearly on cleavage

Translucent to transparent

Translucent to transparent

Good

Perfect

Mohs Hardness

1.5 to 2

1.5 to 2

Specific Gravity

3.56

3.49

Diaphaneity Cleavage

Diagnostic

Color, streak, resinous luster. Association with

Properties

orpiment.

Chemical Composition Crystal System

Uses

Color, streak, luster, foliated appearance. Association with realgar.

As4S4

As2S3

Monoclinic

Monoclinic

An ore of arsenic. Historically used as a pigment,

An ore of arsenic. Used in the production of oil cloth,

depilatory, poison, ingredient in explosives and

semiconductors, photoconductors. Historically used as a pigment,

fireworks, ritualistic "medicine," cosmetic.

poison, ingredient in fireworks and explosives.

What is Realgar? Realgar is a monoclinic arsenic sulfide mineral with a brilliant red color and a chemical composition of As4S4. Well-formed realgar crystals can look so much like red gemstones that the mineral was often called "ruby sulfur" and "ruby arsenic." However, realgar is not used as a gemstone because it is very soft, with a Mohs hardness of just 1-1/2 to 2. It is easily ground into a fine, bright red powder. Those properties caused it to become a favorite pigment in many parts of the ancient world. It was traded over great distances to make paints, inks, and dyes - until people realized that it was toxic. The arsenic in realgar was the source of its toxicity. After its toxicity was realized in the Middle Ages, the mineral was used as a poison to kill rodents, insects, and weeds. Realgar was also used in leather processing to remove the hair from hides. Today it is rarely used for any of these purposes because more effective and less toxic substitutes have been developed. However, some use of realgar in ritualistic cosmetics and "medicines" continues in a few parts of the world. Today the main use of realgar is as an ore of arsenic metal. The chemical formula of realgar is often written as As4S4instead of the simpler AsS. This is done because As4S4represents a structural unit of the mineral. A compound of As+3 and S-2 would be out of electrical balance. In realgar, three of the arsenics are joined in a chain by covalent bonds. This gives the arsenics an effective electrical charge of +8. That combines with four S-2 ions to produce an electrically neutral molecule. This is why realgar's chemical composition is often presented as As4S4 instead of AsS.

Orpiment: Bright orange orpiment from the Jiepaiyu Mine in Hunan Province, China. This specimen shows the botryoidal habit of orpiment. It measures 12.7 centimeters in length. Specimen and photo by Arkenstone / www.iRocks.com.

What is Orpiment? Orpiment is a monoclinic arsenic sulfide with a yellow-orange color and a chemical composition of As2S3. It has a Mohs hardness of 1-1/2 to 2 and is easily ground into a yellow to yellow-orange powder. Like realgar, its earliest widespread use was as a pigment for paints, inks, and dyes, and it was traded over great distances. After its toxicity was discovered, its use as a pigment declined. People took advantage of its toxicity to use it as a poison for insects and rodents. Some people continued to use it as a ritualistic cosmetic and "medicine" even after its toxicity was known, and that practice continues today in some parts of the world. Today the primary use of orpiment is as an ore of arsenic. It is also used in manufacturing oil cloth, semiconductors, and photoconductors.

Realgar on calcite: Red realgar crystals on white calcite, from the Jiepaiyu Mine in Hunan Province, China. Specimen and photo by Arkenstone / www.iRocks.com.

Geologic Occurrence Realgar and orpiment are mainly found associated with hydrothermal and volcanic activities. They are sublimation products at volcanic vents and crystallization products at hot springs. These are among the

earliest deposits exploited in the middle ages. Underground deposits of realgar and orpiment are in veins and fractures. There they are associated with lead, silver, gold, and other arsenic minerals. Contributor: Hobart King

Orthoclase Mineral Properties and Uses

Physical Properties of Orthoclase Chemical Classification

silicate

Color

white, gray, flesh pink, reddish, yellow, green

Streak

white

Luster

vitreous

Diaphaneity

translucent to subtranslucent

Cleavage

perfect

Mohs Hardness

6 to 6.5

Specific Gravity

2.5 to 2.6

Diagnostic Properties

cleavage, color

Chemical Composition

KAlSi3O8

Crystal System Uses

monoclinic gemstones, ceramics

Plagioclase A group of common rock-forming minerals. It is sometimes used as a gem material.

Albite: An igneous rock composed almost entirely of albite. This specimen is from the Petaca District of New Mexico and measures about 4 inches (10 cm) across.

Feldspar classification: This diagram shows how feldspar minerals are classified on the basis of their chemical composition. The sequence of minerals along the base of the triangle represents the solid solution series of plagioclase between albite and anorthite.

What is Plagioclase? “Plagioclase” is the name of a group of feldspar minerals that form a solid solution series ranging from pure albite, Na(AlSi3O8), to pure anorthite, Ca(Al2Si2O8). Minerals in this series are a homogenous mixture of

albite and anorthite. The names of the minerals in the series are arbitrarily given based upon their relative abundance of albite and anorthite. The minerals of the plagioclase series are listed in the table below along with their relative abundance of albite (Ab) and anorthite (An). Plagioclase Group Minerals Mineral Albite

% Albite

% Anorthite

100-90% Ab 0-10% An

Oligoclase 90-70% Ab 10-30% An Andesine

70-50% Ab 30-50% An

Labradorite 50-30% Ab 50-70% An Bytownite 30-10% Ab 70-90% An Anorthite

10-0% Ab 90-100% An

The name “plagioclase” is frequently used instead of one of the more specific names in the table above. This is because the minerals of the plagioclase series are very similar and difficult to tell apart without laboratory testing. Thus the name “plagioclase” is commonly used in many field and classroom situations.

Geologic Occurrence of Plagioclase Members of the plagioclase group are the most common rock-forming minerals. They are important to dominant minerals in most igneous rocks of the Earth’s crust. They are major constituents in a wide range of intrusive and extrusive igneous rocks including granite, diorite, gabbro, rhyolite, andesite, and basalt. Plagioclase minerals are important constituents of many metamorphic rocks, such as gneiss, where they can be inherited from an igneous protolith or formed during the regional metamorphism of sedimentary rocks. Plagioclase is a common clast produced during the weathering of igneous and metamorphic rocks. It can be the most abundant clast in sediments located close to their source area and decreases in abundance downstream. This decrease is partly because quartz is more physically and chemically durable than feldspar and persists in greater relative quantities downstream in eroded sediments.

Bytownite: An igneous rock composed almost entirely of bytownite. This specimen is from Crystal Bay, Minnesota, and measures about 4 inches (10 cm) across.

Oligoclase: A cleavage fragment of oligoclase. This specimen is from Mitchell County, North Carolina. It measures about 4 inches (10 cm) across.

Physical Properties of Plagioclase Minerals All feldspar minerals have two directions of perfect cleavage. It is usually easy to distinguish plagioclase feldspars because their two planes of cleavage intersect at 90-degree angles, and their cleavage faces often display striations. These properties make plagioclase feldspars relatively easy to identify with a hand lens in coarse-grained igneous and metamorphic rocks. Plagioclase in granitic rocks is normally white, pink, or red in color. In basaltic rocks it is normally gray to black. Physical Properties of Plagioclase Chemical Classification

Silicate

Color

Usually white or gray. Also colorless, yellow, orange, pink, red, brown, black, blue, green.

Streak

White

Luster

Vitreous. Pearly on some cleavage faces.

Diaphaneity Cleavage

Translucent to transparent Perfect in two directions that intersect at approximately 90 degrees.

Mohs Hardness

6 to 6.5

Specific Gravity

2.6 to 2.8

Diagnostic

Perfect cleavage, with cleavage faces intersecting at right angles and striations often present on cleavage faces. Well-

Properties

defined crystals are extremely rare.

Chemical Composition

NaAlSi3O8 - CaAl2Si2O8

Crystal System

Triclinic Plagioclase feldspars are important components of many building stones. Labradorite, spectrolite, sunstone, and

Uses

moonstone are gem-quality plagioclase feldspars that are popular because of their optical phenomena. Transparent plagioclase of high clarity is sometimes faceted as a collector gem but lacks the durability needed for use in jewelry.

Labradorite: An igneous rock composed almost entirely of iridescent plagioclase. This specimen was found near the town of Nain in Labrador, Canada. It measures about 4 inches (10 cm) across.

Oregon Sunstone as a faceted stone and a cabochon. The stone on the left is a beautiful orange 7x5 mm oval faceted stone weighing 1.01 carats. The stone on the right is a 7 mm round cabochon with abundant copper platelets weighing 2.29 carats. Both stones are from the Spectrum Sunstone Mine near Plush, Oregon.

Spectrolite: Translucent labradorite with the best exhibit of spectral color is known in the gemstone trade as "spectrolite." This spectrolite free-form cabochon is about 38 millimeters across.

Uses of Plagioclase Construction, Decorative and Architectural Stone Plagioclase minerals are important constituents of some building stone and crushed stone such as granite and trap rock. These rocks are also cut and polished for use as countertops, stair treads, wall panels, building facing, monuments, and many other types of decorative and architectural stone. Plagioclase as a Gemstone Some rare specimens of plagioclase exhibit optical phenomena that make them highly desirable gem materials. Many people enjoy the adularescence of moonstone, the aventurescence of sunstone, and the labradorescence of labradorite. Moonstone Moonstone is a name given to a gem material that consists of very thin, alternating layers of orthoclase (an alkali feldspar) and albite (a plagioclase feldspar). When light enters the stone, it interacts with these thin layers to produce a phenomenon known as "adularescence" (a white-to-bluish light that floats under the surface of the stone when it is turned under a source of light). Sunstone The name sunstone has traditionally been given to a transparent labradorite feldspar that contains plateshaped copper inclusions which share a common alignment within the mineral. When cabochons or faceted stones cut from this material are moved under a source of incident light, bright flashes of reflected light are produced as the incident rays strike platelets being moved to the angle at which they reflect the incident rays. These flashes from reflective particles are known as “aventurescence.” In Oregon, transparent gemquality labradorite with a yellow, orange, red, blue, or green color is also called “sunstone” when it is mined from the same deposit as the aventurescent material. Labradorite Some specimens of labradorite exhibit a schiller effect, which is a strong play of iridescent blue, green, red, orange, and yellow colors when moved under a source of incident light. Labradorite is so well known for

these spectacular displays of color that the phenomenon is known as "labradorescence." Pieces of labradorite with exceptional play-of-color are known as “spectrolite.” These specimens sell for premium prices. Collector Gems Plagioclase minerals are rarely found in transparent crystals of exceptional clarity. Well-formed crystals are prized by mineral specimen collectors because of their beauty and rarity. They can sell for thousands of dollars. Transparent material of high quality is also cut into faceted gemstones which are often sold as “collector gems.” With a Mohs hardness of 6 and perfect cleavage, these stones are usually considered to be too fragile for use in jewelry.

Lunar plagioclase: This rock was collected from the surface of the Moon and brought back to Earth by Apollo 11 astronauts in July, 1969. It is a vesicular basalt made up of approximately 50% pyroxene, 30% plagioclase, and 20% other minerals. The rock has many vesicles, some of which contain well-defined crystals. The specimen is approximately 6.2 x 5.9 x 4.0 centimeters in size and weighs 173 grams. NASA image.

Extraterrestrial Plagioclase As with many minerals, plagioclase occurs in other parts of the solar system. Many of the rocks brought back to Earth from the Moon by Apollo 11 astronauts are lunar basalts rich in plagioclase. Basalt is one of the most common rock types present on the surface of the Moon, and much of that basalt is thought to contain plagioclase. Large areas of Mars are covered with basalt flows and ejecta produced by asteroid impacts. Plagioclase has been identified in many of these basalts. Data from the thermal emission spectrometer onboard the Mars Global Surveyor suggests that plagioclase is the most abundant mineral in the crust of Mars. Several meteorites have been found on Earth that are thought to be pieces of Mars. They are thought to be pieces of Martian bedrock, ejected beyond the influence of the planet's gravity by a large asteroid impact. Some of these meteorites contain abundant plagioclase.

Prehnite Mineral Properties and Uses

Beads of prehnite. Each bead is approximately 6 millimeter across.

Physical Properties of Prehnite Chemical Classification

silicate

Color

white, green, gray, yellow

Streak

white

Luster

vitreous

Diaphaneity

transparent to translucent

Cleavage

basal

Mohs Hardness

6 to 6.5

Specific Gravity Diagnostic Properties Chemical Composition Crystal System Uses

2.8 to 3.0

color, cleavage Ca2Al(AlSi3O10)(OH)2 orthorhombic ornamental material, semiprecious gem

Pyrite With a nickname of "Fool's Gold," it is surprising that pyrite often contains significant amounts of gold

Pyrite crystals: Cubic crystals of pyrite on a marlstone from Navajún, Rioja, Spain. Specimen is approximately 4 inches (9.5 centimeters) across. Image by Carles Millan and used under a Creative Commons license.

What is Pyrite? Pyrite is a brass-yellow mineral with a bright metallic luster. It has a chemical composition of iron disulfide (FeS2) and is the most common sulfide mineral. It forms at high and low temperatures and occurs, usually in small quantities, in igneous, metamorphic, and sedimentary rocks worldwide. The name "pyrite" is after the Greek "pyr" meaning "fire." This name was given because pyrite can be used to create the sparks needed for starting a fire if it is struck against metal or another hard material. Pieces of pyrite have also been used as a spark-producing material in flintlock firearms. Pyrite has a nickname that has become famous - "Fool's Gold." The mineral's gold color, metallic luster, and high specific gravity often cause it to be mistaken for gold by inexperienced prospectors. However, pyrite is often associated with gold. The two minerals often form together, and in some deposits pyrite contains enough included gold to warrant mining. Physical Properties of Pyrite Chemical Classification

Sulfide

Color

Brass yellow - often tarnished to dull brass

Streak

Greenish black to brownish black

Luster

Metallic

Diaphaneity

Opaque

Cleavage

Breaks with a conchoidal fracture

Mohs Hardness

6 to 6.5

Specific Gravity

4.9 to 5.2

Diagnostic Properties

Color, hardness, brittle, greenish black streak

Chemical Composition

Iron sulfide, FeS2

Crystal System Uses

Isometric Ore of gold

Pyrite with hematite: Pyrite with hematite from Rio Marina, Isle of Elba, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Identifying Pyrite Hand-specimens of pyrite are usually easy to identify. The mineral always has a brass-yellow color, a metallic luster and a high specific gravity. It is harder than other yellow metallic minerals, and its streak is black, usually with a tinge of green. It often occurs in well-formed crystals in the shape of cubes, octahedrons, or pyritohedrons, which often have striated faces. The only common mineral that has properties similar to pyrite is marcasite, a dimorph of pyrite with the same chemical composition but an orthorhombic crystal structure. Marcasite does not have the same brassy yellow color of pyrite. Instead it is a pale brass color, sometimes with a slight tint of green. Marcasite is more brittle than pyrite and also has a slightly lower specific gravity at 4.8. Pyrite and gold can easily be distinguished. Gold is very soft and will bend or dent with pin pressure. Pyrite is brittle, and thin pieces will break with pin pressure. Gold leaves a yellow streak, while pyrite's streak is greenish black. Gold also has a much higher specific gravity. A little careful testing will help you avoid the "Fool's Gold" problem.

Massive pyrite: Massive pyrite from Rico, Colorado. Specimen is approximately 3 inches (7.6 centimeters) across.

Pyrite: Pyrite with hematite from Rio Marina, Isle of Elba, Italy. Specimen is approximately 3 inches (7.6 centimeters) across.

Uses of Pyrite Pyrite is composed of iron and sulfur; however, the mineral does not serve as an important source of either of these elements. Iron is typically obtained from oxide ores such as hematite and magnetite. These ores occur in much larger accumulations, the iron is easier to extract and the metal is not contaminated with sulfur, which reduces its strength. Pyrite used to be an important ore for the production of sulfur and sulfuric acid. Today most sulfur is obtained as a byproduct of natural gas and crude oil processing. Some sulfur continues to be produced from pyrite as a byproduct of gold production. The most important use of pyrite is as an ore of gold. Gold and pyrite form under similar conditions and occur together in the same rocks. In some deposits small amounts of gold occur as inclusions and substitutions within pyrite. Some pyrites can contain 0.25% gold by weight or more. Although this is a tiny fraction of the ore, the value of gold is so high that the pyrite might be a worthwhile mining target. If pyrite contains 0.25% gold and the gold price is $1500 per troy ounce, then one ton of pyrite will contain about 73 troy ounces of gold worth

over $109,000. That is not a guaranteed money-maker. It depends upon how efficiently the gold can be recovered and the cost of the recovery process. Pyrite is occasionally used as a gemstone. It is fashioned into beads, cut into cabochons, faceted, and carved into shapes. This type of jewelry was popular in the United States and Europe in the mid- to late1800s. Most of the jewelry stones were called "marcasite," but they are actually pyrite. (Marcasite would be a poor choice for jewelry because it quickly oxidizes, and the oxidation products cause damage to anything that they contact. Pyrite is not an excellent jewelry stone because it easily tarnishes.)

Pyrite framboid: One of the most interesting forms of pyrite is the "framboid." These tiny spheres of euhedral pyrite crystals are often found in organic muds, coal, shale, and other types of rocks. This is a framboid from the Waynesburg coal of northern West Virginia. It is a sphere about 15 microns in diameter that is composed of cubic crystals of pyrite about one micron on a side.

Pyrite and Coal Mining Sulfur occurs in coal in three different forms: 1) organic sulfur, 2) sulfate minerals, and 3) sulfide minerals (mostly pyrite with minor amounts of marcasite). When the coal is burned, these forms of sulfur are converted into sulfur dioxide gas and contribute to air pollution and acid rain unless they are removed from the emissions. The sulfide mineral content of the coal can be reduced by heavy mineral separation, but this removal is expensive, results in a loss of coal, and cannot be done with 100% efficiency. The sulfide minerals in coal and its surrounding rocks can produce acid mine drainage. Before mining, these minerals are deep within the ground and below the water table where they are not subject to oxidation. During and after mining the level of the water table often falls, exposing the sulfides to oxidation. This oxidation produces acid mine drainage which contaminates groundwater and streams. Mining also breaks the rocks above and below the coal. This creates more pathways for the movement of oxygenated waters and exposes more surface area to oxidation.

Pyrite crystals: Pyrite, cubic crystals in schist from Chester, Vermont. Specimen is approximately 4 inches (10 centimeters) across.

Pyrite and Construction Projects Aggregates used to make concrete, concrete block, and asphalt paving materials must be free of pyrite. Pyrite will oxidize when it is exposed to air and moisture. That oxidation will result in the production of acids and a volume change that will damage the concrete and reduce its strength. This damage can result in failure or maintenance problems. Pyrite should not be present in the base material, subsoil or bedrock under roads, parking lots, or buildings. Oxidation of pyrite can result in damage to pavement, foundations, and floors. In parts of the country where pyrite is commonly found, construction sites should be tested to detect the presence of pyritic materials. If pyrite is detected, the site can be rejected or the problem materials can be excavated and replaced with quality fill.

Pyrite fossils: Fossil ammonite in which the shell was replaced by pyrite. External view on left and cross-sectional view on right. External view by asterix0597 and cross-sectional view by Henry Chaplin. Both images © iStockphoto.

Pyrite and Organic Material The conditions of pyrite formation in the sedimentary environment include a supply of iron, a supply of sulfur, and an oxygen-poor environment. This often occurs in association with decaying organic materials. Organic decay consumes oxygen and releases sulfur. For this reason, pyrite commonly and preferentially

occurs in dark-colored organic-rich sediments such as coal and black shale. The pyrite often replaces organic materials such as plant debris and shells to create interesting fossils composed of pyrite. Contributor: Hobart King

Pyrophyllite Mineral Properties and Uses

Pyrophyllite from Moore County, North Carolina. Specimen is approximately 4 inches (10 centimeters) across.

Physical Properties of Pyrophyllite Chemical Classification

silicate

Color

white, pale blue, yellow, grayish green, brownish green

Streak

white

Luster

pearly to greasy

Diaphaneity Cleavage

translucent perfect

Mohs Hardness

1 to 2

Specific Gravity

2.7 to 2.9

Diagnostic Properties

cleavage, greasy feel

Chemical Composition

Al2Si4O10(OH)2

Crystal System Uses

monoclinic ceramics, refractory materials

Layering of pyrophyllite from Moore County, North Carolina. Specimen is approximately 3/4 inch (1.9 centimeters) thick.

Pyrrhotite Mineral Properties and Uses

Pyrrhotite from Falconbridge, Ontario, Canada. Specimen is approximately 3 inches (7.6 centimeters) across.

Nickeliferous pyrrhotite from Falconbridge, Ontario, Canada. Specimen is approximately 5 inches (12.7 centimeters) across.

What is Pyrrhotite? Pyrrhotite is an iron sulfide mineral found in basic igneous rocks, pegmatites, vein deposits, and contact metamorphic deposits. It is slighly magnetic.

Physical Properties of Pyrrhotite Chemical Classification

sulfide

Color

bronze yellow, brownish bronze, reddish bronze

Streak

dark grayish black

Luster

metallic

Diaphaneity

opaque

Cleavage

none

Mohs Hardness

3.5 to 4.5

Specific Gravity

4.6 to 4.7

Diagnostic Properties

color, magnetism

Chemical Composition

iron sulfide, Fe1-xS

Crystal System Uses

monoclinic a very minor ore of iron

Uses of Pyrrhotite? Pyrrhotite is mined primarily because it is associated with pentlandite, sulfide mineral that can contain significant amounts of nickel and cobalt.

Pyrrhotite from Falconbridge, Ontario, Canada. Specimen is approximately 21/2 inches (6.4 centimeters) across.

Quartz A ubiquitous mineral with an enormous number of uses

Quartz crystals: Herkimer "Diamond" quartz crystals. A clear, "rock crystal" variety of quartz.

What is Quartz? Quartz is a chemical compound consisting of one part silicon and two parts oxygen. It is silicon dioxide (SiO2). It is the most abundant mineral found at Earth's surface, and its unique properties make it one of the most useful natural substances.

Rock crystal quartz: Transparent "rock crystal" quartz. This specimen shows the conchoidal fracture (fracture that produces curved surfaces) that is characteristic of the mineral. Specimen is about four inches (ten centimeters) across and is from Minas Gerais, Brazil.

Where is Quartz Found? Quartz is the most abundant and widely distributed mineral found at Earth's surface. It is present and plentiful in all parts of the world. It forms at all temperatures. It is abundant in igneous, metamorphic, and sedimentary rocks. It is highly resistant to both mechanical and chemical weathering. This durability

makes it the dominant mineral of mountaintops and the primary constituent of beach, river, and desert sand. Quartz is ubiquitous, plentiful and durable. Minable deposits are found throughout the world.

Amethyst quartz: Purple crystalline quartz is known as "amethyst." When transparent and of high quality, it is often cut as a gemstone. This specimen is about four inches (ten centimeters) across and is from Guanajuato, Mexico.

Physical Properties of Quartz Chemical Classification

Color

Silicate

Quartz occurs in virtually every color. Common colors are clear, white, gray, purple, yellow, brown, black, pink, green, red.

Streak

Colorless (harder than the streak plate)

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent None - typically breaks with a conchoidal fracture

Mohs Hardness

7

Specific Gravity

2.6 to 2.7

Diagnostic Properties

Conchoidal fracture, glassy luster, hardness

Chemical Composition

SiO2

Crystal System Uses

Hexagonal Glass making, abrasive, foundry sand, hydraulic fracturing proppant, gemstones

Flint: Flint is a variety of microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses and less frequently as a layered deposit. It breaks consistently with a conchoidal fracture and was one of the first materials used to make tools by early people. They used it to make cutting tools. After thousands of years, people continue to use it. It is presently used as the cutting edge in some of the finest surgical tools. This specimen is about four inches (ten centimeters) across and is from Dover Cliffs, England.

Quartz flint arrowheads: One of the first uses of quartz, in the form of flint, was the production of sharp objects such as knife blades, scrapers, and projectile points such as the arrowheads shown above. © iStockphoto / Leslie Banks.

What are the Uses for Quartz? Quartz is one of the most useful natural materials. Its usefulness can be linked to its physical and chemical properties. It has a hardness of seven on the Mohs Scalewhich makes it very durable. It is chemically inert in contact with most substances. It has electrical properties and heat resistance that make it valuable in electronic products. Its luster, color, and diaphaneity make it useful as a gemstone and also in the making of glass.

Uses of Quartz in Glass Making Geological processes have occasionally deposited sandsthat are composed of almost 100% quartz grains. These deposits have been identified and produced as sources of high purity silica sand. These sands are used in the glassmaking industry. Quartz sand is used in the production of container glass, flat plate glass, specialty glass, and fiberglass.

Quartz glass windows: Glassmaking is one of the primary uses of quartz. © iStockphoto / Chinaface.

Jasper beads: Quartz is often used in jewelry or as a gemstone. These jasper beads are an example of quartz used as a gemstone.

Quartz glass sand: High-purity quartz sandstone suitable for the manufacture of high-quality glass. "Glass sand" is a sandstone that is composed almost entirely of quartz grains. Pictured here is a specimen of the Oriskany Sandstone from Hancock, West Virginia. In a few locations, the Oriskany is over 99% pure quartz. Much of it has been used for container glass, but some of it has been selected for use in making lenses for the largest telescopes. Specimen is about four inches (ten centimeters across).

Blue aventurine quartz: Aventurine is colorful variety of quartz that contains abundant shiny inclusions of minerals such as mica or hematite. It is often cut and polished for use as an ornamental stone. Common colors for aventurine are green, orange, and blue. This specimen is about four inches (ten centimeters) across and is from India.

Uses of Quartz as an Abrasive The high hardness of quartz, seven on the Mohs Scale, makes it harder than most other natural substances. As such it is an excellent abrasive material. Quartz sands and finely ground silica sand are used for sand blasting, scouring cleansers, grinding media, and grit for sanding and sawing.

Uses of Quartz as a Foundry Sand Quartz is very resistant to both chemicals and heat. It is therefore often used as a foundry sand. With a melting temperature higher than most metals, it can be used for the molds and cores of common foundry work. Refractory bricks are often made of quartz sand because of its high heat resistance. Quartz sand is also used as a flux in the smelting of metals.

Silicified wood: Silicified "petrified" wood is formed when buried plant debris is infiltrated with mineral-bearing waters which precipitate quartz. This quartz infills the cavities within the wood and often replaces the woody tissues. This specimen is about four inches (ten centimeters) across and is from Yuma County, Arizona.

Uses in the Petroleum Industry Quartz sand has a high resistance to being crushed. In the petroleum industry, sand slurries are forced down oil and gas wells under very high pressures in a process known as hydraulic fracturing. This high pressure fractures the reservoir rocks, and the sandy slurry injects into the fractures. The durable sand grains hold the fractures open after the pressure is released. These open fractures facilitate the flow of natural gas into the well bore.

Chert: Chert is a microcrystalline or cryptocrystalline quartz. It occurs as nodules and concretionary masses and less frequently as a layered deposit. This specimen is about four inches (ten centimeters) across and is from Joplin, Missouri.

Many Other Quartz Sand Uses Quartz sand is used as a filler in the manufacture of rubber, paint, and putty. Screened and washed, carefully sized quartz grains are used as filter media and roofing granules. Quartz sands are used for traction in the railroad and mining industries. These sands are also used in recreation on golf courses, volleyball courts, baseball fields, children's sand boxes and beaches.

Quartz crystal: A Herkimer "Diamond" quartz crystal in dolostone. This specimen is about six inches (fifteen centimeters) across and is from Middleville, New York.

Uses of Quartz Crystals High-quality quartz crystals are single-crystal silica with optical or electronic properties that make them useful for specialty purposes. USGS estimates that about ten billion quartz crystals are used every year. Electronics-grade crystals can be used in filters, frequency controls, timers, electronic circuits that become important components in cell phones, watches, clocks, games, television receivers, computers, navigational instruments, and other products. Optical-grade crystals can be used as lenses and windows in lasers and other specialized devices. Although some natural quartz crystals are used in these applications, most of these special crystals are now manufactured.

Ametrine: A bicolor stone combining golden citrine and purple amethyst. This gem measures about 8x10 mm.

Quartz as a Gemstone Quartz makes an excellent gemstone. It is hard, durable, and usually accepts a brilliant polish. Popular varieties of quartz that are widely used as gems include: amethyst, citrine, rose quartz, and aventurine. Agate and jasper are also varieties of quartz with a microcrystalline structure.

Rose quartz: Translucent rose quartz in the rough.

Rose quartz beads: Translucent rose quartz - cut and polished beads. Each bead is about ten millimeters in diameter.

Novaculite is a dense, cryptocrystalline variety of quartz with a fine-grained and very uniform texture. As quartz, it has a hardness of 7 (harder than steel) and is used as a "whetstone" for sharpening knives.

Special Silica Stone Uses "Silica stone" is an industrial term for materials such as quartzite, novaculite, and other microcrystalline quartz rocks. These are used to produce abrasive tools, deburring media, grinding stones, hones, oilstones, stone files, tube-mill liners, and whetstones.

Tripoli Tripoli is crystalline silica of an extremely fine grain size (less than ten micrometers). Commercial tripoli is a nearly pure silica material that is used for a variety of mild abrasive purposes which include: soaps, toothpastes, metal-polishing compounds, jewelry-polishing compounds, and buffing compounds. It can be used as a polish when making tumbled stones in a rock tumbler. Tripoli is also used in brake friction products, fillers in enamel, caulking compounds, plastic, paint, rubber, and refractories. Contributor: Hobart King

Realgar and Orpiment Arsenic Sulfide Minerals

Realgar: Realgar crystals from the Royal Reward Mine in King County, Washington. Specimen measures about 2.2 x 1.1 x 0.8 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Toxic Arsenic Sulfide Minerals Realgar and orpiment are very similar minerals. They are both arsenic sulfides and members of the monoclinic crystal system. They form in the same geological environments and can be closely associated in the same deposits. They have similar physical properties and similar histories of use by man. Because of these similarities, we decided to describe realgar and orpiment in a single article. Realgar and orpiment are both toxic minerals, and contact with them should be avoided. They are not suitable for classroom specimens. Physical Properties: Realgar / Orpiment

Chemical Classification Color

Realgar

Orpiment

Sulfide

Sulfide

Red to orange, gray

Yellow, golden, yellow-brown

Streak

Red-orange to red

Light yellow

Luster

Resinous to pearly on cleavage

Resinous to pearly on cleavage

Translucent to transparent

Translucent to transparent

Good

Perfect

Mohs Hardness

1.5 to 2

1.5 to 2

Specific Gravity

3.56

3.49

Diaphaneity Cleavage

Diagnostic

Color, streak, resinous luster. Association with

Properties

orpiment.

Chemical Composition Crystal System

Uses

Color, streak, luster, foliated appearance. Association with realgar.

As4S4

As2S3

Monoclinic

Monoclinic

An ore of arsenic. Historically used as a pigment,

An ore of arsenic. Used in the production of oil cloth,

depilatory, poison, ingredient in explosives and

semiconductors, photoconductors. Historically used as a pigment,

fireworks, ritualistic "medicine," cosmetic.

poison, ingredient in fireworks and explosives.

What is Realgar? Realgar is a monoclinic arsenic sulfide mineral with a brilliant red color and a chemical composition of As4S4. Well-formed realgar crystals can look so much like red gemstones that the mineral was often called "ruby sulfur" and "ruby arsenic." However, realgar is not used as a gemstone because it is very soft, with a Mohs hardness of just 1-1/2 to 2. It is easily ground into a fine, bright red powder. Those properties caused it to become a favorite pigment in many parts of the ancient world. It was traded over great distances to make paints, inks, and dyes - until people realized that it was toxic. The arsenic in realgar was the source of its toxicity. After its toxicity was realized in the Middle Ages, the mineral was used as a poison to kill rodents, insects, and weeds. Realgar was also used in leather processing to remove the hair from hides. Today it is rarely used for any of these purposes because more effective and less toxic substitutes have been developed. However, some use of realgar in ritualistic cosmetics and "medicines" continues in a few parts of the world. Today the main use of realgar is as an ore of arsenic metal. The chemical formula of realgar is often written as As4S4instead of the simpler AsS. This is done because As4S4represents a structural unit of the mineral. A compound of As+3 and S-2 would be out of electrical balance. In realgar, three of the arsenics are joined in a chain by covalent bonds. This gives the arsenics an effective electrical charge of +8. That combines with four S-2 ions to produce an electrically neutral molecule. This is why realgar's chemical composition is often presented as As4S4 instead of AsS.

Orpiment: Bright orange orpiment from the Jiepaiyu Mine in Hunan Province, China. This specimen shows the botryoidal habit of orpiment. It measures 12.7 centimeters in length. Specimen and photo by Arkenstone / www.iRocks.com.

What is Orpiment? Orpiment is a monoclinic arsenic sulfide with a yellow-orange color and a chemical composition of As2S3. It has a Mohs hardness of 1-1/2 to 2 and is easily ground into a yellow to yellow-orange powder. Like realgar, its earliest widespread use was as a pigment for paints, inks, and dyes, and it was traded over great distances. After its toxicity was discovered, its use as a pigment declined. People took advantage of its toxicity to use it as a poison for insects and rodents. Some people continued to use it as a ritualistic cosmetic and "medicine" even after its toxicity was known, and that practice continues today in some parts of the world. Today the primary use of orpiment is as an ore of arsenic. It is also used in manufacturing oil cloth, semiconductors, and photoconductors.

Realgar on calcite: Red realgar crystals on white calcite, from the Jiepaiyu Mine in Hunan Province, China. Specimen and photo by Arkenstone / www.iRocks.com.

Geologic Occurrence Realgar and orpiment are mainly found associated with hydrothermal and volcanic activities. They are sublimation products at volcanic vents and crystallization products at hot springs. These are among the

earliest deposits exploited in the middle ages. Underground deposits of realgar and orpiment are in veins and fractures. There they are associated with lead, silver, gold, and other arsenic minerals. Contributor: Hobart King

Rhodochrosite A pink manganese carbonate mineral used as an ore, a gemstone, and a crystal specimen.

Rhodochrosite: Rhodochrosite cabochons illustrating the banded pink colors that are characteristic of this mineral. The specimen in the upper right is a slice from a stalactite. All stones cut from material mined in Argentina.

What is Rhodochrosite? Rhodochrosite is a manganese carbonate mineral that ranges in color from light pink to bright red. It is found in a small number of locations worldwide where other manganese minerals are usually present. Rhodochrosite is sometimes used as an ore of manganese but is rarely found in economic quantities. Specimens with a wonderful pink color are used to produce highly desirable gemstones. Rhodochrosite is rarely found as well-formed crystals, so crystals can be extremely valuable as mineral specimens. Physical Properties of Rhodochrosite Chemical Classification

Carbonate

Color

Pink, red, yellow, gray, brown

Streak

White

Luster

Vitreous to pearly

Diaphaneity Cleavage

Transparent to translucent Perfect, rhombohedral, in three directions

Mohs Hardness

3.5 to 4

Specific Gravity

3.5 to 3.7

Diagnostic Properties

Pink color, cleavage, hardness, effervescence in cold dilute hydrochloric acid

Chemical Composition

(Mn,Fe,Mg,Ca)CO3

Crystal System Uses

Hexagonal Ore of manganese, gemstone, ornamental stone

Physical and Chemical Properties Rhodochrosite has a variable chemical composition. It is a manganese carbonate, but the manganese is frequently replaced by iron, magnesium and/or calcium as shown in this formula: (Mn,Fe,Mg,Ca)CO3. These substitutions of other elements for manganese change the composition and alter the specific gravity, hardness, and color of the mineral. The bright pink color can become grayish, yellowish, or brownish in response to this chemical variability. A complete solid solution series exists between rhodochrosite and siderite (FeCO3). Rhodochrosite is generally easy to identify and is rarely confused with other minerals. Its pink color, perfect cleavage in three directions, low hardness, and weak effervescence with cold dilute hydrochloric acid are rarely seen in other minerals. The most common confusion is between the names "rhodochrosite" and "rhodonite" -- both are pink, manganese-rich minerals with very similar names that people have a hard time remembering.

Colorado Rhodochrosite: Rhodochrosite is the official state mineral of Colorado. Sometimes, nice transparent specimens can be found that are suitable for cutting faceted stones. This faceted cushion was cut from material obtained from the famous Sweet Home Mine near Alma, Colorado. It has a nice orangish pink color, measures 6.7 x 6.2 millimeters and weighs 1.52 carats. Photo by Bradley Payne, TheGemTrader.com.

Geologic Occurrence The formation of rhodochrosite usually occurs in fractures and cavities of metamorphic and sedimentary rocks. It is often associated with silver deposits, and a few silver mines produce rhodochrosite as a byproduct. Some of the common modes of occurrence and their lapidary uses are described below. In metamorphic rocks, rhodochrosite is found as a vein and fracture-filling mineral where it precipitates from ascending hydrothermal solutions. Repeated episodes of crystallization allow it to build up in layers on the walls of the fracture. Each layer can be a unique precipitation event and produce material with a slightly different pink color. This gives character to the material for lapidary use. Miners usually remove the rhodochrosite from the wall rock of these veins and cut it into thin slabs with a diamond saw. The slabs can then be used to make cabochons, small boxes, or other lapidary projects.

Some rhodochrosite forms in cavities in sedimentary and metamorphic rocks when descending solutions deliver a supply of dissolved materials. In these deposits, the rhodochrosite accumulates in layers on the walls of the cavity and may form stalactites and stalagmites on the roof and floor of the cavity - just like speleothems in a cavern. These formations are often removed and slabbed to produce material with concentric pink banding. Some of the best examples of this form of rhodochrosite are found at the Capillitas and Catamarca deposits in Argentina. Rhodochrosite is extremely rare as well-formed crystals. One of the few locations in the world where they are found is the Sweet Home Mine, near Alma, Colorado. Originally opened as a silver mine in 1873, the rhodochrosite was disregarded at that time. Then, as the popularity of mineral collecting increased, the wellformed crystals found at the Sweet Home Mine became many times more valuable than the lapidary material. Excellent, small, hand-size specimens currently sell for five-digit numbers. Broken or damaged crystals are sometimes used as faceting rough. Rhodochrosite for lapidary and mineral specimen use is only found in a few locations worldwide. These include Argentina, South Africa, Peru, Montana, Colorado, Russia, Romania, Gabon, Mexico, and Japan.

Superb Rhodochrosite Crystals: Crystals of transparent red rhodochrosite on tetrahedrite with a few clear quartz crystals. This specimen is about 5 x 4 x 2 centimeters in size and was taken from the Sweet Home Mine. Crystals this transparent and of such wonderful color would make beautiful faceted stones, but because this is such a fine crystal specimen, there would be a huge financial loss if that were done. Specimen and photo by Arkenstone / www.iRocks.com.

Rhodochrosite as a Gemstone Rhodochrosite is a favorite gemstone of many people. It is often slabbed to show off its banded or concentric patterns. Most of the slabs are used to cut cabochons. Cutting rhodochrosite is a difficult job because the material has perfect cleavage, and it is so soft that it is hard to polish. Nice, stable, slabbed material is sometimes used to make small boxes and other ornaments. The rare transparent material that is not suitable as a mineral specimen is sometimes faceted into attractive pink and red gems. The beautiful stones produced are mainly for collectors because faceted rhodochrosite is too fragile for most jewelry use. Rhodochrosite has a hardness of only 3.5 to 4 and has perfect cleavage in three directions. This eliminates it as a good choice as a ring or bracelet stone which might be subject to abrasion or impact. It does well in earrings, pins, and pendants, which are generally not subject to as much abuse as a ring. Contributor: Hobart King

Rhodonite A pink to red manganese silicate used as a gemstone and minor ore of manganese.

Rhodonite from Nevada: Rhodonite with its characteristic matrix and fracture-filling of manganese oxide. This specimen from Humboldt County, Nevada was photographed by Chris Ralph of Nevada-Outback-Gems.com and is used here as a public domain image.

What is Rhodonite? Rhodonite is a pink manganese silicate of variable composition that often contains significant amounts of iron, magnesium, and calcium. It has a generalized chemical composition of (Mn,Fe,Mg,Ca)SiO3. Rhodonite is often associated with black manganese oxides which may occur as dendrites, fracture-fillings, or matrix within the specimen. Other names for rhodonite include "manganese spar" and "manganolite."

Rhodonite cabochon: A cabochon cut from pink rhodonite in a matrix of black manganese oxide. The pink color of this specimen is especially nice, and the manganese oxide is typical. This cabochon is about 28 millimeters in diameter.

Geologic Occurrence

Rhodonite is usually found in metamorphic rocks associated with other manganese minerals. It is also found in rocks that have been altered by contact metamorphism, hydrothermal and metasomatic processes. It is usually massive to granular in occurrence. Rarely, it is found as red triclinic crystals. Rhodonite is an uncommon mineral. It is found in a few small deposits across the world. Sources of rhodonite include: Russia, Canada, Australia, Brazil, Sweden, Peru, and England. In the United States it has been found in North Carolina, Colorado, New Jersey, and has been named as the state gem of Massachusetts. Physical Properties of Rhodonite Chemical Classification

Silicate

Color

Pink, red, reddish brown to brown when weathered

Streak

White

Luster

Pearly to vitreous

Diaphaneity Cleavage

Transparent to translucent Perfect, two directions, 90 degrees

Mohs Hardness

5.5 to 6.5

Specific Gravity

3.5 to 3.7

Diagnostic Properties

Pink color, cleavage, specific gravity, frequent association with black manganese oxide

Chemical Composition

(Mn2+,Fe2+,Mg,Ca)SiO3

Crystal System Uses

Triclinic Decorative stone, gemstones

Physical Properties Rhodonite's diagnostic properties are its pink to red color, hardness, high specific gravity, perfect cleavage, and its close association with black manganese oxides. It is sometimes confused with rhodochrosite, which is softer and effervescent in hydrochloric acid, or thulite, which is usually not associated with black manganese oxides. The physical properties of rhodonite are summarized in the table on this page.

Faceted rhodonite: Rhodonite is rarely seen as a faceted stone. Excellent crystals of rhodonite are very rare and usually sell for such high prices to mineral collectors that very few are faceted. Occasionally, broken or second-quality crystals are faceted. Because of their cleavage and low hardness, faceted specimens of rhodonite are more in demand as "collector gems" than for use in jewelry.

Uses of Rhodonite Rhodonite was once used as ore of manganese in India. Today its only uses are as lapidary materials and as mineral specimens. High-quality crystals of rhodonite can sell for very high prices. Good massive pink- to red-colored material is used as an ornamental stone or gem rough. It is typically used to make cabochons, beads, small sculptures, tumbled stones, and other lapidary projects. Rare, well-formed, transparent crystals are highly sought after by mineral collectors. Damaged crystals of good quality are sometimes cut into faceted stones. Most of these are acquired by collectors because their cleavage and low hardness make them too fragile for use in jewelry.

Rhodonite crystals: A nice cluster of rhodonite crystals from Minas Gerais, Brazil. Specimen is about 2.5 centimeters in height. Specimen and photo by Arkenstone / www.iRocks.com.

Tumbled rhodonite: Rhodonite is sometimes used to make tumbled stones in a rock tumbler. Many people enjoy the raspberry color of rhodonite against the black manganese oxide. Rhodonite is a good tumbling rough for experienced tumblers. The material most often offered as a tumbling rough is inexpensive and with significant amounts of black manganese oxide. It can be challenging to polish because the manganese oxide often has a hardness that is different from the rhodonite. This results in overcutting of one material and undercutting of the other.

Compositional and Structural Variations Specimens of rhodonite that contain up to 20% calcium oxide are usually grayish brown in color and are known as "bustamite." "Fowlerite" is the name given to specimens that contain up to 7% zinc oxide. Specimens with a brown color have usually been altered by weathering. Rhodonite is one of two minerals with a chemical composition of manganese silicate. The other is a hightemperature, low-pressure polymorph known as "pyroxmangite." Contributor: Hobart King

Rutile Mineral Properties and Uses What is Rutile? Rutile is a titanium oxide mineral that is most commonly found in granites, pegmatites and metamorphic rocks. It is also found in sands derived from the weathering of these rocks. Rutile also forms as slender crystals within quartz and micas. It is a common mineral in the alluvial sands that are dredged for magnetite and ilmenite.

Physical Properties of Rutile Chemical Classification

oxide

Color

red, reddish brown, reddish black

Streak

pale brown

Luster

adamantine to submetallic

Diaphaneity Cleavage

transparent to subtranslucent good

Mohs Hardness

6 to 6.5

Specific Gravity

4.2 to 4.4

Diagnostic Properties Chemical Composition Crystal System Uses

Rutile from Oaxaca, Mexico. Specimen is approximately 1 inch (2.5 centimeters) across.

luster, color

titanium oxide, TiO2 tetragonal an ore of titanium, pigments, inert coating on welding rods

Uses of Rutile? Rutile is used as a coating on welding rods. It is also used as an ore of titanium, a metal used where light weight and high strength are needed. Some rutile used in the production of pigments for paints.

Ilmenorutile, a variety of rutile from Iveland, Norway. Specimen is approximately 2 inches (5 centimeters) across.

Sand made of rutile ilmenite zircon from Georgia. Specimens are sand size particles.

Cabochons of rutilated quartz - an example of how long thin crystals of rutile can form in quartz. Each gem is approximately 12 millimeters by 10 millimeters.

Nigrine, a variety of rutile from Magnet Cove, Arkansas. Specimens are approximately 1/2 inch to 1 inch (1.3 centimeters to 2.5 centimeters) across.

Scapolite A name used for the solid-solution mineral pair of marialite and meionite.

Cat's-eye scapolite: Some scapolite has an internal silk that causes it to form a cat's-eye or a chatoyance. The stone on the left is a 10 x 7 millimeter oval with a very coarse silk. The silk can be seen in the stone as linear bands of black inclusions that cross the stone from left to right. The cat's-eye forms at right angles to the silk. In the stone on the right, the silk has just the right spacing to serve as a diffraction grating and produce a beautiful display of iridescent color. Both gems were cut from material produced in India.

What is Scapolite? Scapolite is a name used for a group of aluminosilicate minerals that includes meionite, marialite, and silvialite. Meionite and marialite are end members of a solid solution series. Silvialite is a mineral that is very similar to meionite. These minerals have very similar compositions, crystal structures, and physical properties. They cannot be easily distinguished from one another in the field or during hand specimen examination in a laboratory. The name "scapolite" is a term used for convenient communication. These minerals are found in small quantities in some metamorphic and igneous rocks. Their compositions are compared in the table below. Mineral

Chemical Composition

Meionite

Ca4(Al2Si2O8)3(CO3,SO4)

Marialite

Na4(AlSi3O8)3Cl

Silvialite (Ca,Na)4(Al2Si2O8)3(CO3,SO4)

Scapolite crystals: Scapolite crystals of about 1 inch in length on matrix. Scapolite is one of a small number of minerals that have crystals with a square cross-section. These light purple crystals were found in Pakistan. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Scapolite Scapolite has an appearance that is very similar to many feldspars. As a result, it can easily be overlooked in the field and during hand specimen examination in a laboratory. Massive scapolite is found in regionally metamorphosed rocks such as marble, gneiss, and schist. These massive specimens often exhibit a wood-grain or fibrous texture which facilitates their identification. Wellformed, gem-quality, prismatic crystals with a square cross-section are sometimes found in marbles. In metamorphosed igneous rocks, especially gabbro andbasalt, scapolite often occurs as complete or partial replacements of the feldspar grains. Crystals of scapolite are sometimes found in pegmatites and rocks altered by contact metamorphism. Scapolite minerals are easily attacked by weathering. They are some of the first minerals attacked in their host rocks and easily alter to micas and clay minerals. As weathering begins, the mineral grains lose their transparency, become opaque, and have a reduced hardness. Physical Properties of Scapolite Chemical Classification

Silicate

Color

Colorless, white, gray, yellow, orange, pink, purple

Streak

White

Luster

Vitreous

Diaphaneity

Transparent to translucent

Cleavage

Good

Mohs Hardness

5 to 6

Specific Gravity

2.5 to 2.7

Diagnostic

Luster, specific gravity, massive specimens often have a wood-grain or fibrous appearance, prismatic crystals have a

Properties

square cross-section

Chemical Composition Crystal System Uses

A solid solution between marialite (Na4(AlSi3O8)3Cl) and meionite (Ca4(Al2Si2O8)3(CO3,SO4))

Tetragonal Faceted gemstones and cat's-eye cabochons

Faceted scapolite: Transparent scapolite can be cut into beautiful faceted gems that are often clear, yellow, pink, or purple in color. It is rarely seen in jewelry because it is not commonly available, and the public is unfamiliar with the gem. It has a hardness of only 5 to 6, which is soft for a ring stone. This stone is a 13 x 10 millimeter oval cut from material produced in India.

Uses of Scapolite Scapolite does not have a role as an industrial mineral. It is rarely found in minable quantities and does not have a composition or physical properties that make it of industrial use. The only use of scapolite is as a minor gemstone; however, in that use it can be beautiful and interesting. Yellow and pink transparent scapolite can be cut into very attractive gems like the yellow scapolite shown on this page. Some specimens contain tiny fibrous inclusions that produce a "silk" within the stone that reflects light to form a cat's-eye. A specimen with a coarse silk that forms both a cat's-eye and a diffraction grating is shown in the photo at the top of this page. Scapolite has a Mohs hardness of between 5 and 6, which is too soft to serve as a ring stone. Its use is therefore limited to being a collector's stone and being mounted in jewelry such as earrings and pendants that have a low risk of impact or abrasion. Contributor: Hobart King

Serpentine A group of minerals used as architectural, ornamental, and gem materials. A source of asbestos.

Lizardite: This is a specimen of lizardite, a serpentine-group mineral. This specimen has a gemmy green color and a very smooth texture. This specimen is suitable for cutting into a few gemstones. This specimen is about four centimeters across. From Warren County, New York.

What is Serpentine? Serpentine is not the name of a single mineral. Instead it is a name used for a large group of minerals that fit this generalized formula: (X)2-3(Y)2O5(OH)4 In this formula, X will be one of the following metals: magnesium, iron, nickel, aluminum, zinc, or manganese; and, Y will be silicon, aluminum, or iron. The appropriate generalized formula is thus (Mg,Fe,Ni, Mn,Zn)2-3(Si,Al,Fe)2O5(OH)4. Chrysotile, antigorite, and lizardite are three of the primary serpentine minerals. There are many other serpentine minerals, most of which are rare. Serpentine group minerals have similar physical properties and form by similar processes. They often occur as fine-grained admixtures and can be difficult to distinguish within a rock. Geologists usually call these materials "serpentine" rather than more specific names to simplify communication.

Ophiolite: Surface exposure of an Ordovician ophiolite in Gros Morne National Park, Newfoundland. Ophiolites are occurrences of oceanic plate and/or mantle rock exposed at the surface. They usually consist of serpentinite and associated rocks. (GNU Free Documentation License Image).

Serpentinites and Serpentine Formation Serpentine minerals form where peridotite, dunite, and other ultramafic rocks undergo hydrothermal metamorphism. Ultramafic rocks are rare at Earth's surface but are abundant at the oceanic moho, the boundary between the base of the oceanic crust and the upper mantle. They are metamorphosed at convergent plate boundaries where an oceanic plate is pushed down into the mantle. This is where they are subjected to hydrothermal metamorphism. The source of water for this process is seawater entrained in the rocks and sediments of the oceanic slab. During hydrothermal metamorphism, olivine and pyroxene minerals are transformed into or are replaced by serpentine minerals. Some of the metamorphic rocks produced here are composed almost entirely of serpentine minerals. These serpentine-rich rocks are known as "serpentinites." Extensive areas of Earth's surface are underlain by serpentinites. These areas occur near present or ancient convergent plate boundaries. They are locations where remnants of an oceanic plate is exposed at the surface. The remnant portion of the plate was either thrusted up onto land, accreted onto the edge of a land mass, or exposed by uplift and deep weathering. These areas of exposed oceanic plate are known as ophiolites. They are often the source of valuable minerals that might include magnetite, chromite, chrysoprase, jade, and serpentine. Physical Properties of Serpentine Chemical Classification

Silicate

Color

Usually various shades of green, but can be yellow, black, white, and other colors.

Streak

White

Luster

Greasy or waxy

Diaphaneity

Translucent to opaque, rarely transparent

Cleavage

Poor to perfect

Mohs Hardness

Variable between 3 and 6

Specific Gravity

2.5 to 2.6

Diagnostic Properties

Color, luster, fibrous habit, hardness, slippery feel

Chemical Composition

(Mg,Fe,Ni,Al,Zn,Mn)2-3(Si,Al,Fe)2O5(OH)4

Crystal System Uses

Most serpentine minerals are monoclinic. A source of asbestos, architectural stone, ornamental stone, gem material.

Physical Properties of Serpentine The most obvious physical properties of serpentine are its green color, patterned appearance, and slippery feel. These remind the observer of a snake and that is where the name "serpentine" was derived. Serpentine is also known for its translucent diaphaneity, waxy luster, ease of being cut into shapes, and its ability to accept a polish. These properties make it a popular gemstone, architectural material, and ornamental stone. Last is serpentine's ability to resist the transfer of heat. That makes it a valuable insulator. Fibrous varieties of serpentine, such as chrysotile, have been used to make asbestos, which has many industrial uses. Its use today is limited because the fibers have been associated with respiratory disease.

Architectural Serpentine: Serpentine has a long history of use as an architectural stone. It is usually green in color, cuts easily, polishes well, and has an attractive appearance. It was popular in the first half of the 20th century but is used less today, partly out of concern that it might contain asbestos. Enlarge image. Images copyright by iStockphoto and, clockwise from top left, Vladvg, Violetastock, AlexanderCher, and AlexanderCher.

Use of Serpentine: Architectural Material Serpentine has been used as an architectural stone for thousands of years. It is available in a wide variety of green and greenish colors, often has an attractive pattern, works easily, and polishes to a nice luster. It has a Mohs hardness of 3 to 6 which is softer than granite, and usually harder than most marble. This low hardness limits its appropriate use to surfaces that will not receive abrasion or wear, such as facing stone, wall tiles, mantles, and window sills.

Serpentine was popular in the United States during the first half of the 20th century and is less popular today. The decline in popularity is partly related to concerns about worker safety and the possible asbestos content of the stone. In the dimension stone trade, serpentine is often sold as "marble." It might also be described as "serpentine marble" or given a trade name that does not include the word "serpentine." This is a tradition of the industry and is usually not a misidentification of the material. This practice severely irritates some geologists. :-)

Chrysotile: A rock containing chrysotile, a serpentine group mineral, with a fibrous habit in fractures. Specimen is approximately five centimeters across. From Easton, Pennsylvania.

Use of Serpentine: Asbestos Some varieties of serpentine have a fibrous habit. These fibers resist the transfer of heat, do not burn, and serve as excellent insulators. The serpentine mineral chrysotile is common, found in many parts of the world, is easily mined, and can be processed to recover the heat-resistant fibers. The use of chrysotile and other serpentine minerals with an asbestiform habit as insulators has been widespread. They were widely available, effective in their applications and inexpensive to produce. By the middle of the 20th century, they could be found in most buildings and vehicles. They were used to make wall and ceiling tiles, flooring, shingles, facing material, pipe insulation, stoves, paints, and many other common construction materials and appliances. After they were discovered to be connected to lung and other cancers, their use was mostly discontinued, and a campaign to remove them from many of their uses began. Removal programs have been ongoing for decades and are still being done today. It has been one of the most costly removal programs in history.

Serpentine Cabochons: Three interesting cabochons cut from various types of serpentine. This is only a small example of the infinite diversity of serpentine gem materials.

Lime Green Serpentine: Rare specimens of serpentine have a wonderful green color, clarity, and translucence. These specimens have the appearance of nice jade and are sometimes confused with it in retail products.

Use of Serpentine: Gemstones Attractive serpentine can be cut into a wide variety of gemstones. It is most often cut into cabochons and beads. They usually display a range of green, yellow, and black colors and often have magnetite, chromite, or other minerals as interesting inclusions. The lower left side of the green and black cabochon in the center of the photo on this page contains enough included magnetite that the cab can be moved with a small hand magnet. Gemstone-quality serpentine is easy to polish, and beautiful finishes are possible. However, it usually polishes to a waxy luster rather than the brilliant glassy luster of much harder materials such as agate, jasper, and faceted stones. Rockhounds who polish their first piece of serpentine and know this have their expectations calibrated in advance. The waxy luster is a beautiful and common characteristic of the material. It does not reflect the skill of the operator. Extra polishing time and effort will still produce a waxy luster. Serpentine has some durability concerns. It has a hardness that ranges from 3 to 6 on the Mohs scale. Three is far too soft for anything but the most gently-worn jewelry such as earrings, brooches, or pendants. A

Mohs hardness of six is not hard enough for confident use in a ring or bracelet. Beads can be made from the more durable serpentine. Some specimens of serpentine have a wonderful green color, clarity, and translucence. They are easily mistaken for fine jade by inexperienced buyers. The experienced buyer knows that serpentine polishes to a soft waxy luster rather than a bright glassy luster. Cabochons or beads with a waxy luster are not jade -- or they are jade with a poor polishing job.

Serpentine Sculpture: A small lion sculpture made of beautiful green serpentine by the House of Fabergé. This image by Shakko/Photos is displayed here under a Creative Commons License.

Use of Serpentine: Sculptures Some varieties of serpentine can be carved into beautiful stone sculptures. Fine-grained, translucent material with a uniform texture and without voids and fractures is preferred. Serpentine is relatively soft and carves easily. It also accepts a nice polish. Serpentine sculptures range in size from under one centimeter to several meters in height. Bowls, vases, desk sets, clock bases, animals, fruit, flowers, legendary figures, deities, busts, and statues are all common objects made by artists working with serpentine.

Use of Serpentine: CO2Sequestration Serpentinite rock units have been considered as repositories for the disposal of waste carbon dioxide produced when fossil fuels are burned. Injecting carbon dioxide into subsurface rock units in the presence of water can produce magnesium carbonate and quartz in an exothermic reaction similar to the one shown below. Mg3Si2O5(OH)4 + 3CO2 + H2O --> 3MgCO3 + 2SiO2 + 3H2O

Numerous studies and small scale tests of geological sequestration of CO2 have produced promising results, but the procedure has not been placed into commercial practice.

Siderite Mineral Properties and Uses Physical Properties of Siderite Chemical Classification

carbonate

Color

yellowish, reddish, grayish, brown

Streak

white

Luster

vitreous

Diaphaneity Cleavage

transparent to translucent perfect

Mohs Hardness

3.5 to 4.5

Specific Gravity

3.8 to 4

Diagnostic Properties

color, specific gravity, dissolves in HCl

Chemical Composition

FeCO3

Crystal System Uses

hexagonal iron ore, pigments

Siderite from Roxbury, Connecticut. Specimen is approximately 4 inches (10 centimeters) across.

Siderite from Roxbury, Connecticut. Specimen is approximately 3 inches (7.6 centimeters) across.

Sillimanite Mineral Properties and Uses

Physical Properties of Sillimanite Chemical Classification

silicate

Color

colorless, white, yellow, brown, blue, green

Streak

colorless

Luster

vitreous

Diaphaneity Cleavage

transparent to translucent perfect

Mohs Hardness

6.5 to 7.5

Specific Gravity

3.2 to 3.3

Diagnostic Properties

slender crystals, fibrous habit

Chemical Composition

Al2SiO5

Crystal System Uses

orthorhombic no significant commercial use

Sillimanite with magnetite from Benson Mines, New York. Specimen is approximately 4 inches (10 centimeters) across.

Sillimanite with magnetite from Benson Mines, New York. Specimen is approximately 4 inches (10 centimeters) across.

Sillimanite from Williamstown, South Australia. Specimen is approximately 4 inches (10 centimeters) across.

Sillimanite from Dillon, Montana. Specimen is approximately 4 inches (10 centimeters) across.

Silver The soft, white, native metallic element with a diversity of uses.

Silver crystals: Crystals of native silver on calcite from the New Nevada Mine, Batopilas, Chihuahua, Mexico. Specimen is approximately 11 x 7 x 6 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What is Silver? Silver is a soft, white metal that usually occurs in nature in one of four forms: 1) as a native element; 2) as a primary constituent in silver minerals; 3) as a natural alloy with other metals; and, 4) as a trace to minor constituent in the ores of other metals. Most of the silver produced today is a product of the fourth type of occurrence. Silver is known as a "precious metal" because it is rare and because it has a high economic value. It is valuable because it has a number of physical properties that make it the best possible metal for many different uses. Silver has an electrical and thermal conductance that is higher than any other metal. It has a higher reflectivity at most temperatures than any other metal. It has an attractive color and luster that resist tarnish and make the metal desirable in jewelry, coins, tableware, and many other objects. These are just a few of silver's important properties. When performance is more important than price, silver is often the material of choice.

Silver wire: A specimen of wire silver with a heavy tarnish of acanthite on a calcite matrix. Specimen is approximately 6 x 4 x 3 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Silver Chemical Classification

Native element

Color

Silvery white

Streak

Silvery white

Luster

Metallic

Diaphaneity

Opaque

Cleavage

None

Mohs Hardness

2.5 to 3

Specific Gravity

10.0 to 11.0

Diagnostic Properties

Color, specific gravity

Chemical Composition

Ag

Crystal System Uses

Isometric Jewelry, tableware, coins, electronics, photographic films, ornaments

Related: The Many Uses of Silver

The Many Uses of Silver

Uses of silver: Historically, silver has been used in coins, silverware, and jewelry, but today, these uses account for less than half of all silver consumption. Silver has become a material of innovation that appears in many unexpected places. Images copyright iStockphoto / Jorge Farres Sanchez, Tatiana Buzuleac, Nigel Spooner, and Stephanie Frey.

The White Metal Silver, the white metal, has an illustrious reputation for its use in jewelry and coins, but today, silver's primary use is industrial. Whether in cell phones or solar panels, new innovations are constantly emerging to take advantage of silver's unique properties. Silver is a precious metal because it is rare and valuable, and it is a noble metal because it resists corrosion and oxidation, though not as well as gold. Because it is the best thermal and electrical conductor of all the metals, silver is ideal for electrical applications. Its antimicrobial, non-toxic qualities make it useful in medicine and consumer products. Its high luster and reflectivity make it perfect for jewelry, silverware, and mirrors. Its malleability, which allows it to be flattened into sheets, and ductility, which allows it to be drawn into thin, flexible wire, make it the best choice for numerous industrial applications. Meanwhile, its photosensitivity has given it a place in film photography. Because it is more abundant, silver is much less expensive than gold. Silver can be ground into powder, turned into paste, shaved into flakes, converted into a salt, alloyed with other metals, flattened into printable sheets, drawn into wires, suspended as a colloid, or even employed as a catalyst. These qualities ensure that silver will continue to shine in the industrial arena, while its long history in coinage and jewelry will sustain its status as a symbol of wealth and prestige.

Silver paste in electronics: Printed electronics like RFID tags rely on silver paste. Image copyright iStockphoto / JacobH.

Uses of silver in the U.S.: The United States Geological Survey has tabulated the amount of silver used in the United States by category of use. This data is shown in the graphic above. The "Other" category accounts for almost a quarter of the silver used and is fragmented into hundreds of different uses. Many of these are described below.

Uses of Silver in Electronics The number one use of silver in industry is in electronics. Silver's unsurpassed thermal and electrical conductivity among metals means it cannot easily be replaced by less expensive materials. For example, small quantities of silver are used as contacts in electrical switches: join the contacts, and the switch is on; separate them and the switch is off. Whether turning on a bedroom light using a conventional switch or turning on a microwave using a membrane switch, the result is the same: the current can pass through only when the contacts are joined. Automobiles are full of contacts that control electronic features, and so are consumer appliances. Industrial strength switches use silver, too. How does silver get from the earth to these electronic devices? Silver comes from silver mines or from lead and zinc mines from which silver is a by-product. Smelting and refining removes silver from the ore. Then, the silver is usually shaped into bars or grains. Electronics demand silver of the highest purity: 99.99% pure, also known as having a fineness of 999.9.

Dissolving pure silver in nitric acid produces silver nitrate, which can be formed into powder or flakes. This material, in turn, can be fabricated into contacts or silver pastes, like conductive paste made with a silverpalladium alloy.

Related: The Geology of Silver

Silver paste has many uses, such as the membrane switch already mentioned and the rear defrost in many cars. In electronics, circuit paths, as well as passive components called multilayer ceramic capacitors (MLCCs), rely on silver paste. One of the fastest growing uses of silver paste is in photovoltaic cells for the production of solar energy. Nanosilver, silver with an extremely small particle size (1-100 nanometers, that is 1-100 billionths of a meter), provides a new frontier for technological innovation, requiring much smaller amounts of silver to get the job done. Printed electronics work by using nanosilver conductive inks. One example of a printed electronic is the electrode in a supercapacitor, which can charge and discharge repeatedly and quickly. Regenerative breaking is an automotive innovation that allows the kinetic energy of a slowing vehicle to be stored in a supercapacitor for reuse. Radio frequency identification (RFID) tags offer another powerful application of printed electronics. These tags are better than bar codes for tracking inventory because they store more information and can be read from a greater distance, even without a direct line of sight. Silver has its place in consumer electronics, too. Your plasma television set may rely on silver for more than just the on-off switch if it contains a silver electrode aimed at giving a higher quality image. Light emitting diodes (LED) also use silver electrodes to produce low-level, energy efficient light. Meanwhile the DVDs and CDs you play probably have a thin silver recording layer. Another electronic application of silver is in batteries that employ silver oxide or silver zinc alloys. These light-weight, high-capacity batteries perform better at high temperature than other batteries. Silver-oxide is used in button batteries that power cameras and watches, as well as in aerospace and defense applications. Silver-zinc batteries offer an alternative to lithium batteries for laptop computers and electric cars. On the cutting edge of technology are superconductors. Silver is not a superconductor, but when paired with one, the two together can transmit electricity even faster than the superconductor alone. At very low temperatures, superconductors carry electricity with little or no electrical resistance. They can be used to generate magnetic energy for turning motors or propelling magnetic levitation trains. The myriad applications of silver in electronics offer an eye-opening view into how one of the most famous metals in history has become a cutting edge material of the future. Due in part to its unique property of having the highest thermal and electrical conductivity of all metals, silver is often a must-have over other, less expensive materials.

Silver in solar panels: Silver paste contacts form bus bars and grid lines that draw electrical current from the semiconducting surface of a photovoltaic cell. Image copyright iStockphoto / Gyuszko.

Uses of Silver in Energy As mentioned previously, silver paste is used to make solar panels. Silver paste contacts printed onto photovoltaic cells capture and carry electrical current. This current is produced when energy from the sun impacts the semiconducting layer of the cell. Photovoltaic cells are one of the fastest growing uses of silver. Silver's reflectivity gives it another role in solar energy. It reflects solar energy into collectors that use salts to generate electricity. Nuclear energy also uses silver. The white metal is often employed in control rods to capture neutrons and slow the rate of fission in nuclear reactors. Inserting the control rods into the nuclear core slows the reaction, while removing them speeds it up.

Silver brazing and soldering: Silver brazing and soldering ensures a tight joint between metal pipes. Image copyright iStockphoto / bypicart.

Uses of Silver in Brazing and Soldering Brazing and soldering make use of silver's high tensile strength and ductility to create joints between two metal pieces. Brazing takes place at temperatures above 600°C, while soldering takes place at temperatures below 600°C. Silver scrap can be used in brazing and soldering because these processes do not require very pure silver. Brazing and soldering produce tight joints for everything from heating and air conditioning vents

to plumbing. Silver's antibacterial properties and non-toxicity to humans make it a great replacement for lead-based bonds between water pipes.

Silver wire: Silver wire can be woven into a metal mesh and used as a catalyst. Granular silver also makes a good catalyst. Image copyright iStockphoto / Greg801.

Uses of Silver in Chemical Production Silver acts as a catalyst to produce two important chemicals: ethylene oxide and formaldehyde. Ethylene oxide is used to produce molded plastics, such as plastic handles, and flexible plastics, such as polyester. It is also a major ingredient in antifreeze. Formaldehyde is used to make solid plastics and resins and as a protective coating. It is also used as a disinfectant and embalming agent. As a catalyst, silver increases the speed of reactions without getting used up.

Silver coins and bullion: Silver coins have been minted for thousands of years. Silver bullion is still a popular investment choice today. Image copyright iStockphoto / shakzu.

Related: The Many Uses of Gold

Uses of Silver in Coins and Investments Silver has traditionally served, with gold, as the metal used in coins. As a precious metal, silver is rare and valuable, making it a convenient store of wealth. In the past, people accumulated their wealth in the form of silver coins; today, they invest in investment-grade silver bullion. The fact that silver does not corrode and

only melts at a relatively high temperature, means that it can last, and the fact that it has high luster makes it attractive. Its malleability makes silver a good choice for designing and minting local currency. In greater abundance, and therefore less expensive, than gold, silver has been used more prevalently as currency. Silver was mined and used in trading several thousands of years BC and was first minted into silver coins in the Mediterranean region many hundreds of years BC. Until the 20th century, many countries used a silver or gold standard, backing up the value of currency with the presence of gold or silver in the treasury. Today, countries use less expensive metals, such as copper and nickel, to produce coins, and they use fiat currency, in which government regulation controls the value, instead of a gold or silver standard. Still, silver retains its value as a commodity. Many individuals choose to invest in silver through financial instruments, like stocks and mutual funds, or by actually buying and storing 99.9% pure silver bullion bars, coins, or medallions. Countries sometime produce silver collector's edition coins, which they sell to buyers at a price exceeding the value of the silver used to make the coin.

Silver rings: Silver, precious and lustrous, makes beautiful, long-lasting jewelry. Image copyright iStockphoto / pixeljuice.

Uses of Silver in Jewelry and Silverware Jewelry and silverware are two other traditional uses of silver. Malleability, reflectivity, and luster make silver a beautiful choice. Because it is so soft, silver must be alloyed with base metals, like copper, as in the case of sterling silver (92.5% silver, 7.5% copper). Even though it resists oxidation and corrosion, silver can tarnish, but with a little polish, it can shine for a lifetime. Because it is less expensive than gold, silver is a popular choice for jewelry and a standard for fine dining. Silver-plated base metals offer a less costly alternative to silver. Silver dishes and plates may accompany silverware, and these can often be ornately crafted works of art. For example, Paul Revere (1734-1818), known to most for his midnight ride at the start of the American Revolution, was a silversmith by trade, and some of his artwork is still on display at the Boston Museum of Fine Arts.

Silver in photography film: Silver halide in photographic film responds to light, leaving a latent image behind. Image copyright iStockphoto / Njari.

Uses of Silver in Photography Photography had been one of the primary industrial uses of silver until the recent rise of digital media. Traditional film photography relies on the light sensitivity of silver halide crystals present in film. When the film is exposed to light, the silver halide crystals change to record a latent image that can be developed into a photograph. The accuracy of this process makes it useful for non-digital consumer photography, film, and X-rays. The silver used in film photography should not be confused with the "silver screen" of cinema. This phrase refers not to the silver in the film itself, but to the silver lenticular screen onto which early films were projected.

Silver is used in ointments: Some ointments take advantage of silver's antibacterial qualities to protect wounds against infection. Image copyright iStockphoto / cglade.

Uses of Silver in Medicine Silver ions act as a catalyst by absorbing oxygen, which kills bacteria by interfering with their respiration. This antibiotic property, along with its non-toxicity, has given silver an essential role in medicine for thousands of years. Before widespread use of antibiotics, silver foil was wrapped around wounds to help

them heal, and colloidal silver and silver-protein complexes were ingested or applied topically to fight illness. Silver has also been used in eye drops and in dental hygiene to cure and prevent infection. While silver is not toxic, repeat intake of small amounts of silver over time can result in argyria. In people with this condition, silver builds up in body tissue, giving it a gray-blue appearance when exposed to the sun. In addition, the ingestion of large amounts of silver can have negative effects on the body. For these reasons, medical doctors discourage the use of colloidal silver, discounting claims by some that colloidal silver is a cure-all dietary supplement. Today, the presence of antibiotic-resistant superbugs increases the demand for silver in hospitals. Small amounts of silver can coat hospital surfaces and medical equipment to prevent the spread of pathogens. Silver in surgical equipment, wound dressings, and ointments protects wounds from infection. Silver sulfadiazine is especially useful for burn victims because it kills bacteria while also allowing the skin to regrow. Silver ion treatments can heal bone infections and allow regeneration of damaged tissue.

Silver in windows of skyscrapers: Many modern skyscrapers have window glass that is coated with silver to reflect sunlight and reduce air conditioning costs. Image copyright iStockphoto / sankai.

Uses of Silver in Mirrors and Glass Silver is almost completely reflective when polished. Since the 19th century, mirrors have been made by coating a transparent glass surface with a thin layer of silver, though modern mirrors also use other metals like aluminum. Many windows of modern buildings are coated with a transparent layer of silver that reflects sunlight, keeping the interior cool in the summer. In aerospace, silver-coated tiles protect spacecraft from the sun.

Silver-coated ball bearings: Silver-coated ball bearings reduce friction in engines. Image copyright iStockphoto / felixR.

Uses of Silver in Engines Engine bearings rely on silver. The strongest bearing is made from steel that has been electroplated with silver. Silver's high melting point allows it to withstand the high temperature of engines. Silver also acts like a lubricant to reduce friction between a ball bearing and its housing. Due to its ability to absorb oxygen, silver is being researched as a possible substitute for platinum to catalyze oxidation of matter collected in diesel engine filters.

Silver awards: Silver often signifies second place and is used in medals and other awards. Image copyright iStockphoto / Vonkara1.

Uses of Silver in Awards Due to its status as a precious metal, ranked second only to gold, silver is often used to award second place. The most famous silver award is the second-place Olympic Silver Medal. Silver also symbolizes honor, valor, and accomplishment, which is why many military organizations, employers, clubs, and associations use silver or silver-colored awards to honor individuals for their contributions.

Silver in water filters: A silver coating prevents bacterial build-up in carbon-based water filtration systems. Image copyright iStockphoto / RaginaQ.

Uses of Silver for Water, Food, Hygiene Silver's antibacterial properties have been applied for thousands of years, long before the discovery of microbial organisms, because silver containers and coins were known to prevent spoilage of liquids. Today,

a silver coating prevents bacterial build-up in carbon-based water filters, while silver ions in water purification systems carry oxygen that oxidizes and kills microbes. Silver-copper ions can even replace corrosive chlorine to sanitize pools and tanks. The antimicrobial properties of silver that make it useful to medicine and water purification are now being applied in food and hygiene. Nanosilver coatings are applied to food packages and refrigerators. And many new consumer products, such as washing machines, clothing, and personal hygiene products tout the benefits of antibacterial silver.

Did You Know? Most of the world's silver is produced as a by-product of lead mining. The mineral galena (shown above) is a combination of lead and sulfur. However, a small amount of silver usually substitutes for lead in the mineral's crystal structure. At many galena mines, enough silver can be present in the galena that the value of the silver greatly exceeds the value of the lead that is mined. The financial reality of this is that the supply of silver is more dependent upon the amount of lead that is being mined than the price of silver. Specimen and photo by Arkenstone / www.iRocks.com. Silver References [1] Silver The Indispensable Metal: The Silver Institute, www.silverinstitute.org. [2] Silver: Mineral Commodity Summaries: United States Geological Survey, www.usgs.gov. [3] Silver: Minerals Yearbook: United States Geological Survey, www.usgs.gov.

Other Uses of Silver Other traditional uses of silver exist. For example, silver is one ingredient in the amalgam used to fill dental cavities, though this approach has been largely replaced by other materials due to the presence of toxic mercury in the amalgam. Silver has also been used to plate instruments, such as flutes. Today, silver is being applied to many new uses. Silver is one of many options for replacing toxic chromated copper arsenate as a wood preservative. Nanosilver inks and coatings on paper tout their ability to prevent the spread of bacterial infection. Silver metal glass, produced by cooling silver quickly, offers durable strength that resists deformation. Silver-based ionic liquids, which are in a liquid state at room temperature,

can be used to clean up petroleum waste products. Silver in fabric allows touch screen users to keep their gloves on during cold weather. Silver seems to have as many uses as the human imagination can develop. Traditional works of silver, like jewelry and silverware, rely on the creativity of artists. Modern uses depend on the creative exploits of scientists and engineers to meet the changing demands of consumers and industries. While some uses rise and fall, such as the use of silver in photographic film, other uses may continue to grow, such as the burgeoning production of photovoltaic cells for solar energy. Silver's unique properties, especially its high thermal and electrical conductivity, its reflectivity, and its antibacterial qualities, make it difficult to replace, like a one-of-a-kind silver ring

Silver as a Native Element Mineral Silver is rarely found as a native element mineral. When found, it is often associated with quartz, gold, copper, sulfides of other metals, arsenides of other metals, and other silver minerals. Unlike gold, it is rarely found in significant amounts in placer deposits. Native silver is sometimes found in the oxidized zones above the ores of other metals. It persists there because silver does not readily react with oxygen or water. It does react with hydrogen sulfide to produce a tarnished surface that is composed of the silver sulfide mineral known as acanthite. Many specimens of native silver that have been exposed to the atmosphere or to hydrothermal activity have an acanthite coating. Most native silver is found associated with hydrothermal activity. In these areas it often occurs in abundance as vein and cavity fillings. A few of these deposits are large enough and rich enough in native silver to support mining. In most cases, the economic viability of the deposit depends upon the presence of other valuable minerals. The mines are usually underground operations that follow the veins and cavities where the native silver occurs. Native silver is usually without a characteristic crystal habit. When it forms in the open spaces of pockets and fractures, some interesting crystal habits sometimes develop. The crystals are rarely the cubes, octahedrons, and dodecahedrons expected of an isometric mineral. Instead the silver's habit is usually thin flakes, plates, and dendritic crystal clusters formed in the narrow spaces of joints and fractures. Filiform and wire-like habits are also seen. Minerals that Contain Silver Acanthite

Ag2S

Aguilarite

Ag4SeS

Allargentum

Ag1-xSbx

Andorite

PbAgSb3S6

Arcubisite

Ag6CuBiS4

Argentite

Ag2S (when above 177°C)

Argyrodite

Ag8GeS6

Arquerite

(Ag,Hg)

Berryite

Pb3(Ag,Cu)5Bi7Si6

Boleite

KPb26Ag9Cu24(OH)48Cl62

Bromargyrite

AgBr

Canfieldite

Ag8SnS6

Chlorargyrite

AgCl

Chrisstanleyite

Ag2Pd3Se4

Crookesite

Cu7(Tl,Ag)Se4

Dyscrasite

Ag3Sb

Empressite

AgTe

Fettelite

Ag16HgAs4S15

Freibergite

(Ag,Cu,Fe)12(Sb,As)4S13

Freieslebenite

AgPbSbS3

Gabrielite

Tl6Ag3Cu6(As,Sb)9S21

Hessite

Ag2Te

Iodargyrite

AgI

Jalpaite

Ag3CuS2

Krennerite

(Au0.8,Ag0.2)Te2

Marrite

PbAgAsS3

Miargyrite

AgSbS2

Moschellandsbergite

Ag2Hg3

Pearceite

Cu(Ag,Cu)6Ag9As2S11

Petzite

Ag3AuTe2

Polybasite

[(Ag,Cu)6(Sb,As)2S7][Ag9CuS4]

Proustite

Ag3AsS3

Pyrargyrite

Ag3SbS3

Samsonite

Ag4MnSb2S6

Stephanite

Ag5SbS4

Stromeyerite

AgCuS

Stützite

Ag5-xTe3 (with x = 0.24 to 0.36) or Ag7Te4-

Sylvanite

(Ag,Au)Te2

Uytenbogaardtite

Ag3AuS2

Minerals that Contain Silver The number of minerals that contain silver as an essential constituent is surprising. The green table on this page contains a partial list of silver minerals that includes 39 different species. Each of these is a distinct silver mineral. All of them are rare, but a few (such as acanthite, proustite, and pyrargyrite) can be found in sufficient quantities to warrant mining. Silver minerals can be sulfides, tellurides, halides, sulfates, sulfosalts, silicates, borates, chlorates, iodates, bromates, carbonates, nitrates, oxides, and hydroxides.

Silver copper nugget: A stream-rounded nugget of silver and copper found in Keweenaw County, Michigan. Specimen is approximately 2.7 x 2.1 x 1.3 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Natural Silver Alloys and Amalgams Most gold found in placer deposits is alloyed with small amounts of silver. If the ratio between gold and silver reaches at least 20% silver, the material is called "electrum." Electrum is the name for an alloy of gold and silver. A significant amount of today's silver production is a refining byproduct of gold mining. Silver also forms a natural alloy with mercury. This silver amalgam is sometimes found in the oxidation zones of silver deposits and is occasionally associated with cinnabar.

Silver as a Constituent in Other Metals and Ores Most of the silver produced today is a byproduct of mining copper, lead, and zinc. The silver occurs within the ores of these metals in one of two ways: 1) substituting for one of the metal ions within the ore mineral's atomic structure; or, 2) occurring as an inclusion of native silver or a silver mineral within the ore mineral. The value of this minor silver within the ore mineral can exceed the value of the primary metal within the ore. The diagram below considers the situation of argentiferous galena (galena that contains up to a few percent by weight of silver substituting for lead in the galena mineral structure).

Galena value: Some mines producing galena produce more revenue from the silver content of their ore than from the lead content. Assume that we have a mine that produces argentiferous galena with an average composition of 86% lead, 13% sulfur, and just 1% silver (as shown in the diagram on the left). If the silver price is $25 per troy ounce and the lead price is $1 per avoirdupois pound, the value of the lead in one ton of ore will be $1720, while the value of the silver in that same ton of ore will be $7292 (as shown in the diagram on the right). The small amount of silver has a huge impact on revenue because at the prices assumed, silver is 364 times more valuable than an equal weight of lead. It is easy to understand why mining companies get excited by argentiferous galena! Even though galena is the ore being removed and lead makes up the bulk of the product, these mines are often called "silver mines."

Map of silver-producing countries: The map above shows the top ten silver-producing countries in the world for calendar year 2013. Data from the USGS Mineral Commodity Summary.

Geographic Distribution of Silver Production 2013 Silver Production Country

Metric Tons

Mexico

5,360

China

3,900

Peru

3,480

Australia

1,730

Russia

1,500

Bolivia

1,210

Chile

1,190

Poland

1,150

United States

1,060

Canada Other Countries

663 4,230

The values above are estimated silver mine production in metric tons from USGS Mineral Commodity Summaries.

Silver and silver-bearing minerals tend to be closely associated with magmatic activity, as that is where hydrothermal activity also occurs. This association holds especially well along western North, Central, and South America, where silver production follows the trend of the Andes Mountain Range. Argentina, Bolivia, Canada, Guatemala, Honduras, Mexico, Peru, and the United States are all significant producers of silver today and in the past. In other parts of the world, silver production is associated with igneous activity of any geologic age. In Europe there is a band of current and geologically ancient volcanic activity that passes from Spain in the west into Turkey in the east. Much of the European silver production has been from this trend. The table and map above show the top ten silver-producing countries in the world during calendar year 2013.

Sodalite The blue gem material, ornamental and architectural stone.

Sodalite: Pieces of polished sodalite. Image by Adam Ognisty, used here under a GNU Free Documentation License. Click to enlarge.

What Is Sodalite? Sodalite is a rare rock-forming mineral best known for its blue to blue-violet color. It has a chemical composition of Na4Al3Si3O12Cl and is a member of the feldspathoid mineral group. High-quality sodalite is used as a gemstone, a sculptural material, and an architectural stone. Sodalite occurs in igneous rocks that crystallized from sodium-rich magmas. This is the origin of the name "sodalite." These magmas also contained so little silicon and aluminum that quartz and feldspar minerals are often absent. Sodalite-bearing rocks include: nepheline syenite, trachyte, and phonolite. These types of rocks are so rare that most geologists never see them in the field. Well-known sources of sodalite include: Litchfield, Maine; Magnet Cove, Arkansas; northern Namibia; Golden, British Columbia; Bancroft, Ontario; Kola Peninsula of Russia; and the Ilimaussaq intrusive complex of Greenland.

Nepheline syenite with sodalite: Nepheline syenite rich in light blue sodalite. This rare material is sold as a dimension stone for interior use under the trade name of "sodalite granite." Found near Ice River, British Columbia, Canada. Specimen is approximately 3 inches (7.6 centimeters) across.

What Are Feldspathoids? Sodalite is a member of a mineral group known as "feldspathoids." They are rare aluminosilicate minerals that contain abundant calcium, potassium, or sodium. Examples are sodalite, nepheline, leucite, nosean, hauyne, lazurite, cancrinite, and melilite. These minerals often occur in igneous rocks, the veins and fractures that cut them. They also occur in contact metamorphic rocks.

Sodalite granite: A close-up of the "sodalite granite" from Ice River, British Columbia, Canada.

Physical Properties of Sodalite Sodalite is usually blue to blue-violet in color and found with nepheline and other feldspathoid minerals. It is usually translucent, with a vitreous luster, and has a Mohs hardness of 5.5 to 6. Sodalite often has white veining, and it can be confused with lapis lazuli. Small amounts of sodalite are present in many specimens of lapis lazuli. If significant pyrite is present, the specimen is not sodalite.

Sodalite is a member of the cubic crystal system, but well-formed crystals are rarely found. It is usually massive in habit and breaks with a conchoidal fracture rather than exhibiting its poor cleavage. Physical Properties of Sodalite Chemical Classification

Silicate

Color

Often bright blue to violet-blue with white veining. Also gray, white, green, colorless, yellow, red.

Streak

White, bluish

Luster

Vitreous to greasy

Diaphaneity Cleavage

Translucent to transparent Poor cleavage in six directions that is usually unnoticed. Breaks by conchoidal fracture.

Mohs Hardness

5.5 to 6

Specific Gravity

2.2 to 2.4

Diagnostic

Blue color, associated with feldspathoid minerals, especially nepheline. Differentiated from lapis by its darker blue color

Properties

and an absence of pyrite. Often produces a weak orange fluorescence under shortwave or longwave ultraviolet light.

Chemical Composition Crystal System

Na8(Al6Si6O24)Cl2

Cubic A semiprecious gemstone often used to produce cabochons, beads, and tumbled stones. An ornamental stone often used

Uses

to make small sculptures. "Sodalite granite" is a rare nepheline syenite with minor to abundant sodalite that is cut as a dimension stone for interior use. A less costly alternative to lapis lazuli.

Sodalite tumbled stones: Tumbled stones of sodalite. Pieces shown are about 5/8" to 1" in diameter.

Sodalite as a Gemstone

Blue is a rare color for rocks and minerals. When was the last time you found a rock with a vivid blue color? Have you ever found one in nature? When a blue rock - especially one capable of serving as a gem material - is found it immediately has a market. Sodalite is one of the only vivid blue materials that is still sold at a reasonable price. But, sodalite can be hard to find in finished jewelry. It is rarely seen in mall jewelry stores, and it is seen less often in high-profile jewelry stores. Most jewelry customers are not familiar with sodalite, and very few of them are asking for it in stores. Stores stock what customers want to buy. The place to find sodalite in jewelry is at craft and lapidary stores and shows. Part of the reason that sodalite is not widely used in commercial jewelry is that cut cabochons vary so much in appearance that it is difficult to impossible to produce a standardized product line. Sodalite is sometimes confused with lapis lazuli. Some specimens have a similar color and the presense of white veining is found in both materials. This gives sodalite the opportunity to be used as a less expensive alternative gem for people who like lapis lazuli but don't want to pay the high price. However, sodalite cabochons, beads and tumbled stones are beautiful and many people enjoy them. They are available at prices that fit almost anyone's budget. A limitation on sodalite's use in jewelry is its hardness of 5.5 to 6 on the Mohs scale. It will be quickly scratched if used in a ring or bracelet. It therefore is best used in earrings, pins, pendants, and other items that will not subject the sodalite to impact or abrasion.

Sphalerite The primary ore of zinc and a gemstone with a "fire" that exceeds diamond.

Sphalerite with galena and chalcopyrite: A typical mineral association of sphalerite with galena and chalcopyrite. From the Huaron Mine of Peru. Specimen is about 4.3 x 3.2 x 1.8 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

What is Sphalerite? Sphalerite is a zinc sulfide mineral with a chemical composition of (Zn,Fe)S. It is found in metamorphic, igneous, and sedimentary rocks in many parts of the world. Sphalerite is the most commonly encountered zinc mineral and the world's most important ore of zinc. Dozens of countries have mines that produce sphalerite. Recent top producers include Australia, Bolivia, Canada, China, India, Ireland, Kazakhstan, Mexico, Peru, and the United States. In the United States, sphalerite is produced in Alaska, Idaho, Missouri, and Tennessee. [1] The name sphalerite is from the Greek word "sphaleros" which means deceiving or treacherous. This name is in response to the many different appearances of sphalerite and because it can be challenging to identify in hand specimens. Names for sphalerite used in the past or by miners include "zinc blende," "blackjack," "steel jack," and "rosin jack."

Geologic Occurrence Many minable deposits of sphalerite are found where hydrothermal activity or contact metamorphism has brought hot, acidic, zinc-bearing fluids in contact with carbonate rocks. There, sphalerite can be deposited in veins, fractures, and cavities, or it can form as mineralizations or replacements of its host rocks. In these deposits, sphalerite is frequently associated with galena, dolomite, calcite, chalcopyrite, pyrite, marcasite, and pyrrhotite. When weathered, the zinc often forms nearby occurrences of smithsonite or hemimorphite.

Sphalerite on dolomite: Crystals of sphalerite on dolomite with minor amounts of chalcopyrite. Specimen from the Joplin Field, Tri-State District of Missouri, USA. Specimen is about 6.5 x 4.5 x 3.5 centimeters. Specimen and photo by Arkenstone / www.iRocks.com.

Chemical Composition The chemical formula of sphalerite is (Zn,Fe)S. It is a zinc sulfide containing variable amounts of iron that substitutes for zinc in the mineral lattice. The iron content is normally less than 25% by weight. The amount of iron substitution that occurs depends upon iron availability and temperature, with higher temperatures favoring higher iron content. Sphalerite often contains trace to minor amounts of cadmium, indium, germanium, or gallium. These rare elements are valuable and when abundant enough can be recovered as profitable byproducts. Minor amounts of manganese and arsenic can also be present in sphalerite.

Sphalerite crystals: Gem-quality crystals of yellow sphalerite from the Balmat-Edwards Zinc District of New York. Specimen is about 2.75 x 1.75 x 1.5 centimeters in size. Specimen and photo by Arkenstone / www.iRocks.com.

Sphalerite: Sphalerite with dolomite from Gilman, Colorado. Specimen is approximately 5 centimeters across.

Physical Properties The appearance and properties of sphalerite are variable. It occurs in a variety of colors, and its luster ranges from nonmetallic to submetallic and resinous to adamantine. Occasionally it will be transparent with a vitreous luster. Sphalerite's streak is white to yellowish brown and sometimes is accompanied by a distinct odor of sulfur. Occasionally it streaks reddish brown. One of the most distinctive properties of sphalerite is its cleavage. It has six directions of perfect cleavage with faces that exhibit a resinous to adamantine luster. Specimens that display this distinctive cleavage are easy to identify. Unfortunately, many specimens have such a fine grain size that the cleavage is difficult to observe. Because sphalerite often forms in veins and cavities, excellent crystals are relatively common. Sphalerite is a member of the isometric crystal system, and cubes, octahedrons, tetrahedrons, and dodecahedrons are all encountered. Physical Properties of Sphalerite Chemical Classification

Sulfide

Color

Yellow, brown, black, red, green, white, colorless

Streak

White to yellowish brown, often with an odor of sulfur

Luster

Nonmetallic, submetallic, resinous or adamantine

Diaphaneity Cleavage

Transparent to translucent Perfect, dodecahedral, in six directions!

Mohs Hardness

3.5 to 4

Specific Gravity

3.9 to 4.1

Diagnostic Properties

Luster, cleavage, streak

Chemical

Zinc sulfide with variable amounts of iron, (Zn,Fe)S

Composition Crystal System

Uses

Isometric The primary ore of zinc. Often mined for minor amounts of indium, cadmium, germanium, or gallium as profitable byproducts. Mineral specimens. Faceted stones for collectors.

Sphalerite gemstones: Sphalerite is occasionally cut as a faceted stone. It is a popular stone with collectors because it has a dispersion that is three times higher than the dispersion of diamond. The stones occur in a spectrum of colors ranging from yellowish green, to yellow, to orange, to red. To exhibit the excellent dispersion the stones must have a very high clarity. Sphalerite's very low hardness (3.5 to 4 on the Mohs scale) and perfect cleavage make the mineral a very poor choice for any jewelry except pieces such as earrings and brooches that will receive very little abrasion or impact. It is considered to be a "collector's stone." Information Sources [1] Zinc: Amy C. Tolcin, U.S. Geological Survey, Mineral Commodity Summaries, February 2014. [2] Zinc: Amy C. Tolcin, U.S. Geological Survey, 2011 Minerals Yearbook, February 2012.

Sphalerite as a Gemstone? Although sphalerite has a hardness of just 3.5 to 4 on the Mohs scale and is not suitable for most jewelry use, specimens with excellent clarity are sometimes cut into gemstones for collectors. Why? Sphalerite has a dispersion that exceeds that of all of the popular gems and is three times higher than the dispersion of diamond. Dispersion is the ability of a material to separate white light into the colors of the spectrum as it passes through the material. Diamond is well known for its exceptional "fire" - flashes of color as the gem is moved under a source of light. These are caused by its high dispersion of 0.044. Common natural gems that have a dispersion higher than diamond are sphene at 0.051 and demantoid garnet at 0.057. Sphalerite has an incredible dispersion of 0.156. The only things that hold specimens of sphalerite back from an incredible display of brilliant "fire" are their less-than-excellent clarity and their obvious body color.

Sphalerite is a difficult stone to cut and polish. It is soft and it has cleavage. Weaknesses in the stone or minor accidents during the cutting or polishing process can easily ruin a stone. Before deciding to cut a transparent specimen of sphalerite into a gemstone, its value as a mineral specimen should be determined. If this is not done, the owner could make a costly mistake. Contributor: Hobart King

Spinel A gemstone that was confused with ruby and sapphire for over 1000 years.

Red and blue spinel: Spinel occurs in a wide variety of colors. The bright reds and deep blues are spectacular specimens. It is easy to understand how early gem traders confused spinel with ruby and sapphire for over 1000 years. Specimens and photos by Arkenstone / www.iRocks.com.

Faceted spinel: Several pretty facet-cut spinels. It is easy to see how spinel can be confused with ruby and sapphire or used as an alternative stone. These spinels are about 4 1/2 millimeters in size and weigh a little less than 1/2 carat each. The top three red and pink stones were cut from material mined in Myanmar. Deep red spinel is rarer than ruby but sells at a fraction of the price. The blue stones below them were cut from material mined in Tanzania.

The Ruby and Sapphire Impostor

Spinel is a gemstone mineral that has been confused with ruby and sapphire for over 1000 years. Several of the most spectacular spinels ever discovered have been mounted in "crown jewels" and other "jewelry of significance" under the assumption that they were rubies or sapphires. Spinel occurs in the same bright red and blue colors as rubies and sapphires. Spinel forms in the same rock units, under the same geological conditions and is found in the same gravels. It is not surprising that ancient gem traders thought that these colorful spinels were rubies and sapphires.

Physical Properties of Spinel Chemical Classification

Oxide

Color

Colorless, pink, red, orange, blue, purple, brown, black

Streak

Colorless (harder than the streak plate)

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent None

Mohs Hardness

7.5 to 8

Specific Gravity

3.5 to 4.1

Diagnostic Properties

Hardness, octahedral crystals, vitreous luster

Chemical Composition

MgAl2O4

Crystal System

Isometric

Uses

The only significant use is as a gemstone.

Why the Confusion? Two thousand years ago, gemstone traders did not know that spinel and corundum (the mineral of ruby and sapphire) have different chemical compositions and different crystal structures. Instead, gem traders thought that every bright red gemstone was a "ruby" and every deep blue gemstone was a "sapphire." As a result, lots of spinels are now in very important jewelry collections based on their incorrect identification as a ruby. The Black Prince's Ruby The most famous example of a spinel being identified as a ruby is a 170-carat bright red spinel named "The Black Prince's Ruby." The first known owner of this beautiful stone was Abu Sa'id, the Moorish Prince of Granada, in the 14th century. The stone passed through several owners and eventually made its way into the Imperial State Crown of the United Kingdom, where it is mounted immediately above the famous Cullinan II diamond. [1, photo]

The Timur Ruby The "Timur Ruby" is a 352.5-carat bright red spinel that is currently in a necklace of The Royal Collection that was made for Queen Victoria in 1853. The stone was found in Afghanistan and is inscribed with the names and dates of its owners back to 1612. It was part of a group of spinels from the Lahore Treasure presented to Queen Victoria by the East India Company in 1849. [2] Diagnostic Properties: Spinel / Corundum Property

Spinel

Corundum

Composition

MgAl2O4

Al2O3

Crystal System

Isometric

Hexagonal

Typical Crystal Form Octahedrons, dodecahedrons Hexagonal prisms Hardness

7.5 to 8

9

Diagnostic Differences (Spinel, Ruby, Sapphire) Today gemologists understand that there are significant differences between spinel and corundum (the mineral of ruby and sapphire). The diagnostic differences are summarized in the chart on this page. Optical properties can also be used to distinguish spinel from corundum. Gem traders in Myanmar were the first to recognize spinel as being distinctly different from ruby in the late 1500s. [3] In Europe, spinel continued to be misidentified as ruby until the mid-1800s.

Alluvial spinel: Much of the spinel used to produce gemstones is obtained from alluvial deposits. These deposits are worked with minimal mechanization in many parts of the world. Workers wash stream sediment and visually search through the coarse sand to fine gravel fraction, looking for colorful mineral grains that might be of value. The photo above shows some alluvial spinel produced in Vietnam. Some of the particles are thoroughly worn into rounded pebbles. Others have experienced so little transport that they still have sharp crystal edges and unworn faces.

What is Spinel?

Spinel is an oxide mineral with a composition of MgAl2O4. It is very hard (7.5 to 8 on the Mohs Hardness Scale) and is often found in octahedral crystals. It is typically found in three geologic situations: 1) as crystals in limestones and dolomites that have been subjected to contact metamorphism; 2) irregularlyshaped grains in basic igneous rocks; and, 3) as water-worn pebbles in alluvial deposits. Spinel is very resistant to chemical and physical weathering. It often occurs in marble, which is much less resistant to weathering. Spinel easily weathers out of the marble and is transported by streams. This places spinel in alluvial deposits which are often worked for gemstones. Most of the spinels of a "ruby-red color" are produced from alluvial deposits in Sri Lanka, Thailand, Cambodia, Vietnam, Myanmar, and other countries. Other countries where spinel is mined include: Afghanistan, Nepal, Tajikistan, Australia, Madagascar, Nigeria, and Tanzania.

Crown of Catherine the Great: The Great Imperial Crown was made for Empress Catherine II the Great's Coronation in 1762. The large red stone at the crest of the crown is the second-largest known spinel, weighing 398 carats. It has been titled: "Catherine the Great's Ruby." [5] Creative Commons image by Hugo Gerard Ströhl.

Samarian Spinel: Photo of the Samarian Spinel, the largest known spinel in the world and part of the Iranian Crown Jewels. It weighs approximately 500 carats. It bears an inscription dating to the mid-1600s attributing its ownership to Jehangir, the Mogul Emperor of India. It was taken from India in the early 1700s, during the Afsharid Conquest. [4] Public domain image.

Uses of Spinel The only significant use of spinel is as a gemstone. It occurs in a variety of colors (colorless, pink, red, orange, blue, purple, brown, black). The colors that imitate ruby and sapphire are the most popular, along with an orange-red color known as "flame spinel." Gem-quality red and blue spinels are very rare. They are much less abundant than rubies and sapphires of similar quality and color. Even with equivalent beauty and greater rarity, their prices are much lower than ruby and sapphire. This is an example of how rarity has not determined the price. Spinel is not as valuable because it is not as popular. Spinel has not been strongly promoted by the gem and jewelry trade because its supply is limited and unreliable. Occasionally an exceptional spinel or a jewelry item of historical significance is sold at auction for a very high price. One necklace containing eleven bright red spinels, totaling 1,132 carats and inscribed by Mughal emperors, sold for over $5 million. [6, photo] United States Birthstones Month

Stone

January

garnet

February

amethyst

March

aquamarine, bloodstone

April

diamond

May

emerald

June

pearl, moonstone, alexandrite

July

ruby

August

peridot, spinel

September October

sapphire opal, tourmaline

November

topaz, citrine

December

turquoise, zircon, tanzanite

Spinel: A New Birthstone for August Many authors link the tradition of associating a birthstone with each month of the year to Aaron's breastplate of the Bible's Book of Exodus. Others link it to customs from 16th century Germany [9] and 18th century Poland [10]. In 1912, the National Association of Jewelers, now the Jewelers of America, adopted and began promoting a modern list of birthstones. Spinel was not used as a birthstone in this list. The Jewelry Council of America, the American Gem Trade Association, and the National Association of Goldsmiths of Britain are all involved in promoting lists of birthstones.

In July 2016, spinel was named a new birthstone for the month of August by the American Gem Trade Association and the Jewelers of America. Before then, peridot served as the August birthstone. Now both spinel and peridot will share the designation. This event and continued promotion of monthly birthstones will bring significant attention to spinel, which occurs in a variety of colors. Consumers will now have a choice beyond the yellow-green color of peridot.

Synthetic Spinel The first synthetic spinel was produced in 1847 by Jacques-Joseph Ebelmen, a French Chemist. [7] Commercial production of synthetic spinels was very limited in the 1800s. However, in the 1930s synthetic spinels in a wide variety of colors were produced to imitate popular gemstones such as aquamarine, zircon, tourmaline, emerald, chrysoberyl, and ruby. The colors were produced by introducing metals in trace amounts into the stone by the addition of: cobalt oxide (blue), manganese (yellow), chromium oxide (green), and iron (pink). Careful chemical procedures allowed the manufacturers to control the color of the stones. [8] These colored synthetic spinels were given trade names such as "Tourmaline Green," mounted in inexpensive settings and sold as "birthstone" jewelry. [8] These synthetic stones were the first encounter with spinel for most of the consumers who purchased them. In addition to its use as a gemstone, synthetic spinel is also used as a refractory. It is used to produce heatresistant coatings on metal tools and as an additive in making refractory bricks and ceramics. Information Sources [1] The Black Prince's Ruby, featured in an article titled: The Imperial State Crown on The Royal Collection Trust website, last accessed July 2016. [2] The 'Timur Ruby' Necklace, an article on The Royal Collection Trust website, last accessed January 2013. [3] Spinel: Collector's Favourite, an article on the International Colored Gemstone Association website, last accessed July 2016. [4] The Samarian Spinel, article on the Wikipedia.com website, last accessed July 2016. [5] The Russian Crown Jewels, article on the "Famous Diamonds" website showing a photo of the Great Imperial Crown of Russia, last accessed July 2016. [6] An Imperial Mughal Spinel Necklace, Christie's auction in Geneva from May 18, 2011, webpage last accessed July 2016. [7] Restoring the Luster to a Once-Loved Gem, article by Victoria Gomelsky on The New York Times website, May 12, 2011. [8] Synthetic Gemstones: Spinel, Gems: Their Sources, Descriptions and Identification, edited by Michael O'Donoghue, Chapter 24, pages 495502. [9] Gems in Myth, Legend and Lore by Bruce G. Knuth, Parachute: Jewelers Press, 2007. [10] The Curious Lore of Precious Stones by George Kunz, Lippincott, 1913.

The Spinel Mineral Group

The name "spinel" is also used for a broad group of minerals with a general chemical composition of XY2O4. In this formula "X" could be filled by: Mg, Fe+2, Zn, Mn+2, Ni, Co, or Cu. "Y" could be filled by: Al, Fe+3, Cr, V+3, Ti+4, Ge, or Sb. Examples of spinel group minerals include gahnite, magnetite, franklinite, chromite, chrysoberyl, and Columbite-Tantalite, as shown in the chart below. Spinel Group Minerals Mineral

Composition

Spinel

MgAl2O4

Gahnite

ZnAl2O4

Magnetite

Fe3O4

Franklinite

(Zn,Fe,Mn)(Fe,Mn)2O4

Chromite

FeCr2O4

Chrysoberyl

BeAl2O4

Columbite-Tantalite (Fe,Mn)Nb2O6--(Fe,Mn)Ta2O6

Contributor: Hobart King

Spodumene An important source of high-purity lithium and a gemstone with collector appeal

Spodumene: Translucent to transparent spodumene with an attractive pink, yellow, or green color is sometimes faceted, cut en cabochon or used to make tumbled stones. Its perfect cleavage limits its use to jewelry that will not be subject to rough wear or handling. Spodumene is primarily a "collector's gemstone." The larger pieces of spodumene in this image are about one inch long.

What is Spodumene? Spodumene is a pyroxene mineral that is found, almost exclusively, in granite pegmatites. It has a chemical composition of LiAlSi2O6 but small amounts of sodium sometimes substitute for lithium. Spodumene is typically found in lithium-rich pegmatites in association with other lithium minerals such as lepidolite, eucryptite, and petalite. In the historical literature, the mineral is often referred to as "triphane." Physical Properties of Spodumene Chemical Classification

Silicate

Color

White, gray, yellow, green, blue, lilac, pink, brown. Sometimes pleochroic

Streak

White, colorless

Luster

Vitreous, pearly

Diaphaneity Cleavage

Transparent to translucent Perfect in two directions with parting

Mohs Hardness

6.5 to 7

Specific Gravity

3.1 to 3.3

Diagnostic Properties

Prismatic crystals with strong striations parallel to their principal axis. Perfect cleavage.

Chemical Composition

LiAl(SiO3)2

Crystal System

Monoclinic (low temperature), tetragonal (high temperature)

Uses

An ore of lithium. Gemstones (kunzite, hiddenite)

Enormous Crystals Spodumene often occurs in extremely large crystals. One of the earliest accounts of large spodumene crystals is from the Etta Mines, Black Hills, Pennington County, South Dakota. The United States Geological Survey, Bulletin 610 reports: "The crystals are often of enormous size. In the Etta Mine, where they are best exposed both in the open cut and tunnel, they frequently attain a diameter of 3 to 4 feet and a length of 30 feet. The largest "log" so far found was 42 feet long and 5 feet 4 inches in maximum diameter. This one log alone would yield 90 tons of spodumene." [1]

Giant spodumene crystals: Molds of giant spodumene crystals at the Etta Mines, Black Hills, Pennington County, South Dakota. Note miner at right center for scale. USGS photo. [1]

Uses of Spodumene Spodumene once served as the most important ore of lithium metal. Although it remains an important source of lithium, today most of the world's lithium is produced from subsurface brines in Chile, Argentina, and China. These sources of lithium have lower production costs and are suitable for most uses. However, when lithium of highest purity is needed, spodumene is the source that is used.

Kunzite spodumene: Pink gem-quality spodumene (kunzite) from the Konar Valley, Afghanistan. Creative Commons image by Didier Descouens.

Spodumene: An ore-grade spodumene crystal section showing cleavage and typical striations. Photograph by Andrew Silver, USGS, BYU Collection.

Lithium battery: One of the primary uses of spodumene is in the production of high-purity lithium for use in lithium-ion batteries. The popularity of small electronic devices such as cell phones, portable computers, and cameras is driving the demand for spodumene. Photo © iStockphoto / Anton Snarikov.

Spodumene as a Gemstone Spodumene sometimes occurs in transparent crystals in pastel shades of pink, green, and yellow. These have been cut into gemstones that are prized by collectors. However, their use in jewelry is limited to pieces that will be subject to limited abuse because of spodumene's perfect cleavage. Kunzite Pink to lilac specimens of gem-quality spodumene are highly prized and known as "kunzite". The color of these specimens is attributed to the presence of manganese as a chromophore. Kunzite is the most commonly encountered spodumene gem. Many specimens of kunzite are strongly pleochroic, with the deepest color observed when the gem is viewed down the principal axis. To take full advantage of its phenomenon, gemstones are cut with the table perpendicular to the principal axis to yield stones of the deepest color. Some kunzite will develop a richer color when heated or irradiated. These procedures have been applied to some stones that enter the marketplace. Some specimens of kunzite will fade over time when exposed to direct sunlight. Valuable stones should be stored away from direct light and, to be conservative, in a closed container. Hiddenite Emerald-green spodumene is known as "Hiddenite." Its vivid green color is very similar to emerald and is attributed to the presence of chromium as a chromophore. It is the rarest gem variety of spodumene. It was first found near the town of Stony Point, North Carolina, which changed its name to "Hiddenite" after the popular gemstone that attracted people to the area. Other Colors Yellow and clear specimens of spodumene have also been cut into gems; however, variety names for spodumene gems have only been given to Kunzite and Hiddenite.

Uses of lithium: Lithium has many diverse uses. This chart shows estimated global uses of lithium by end product. It is mainly used in manufacturing ceramics, specialty glass, rechargeable batteries, high-temperature grease, continuous castings, polymers, aluminum alloys, and pharmaceuticals. USGS data. [2]

Did You Know? Lithium is an active ingredient in some medications. Salts of lithium are used in medication for bipolar disorder. The lithium contributes to a "mood-stabilizing" effect. One product has been named "Lithium." Image © iStockphoto / Paige Foster.

Demand for Spodumene The demand for spodumene is dependent upon the use of lithium in manufacturing. In the past, most lithium compounds and minerals were used to produce ceramics, glass, aluminum alloys, and high-temperature grease. However, an exploding demand for rechargeable batteries to power cell phones, tablet computers, cameras, music players, GPS units, and other portable electronic devices is driving the demand for highpurity lithium - and that drives the demand for spodumene. Lithium batteries have a much higher charge-to-weight ratio and power-to-weight ratio than lead/acid and zinc carbon cells. This makes lithium the battery material of choice. Lithium produced from spodumene has fewer contaminants than lithium produced from brines. These contaminants can interfere with battery performance and make spodumene the preferred choice for battery lithium. A new battery technology could displace the use of lithium; however, most new battery technologies have been lithium-based. Contributor: Hobart King Spodumene Information [1] Mineralogic Notes, Series 3: Waldemar Schaller, Gigantic Crystals of Spodumene, United States Geological Survey, Bulletin 610, 1916. [2] Lithium: Brian Jaskula, United States Geological Survey, Mineral Commodity Summaries, January 2012. [3] Lithium: Brian Jaskula, United States Geological Survey, Minerals Yearbook, December 2011.

Staurolite The metamorphic mineral that has become famous for its twinned crystals

Staurolite: Staurolite crystals forming the typical 60-degree penetration twin from Rubelita, Minas Gerais, Brazil. The specimen is about 1.5 inches tall. Specimen and photo by Arkenstone / www.iRocks.com.

What is Staurolite? Staurolite is a mineral that is commonly found in metamorphic rocks such as schist and gneiss. It forms when shale is strongly altered by regional metamorphism. It is often found in association with almandine garnet, muscovite, and kyanite - minerals that form under similar temperature and pressure conditions.

Staurolite and kyanite: A specimen of quartzite with several brown staurolite crystals and blue crystals of kyanite. This specimen is about three inches wide and was collected in the Bernina Pass area, near Grischun, Switzerland. Specimen and photo by Arkenstone / www.iRocks.com.

Properties of Staurolite Staurolite is a silicate mineral with a generalized chemical composition of (Fe,Mg)2Al9Si4O23(OH). It is usually brown or black in color with a resinous to vitreous luster. It ranges from transparent to opaque in diaphaneity. Staurolite is usually easy to identify when it occurs as visible grains in a metamorphic rock. Grains of staurolite are typically larger than the grains of other minerals in the rock, and they often exhibit an obvious crystal structure. They occur as six-sided crystals, often with penetration twins.

Staurolite: Staurolite in schist from Little Falls, Minnesota. Specimen is approximately 4 inches (10 centimeters) across.

Physical Properties of Staurolite Chemical Classification

Silicate

Color

Usually brown, reddish brown, yellowish brown, brownish black, black, dark gray

Streak

Colorless (harder than the streak plate)

Luster

Vitreous, sometimes resinous

Diaphaneity Cleavage

Translucent to opaque, rarely transparent Poor

Mohs Hardness

7 to 7.5

Specific Gravity

3.7 to 3.8

Diagnostic Properties

Chemical Composition Crystal System

Color, six-sided crystals that are frequently twinned, usually found in schist and gneiss with muscovite mica and almandine garnet

(Fe,Mg)2Al9Si4O23(OH)

Monoclinic

Uses

Little industrial use

Twinned staurolite crystals: Twinned staurolite crystals in muscovite schist from Pestsovye Keivy, Keivy Mountains, Russia. This specimen of schist has one pair of staurolite crystals forming a 90-degree penetration twin (lower right) and another pair forming the more typical 60-degree penetration twin (upper left, partially embedded). Specimen is approximately 4 inches (10 centimeters) across. Specimen and photo by Arkenstone / www.iRocks.com.

Twinning in Staurolite The name "staurolite" is from the Greek word "stauros," which means "cross." The mineral commonly occurs as twinned, six-sided crystals that sometimes intersect at 90 degrees to form a cross. (An intersection angle of 60 degrees is more common.) In some localities these twinned crystals are collected, made into jewelry, and sold under the name "fairy crosses."

Staurolite "fairy crosses": Staurolite crystals are often collected, made into jewelry, and sold as souvenirs or "good luck" charms. Some of these items are genuine twinned staurolite crystals. Others are cross-shaped models manufactured for the tourist trade. If you see a selection of staurolite crosses offered for sale that are all the same size, same shape, and have air bubbles on close examination, they might be manufactured.

Uses of Staurolite There are very few uses for staurolite. It has been used as an abrasive, but that use has been replaced by other minerals and man-made materials. It is used in geologic field work to assess the temperature-pressure conditions of a rock's metamorphic history. In locations where staurolite is found as well-formed cruciform twinned crystals, it is sometimes collected, sold as a souvenir, made into jewelry, and used as an ornament. The cruciform crystals have often stirred religious beliefs and superstitions. Some of these objects are not staurolite; instead they are manufactured. If you see a selection of these for sale that are all the same size, the same shape and containing gas bubbles, they might be manufactured. Staurolite is the official state mineral of the state of Georgia. It is especially abundant in a few localities in Patrick County, Virginia. One of them is now Virginia's "Fairy Stone State Park," named after the stone and the legends that surround it. Contributor: Hobart King

Sulfur Chemical element. Native mineral. Essential to all living things.

Sulfur terminal: Piles of yellow sulfur at a terminal near Vancouver, British Columbia, Canada. The sulfur is brought by rail from oil and natural gas processing facilities in the Province of Alberta. At this terminal it is loaded onto barges and ships for bulk transport. Photo © iStockphoto / teekaygee.

Sulfur fumarole: As hot volcanic gases, rich in sulfur, escape from a volcanic vent, the gases cool and sulfur is deposited as yellow crystals around the vent. This fumarole on the island of Kunashir (in the Kuril Islands, northeast of the Japanese island of Hokkaido) has a significant accumulation of bright yellow sulfur. Photo © iStockphoto / Sergey Dubrovskiy.

Did You Know? Many strong odors are produced by sulfur compounds. The smell of skunks, matches, garlic, grapefruit, and rotten eggs are caused by sulfur. Image © iStockphoto / Florintt, Gio_banfi, Abomb Industries Design, ivelly, and Big_Ryan.

What is Sulfur? Sulfur is a chemical element with an atomic number of 16 and an atomic symbol of S. At room temperature it is a yellow crystalline solid. Even though it is insoluble in water, it is one of the most versatile elements at forming compounds. Sulfur reacts and forms compounds with all elements except gold, iodine, iridium, nitrogen, platinum, tellurium, and the inert gases. Sulfur is abundant and occurs throughout the Universe, but it is rarely found in a pure, uncombined form at Earth's surface. As an element, sulfur is an important constituent of sulfate and sulfide minerals. It occurs in the dissolved ions of many waters. It is an important constituent of many atmospheric, subsurface, and dissolved gases. It is an essential element in all living things and is in the organic molecules of all fossil fuels.

Did You Know? The Chinese discovered sulfur in about 2000 BC, used it to make gunpowder in the 7th century, and used gunpowder to launch rockets, shoot projectiles, and make hand grenades in the 10th century.

Physical Properties of Sulfur Chemical Classification

Color

Native element

Yellow. Brownish yellow to greenish yellow. Red when molten at over 200 degrees Celsius. Burns with a flame that can be difficult to see in daylight but is blue in the dark.

Streak

Yellow

Luster

Crystals are resinous to greasy. Powdered sulfur is dull or earthy.

Diaphaneity Cleavage

Transparent to translucent None

Mohs Hardness

1.5 to 2.5

Specific Gravity

2.0 to 2.1

Diagnostic Properties

Yellow color, low hardness, low specific gravity, extremely flammable burning with a blue flame, low melting temperature

Chemical

S

Composition Crystal System

Orthorhombic About 90% is used to manufacture sulfuric acid. The remainder is used in a variety of products that include hydrogen

Uses

sulfide, insecticides, herbicides, fungicides, pharmaceuticals, soaps, textiles, papers, processed rubber, gunpowder, leather, paint, dyes, food preservatives.

World Sulfur Production: During 2015, an estimated 70 million metric tons of sulfur was produced worldwide. The production was widely divided among a large number of countries. The top 12 producing countries were China, the United States, Russia, Canada, Germany, Japan, Saudi Arabia, India, Kazakhstan, Iran, United Arab Emirates, and Mexico. These countries are where the sulfur was separated from its geologic source material rather than the original source of the sulfur, since most sulfur is separated when fossil fuels are processed or sulfide ores are smelted. Data from the United States Geological Survey. [7]

Sulfur is Abundant and Everywhere! The information below should convince you that sulfur is extremely abundant and present everywhere.      

11th most abundant element in the human body [1] 6th most abundant element in seawater [2] 14th most abundant element in Earth’s crust [3] 9th most abundant element in the entire Earth [4] 10th most abundant element in the solar system [5] 10th most abundant element in the Universe [6]

Sulfur Crystals: Bright yellow sulfur crystal group showing the mineral's characteristic orthorhombic crystal habit and resinous luster. Specimen measures approximately 7.3 x 6.6 x 5.3 centimeters in size and was collected from the Agrigento Province, Sicily, Italy. Specimen and photo by Arkenstone / www.iRocks.com.

Burning sulfur: Pieces of sulfur burning in daylight and in the dark. Photo by Johannes 'volty' Hemmerlein, used here under a GNU Free Documentation License.

Did You Know? Jupiter's moon, Io, has over 400 active volcanoes that emit enormous amounts of sulfur - so much sulfur that the moon has a yellowish color.

“Sulfur” or "Sulphur"? The name "sulphur" has been used in the United Kingdom and throughout the British Empire for hundreds of years. "Sulfur" is the spelling used in common and scientific communication in the United States. In 1990 the International Union of Pure and Applied Chemistry designated "sulfur" as the preferred spelling. How the word is spelled can often reveal the age and origin of publications and authors. Information Sources [1] What Elements Are Found in the Human Body? Article in the Building Blocks of Life section of the Arizona School of Life Sciences website, accessed November 2016. [2] Periodic Table of Elements in the Ocean, article on the Monterey Bay Aquarium Research Institute website, accessed November 2016. [3] List of Periodic Table Elements Sorted by Abundance in Earth's Crust, article on the Israel Science and Technology website, accessed November 2016. [4] The Composition of the Earth, by William F. McDonough, Chapter 1 in Earthquake Thermodynamics and Phase Transformations in the Earth’s Interior, manuscript on the Massachusetts Institute of Technology website, accessed November 2016. [5] Solar System Abundances and Condensation Temperatures of the Elements by Katharina Lodders, article published on The Astrophysical Journal website, accessed November 2016. [6] Abundance in the Universe of the Elements, article on the PeriodicTable.com website, accessed November 2016. [7] Sulfur, by Lori E. Apodaca, United States Geological Survey, Mineral Commodity Summaries, 2016. [8] The International Mineralogical Association Database of Mineral Properties, an online database of minerals along with their chemical and physical properties that can be queried and sorted by anyone with internet access.

Sulfur as a Native Element Mineral As a mineral, sulfur is a bright yellow crystalline material. It forms near volcanic vents and fumaroles, where it sublimates from a stream of hot gases. Small amounts of native sulfur also form during the weathering of sulfate and sulfide minerals. The largest accumulations of mineral sulfur are found in the subsurface. Many of these are in fractures and cavities associated with sulfide ore mineralization. The largest are associated with evaporite minerals, where

gypsum and anhydrite yield native sulfur as a product of bacterial action. Significant amounts of sulfur have been produced from the cap rock of salt domes but this type of production is rarely done today.

Minerals That Contain Sulfur According to the International Mineralogical Association's database, over 1000 minerals contain sulfur as an essential part of their composition. [8] This is a result of sulfur's ability to form compounds with all but a few other elements. The tables below list a small number of sulfide, sulfarsenide, sulfosalt and sulfate minerals. Many of the most common sulfur minerals are included in the list, but the list is not intended to be complete. Sulfide Minerals: Mineral

Composition

Acanthite

Ag2S

Chalcocite

Cu2S

Bornite

Cu5FeS4

Galena

PbS

Sphalerite

ZnS

Chalcopyrite

CuFeS2

Pyrrhotite

Fe1-xS

Millerite

NiS

Pentlandite

(Fe,Ni)9S8

Covellite

CuS

Cinnabar

HgS

Realgar

AsS

Orpiment

As2S3

Stibnite

Sb2S3

Pyrite

FeS2

Marcasite

FeS2

Molybdenite

MoS2

Sulfarsenide Minerals: Mineral Cobaltite

Composition (Co,Fe)AsS

Arsenopyrite

FeAsS

Gersdorffite

NiAsS

Sulfosalt Minerals: Mineral

Composition

Pyrargyrite

Ag3SbS3

Proustite

Ag3AsS3

Tetrahedrite

Cu12Sb4S13

Tennantite

Cu12As4S13

Enargite

Cu3AsS4

Bournonite

PbCuSbS3

Jamesonite

Pb4FeSb6S14

Cylindrite

Pb3Sn4FeSb2S14

Anhydrous Sulfate Minerals: Mineral

Composition

Barite

BaSO4

Celestite

SrSO4

Anglesite

PbSO4

Anhydrite

CaSO4

Hanksite

Na22K(SO4)9(CO3)2Cl

Hydroxide and Hydrous Sulfate Minerals: Mineral

Composition

Gypsum

CaSO4·2H2O

Chalcanthite

CuSO4·5H2O

Kieserite

MgSO4·H2O

Starkeyite

MgSO4·4H2O

Hexahydrite

MgSO4·6H2O

Epsomite

MgSO4·7H2O

Meridianiite

MgSO4·11H2O

Melanterite

FeSO4·7H2O

Antlerite

Cu3SO4(OH)4

Brochantite

Cu4SO4(OH)6

Alunite

KAl3(SO4)2(OH)6

Jarosite

KFe3(SO4)2(OH)6

Sylvite Mineral Properties and Uses

Physical Properties of Sylvite Chemical Classification

halide

Color

colorless, white, blue, yellow, red, gray

Streak

white

Luster

vitreous

Diaphaneity Cleavage

transparent to translucent perfect, cubic

Mohs Hardness

2

Specific Gravity

2

Diagnostic Properties

taste

Chemical Composition

KCl

Crystal System Uses

isometric salt substitute, fertilizer

Talc: The Softest Mineral What is Talc?

How Does it Form?

How is Talc Used?

Uses of talc: Talc is used in a wide variety of products that we see every day. It is an important ingredient in rubber, a filler and whitener in paint, a filler and brightening agent in high-quality papers, and a primary ingredient in many types of cosmetics. Images © iStockphoto and (clockwise) MorePixels, Mark Wragg, Franz-W. Franzelin and High Impact Photography.

Talc: A Mineral in Your Daily Life Most people are familiar with the mineral talc. It can be crushed into a white powder that is widely known as "talcum powder." This powder has the ability to absorb moisture, absorb oils, absorb odor, serve as a lubricant, and produce an astringent effect with human skin. These properties make talcum powder an important ingredient in many baby powders, foot powders, first aid powders, and a variety of cosmetics. A form of talc known as "soapstone" is also widely known. This soft rock is easily carved and has been used to make ornamental and practical objects for thousands of years. It has been used to make sculptures, bowls, countertops, sinks, hearths, pipe bowls, and many other objects. Although talcum powder and soapstone are two of the more visible uses of talc, they account for a very small fraction of talc consumption. Its hidden uses are far more common. Talc's unique properties make it an important ingredient for making ceramics, paint, paper, roofing materials, plastics, rubber, insecticides, and many other products.

Talc: Talc is a phyllosilicate mineral that cleaves into thin sheets. These sheets are held together only by van der Waals bonds, which allows them to easily slip past one another. This characteristic is responsible for talc's extreme softness, its greasy to soapy feel, and its value as a high-temperature lubricant.

What is Talc? Talc is a hydrous magnesium silicate mineral with a chemical composition of Mg3Si4O10(OH)2. Although the composition of talc usually stays close to this generalized formula, some substitution occurs. Small amounts of Al or Ti can substitute for Si; small amounts of Fe, Mn, and Al can substitute for Mg; and, very small amounts of Ca can substitute for Mg. When large amounts of Fe substitute for Mg, the mineral is known as minnesotaite. When large amounts of Al substitute for Mg, the mineral is known as pyrophyllite. Talc is usually green, white, gray, brown, or colorless. It is a translucent mineral with a pearly luster. It is the softest known mineral and is assigned a hardness of 1 on the Mohs Hardness scale. Talc is a monoclinic mineral with a sheet structure similar to the micas. Talc has perfect cleavage that follows planes between the weakly bonded sheets. These sheets are held together only by van der Waals bonds, which allows them to slip past one another easily. This characteristic is responsible for talc's extreme softness, its greasy, soapy feel, and its value as a high-temperature lubricant.

Where is Talc Produced? 2011 Mine Production of Talc Country

Thousand Metric Tons

United States

615

Brazil

420

China

2,000

Finland

500

France

420

India

650

Japan

360

South Korea

700

Other Countries

1,570

The values above are estimates of mine productions in thousands of metric tons from the USGS Mineral Commodity Summaries.

In 2011 talc production was still down in response to the world-wide economic downturn. For most countries, 2011 production was about the same as production in 2010. China, South Korea, India, United States, Finland, Brazil, France, and Japan are the leading producers. The United States is self-sufficient for most types of talc used in manufacturing. Estimated 2011 production was 615,000 metric tons with a value of about $20 million. Three companies in the United States account for nearly 100% of the country's production.

Soapstone: A rock known as "soapstone" is a massive variety of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, and pyroxenes. It is a soft rock that is easy to work, and that has caused it to be used in a wide variety of dimension stone and sculpture applications. It is used for counter tops, electrical panels, hearthstones, figurines, statuary, and many other projects.

How Does Talc Form? Talc is a mineral that is most often found in the metamorphic rocks of convergent plate boundaries. It forms from at least two processes. Most large talc deposits in the United States formed when heated waters

carrying dissolved magnesium and silica reacted with dolomitic marbles. A second process of talc formation occurred when heat and chemically active fluids altered rocks such as dunite and serpentinite into talc. Most of the talc deposits in the United States are in metamorphic rocks on the eastern side of the Appalachian Mountains and in rocks metamorphosed in convergent terranes of Washington, Idaho, Montana, California, Nevada, and New Mexico. Deposits of talc are also found in Texas.

Foliated talc: Talc is a metamorphic mineral that frequently exhibits distinct foliation.

Talc Mining and Processing Most talc in the United States is produced from an open pit mine where the rock is drilled, blasted, and partially crushed in the mining operation. The highest grade ores are produced by selective mining and sorting operations. Great care is taken during the mining process to avoid contaminating the talc with other rock materials. These other materials can have an adverse effect on the color of the product. Contamination can introduce hard particles that cause problems in applications where talc is being used because of its softness or lubricating properties. Partially crushed rock is taken from the mine to a mill, where it is further reduced in particle size. Impurities are sometimes removed by froth flotation or mechanical processing. The mills produce crushed or finely ground talc that meets customer requirements for particle size, brightness, composition, and other properties.

Uses of Talc: Talc is used as a filler, coating, pigment, dusting agent and extender in plastics, ceramics, paint, paper, cosmetics, roofing, rubber and many other products. Data from the United States Geological Survey.

Talc: Foliated talc that has a black color in massive form but cleaves into thin, flexible, inelastic and colorless sheets.

Physical Properties of Talc Chemical Classification

Silicate

Color

Green, white, gray, brown, colorless

Streak

White to pale green

Luster

Pearly

Diaphaneity

Translucent

Cleavage

Perfect

Mohs Hardness

1

Specific Gravity

2.7 to 2.8

Diagnostic Properties

Feel, color, softness, cleavage

Chemical Composition

Mg3Si4O10(OH)2

Crystal System Uses

Monoclinic Used as a filler and anti-stick coating in plastics, ceramics, paint, paper, roofing, rubber, cosmetics

Talc Information [1] Talc and Pyrophyllite: Robert L. Virta, U.S. Geological Survey, Mineral Commodity Summaries, January 2012. [2] Talc and Pyrophyllite: Robert L. Virta, U.S. Geological Survey, 2010 Minerals Yearbook, November 2011. [3] U.S. Talc -- Baby Powder and Much More: U.S. Geological Survey, Fact Sheet FS-065-00, September 2000. [4] Talc resources of the conterminous United States: Robert C. Greene, United States Geological Survey, Open-File Report OF 95-586, 1995.

[5] Talc in Cosmetics: United States Food and Drug Administration, website article, last accessed August 2016.

Uses of Talc Most people use products made from talc every day; however, they don't realize that talc is in the product or the special role that it plays. Talc in Plastics In 2011, about 26% of the talc consumed in the United States was used in the manufacturing of plastics. It is mainly used as a filler. The platy shape of talc particles can increase the stiffness of products such as polypropylene, vinyl, polyethylene, nylon, and polyester. It can also increase the heat resistance of these products and reduce shrinkage. Where the plastic is extruded in the manufacturing process, talc's very low hardness produces less abrasion on equipment than harder mineral fillers. Talc in Ceramics In the United States in 2011, about 17% of the talc consumed was used in the manufacturing of ceramics products such as bathroom fixtures, ceramic tile, pottery, and dinnerware. When used as a filler in ceramics, talc can improve the firing characteristics of the greenware and the strength of the finished product. Talc in Paint Most paints are suspensions of mineral particles in a liquid. The liquid portion of the paint facilitates application, but after the liquid evaporates, the mineral particles remain on the wall. Talc is used as an extender and filler in paints. The platy shape of talc particles improves the suspension of solids in the can and helps the liquid paint adhere to a wall without sagging. Powdered talc is a very bright white color. This makes talc an excellent filler in paint because it simultaneously serves to whiten and brighten the paint. Talc's low hardness is valued because it causes less abrasion damage on spray nozzles and other equipment when paint is applied. In 2011, about 16% of the talc consumed in the United States was used to make paint. Talc in Paper Most papers are made from a pulp of organic fibers. This pulp is made from wood, rags, and other organic materials. Finely ground mineral matter is added to the pulp to serve as a filler. When the pulp is rolled into thin sheets, the mineral matter fills spaces between the pulp fibers, resulting in a paper with a much smoother writing surface. Talc as a mineral filler can improve the opacity, brightness, and whiteness of the paper. Talc also can also improve the paper's ability to absorb ink. In 2011, the paper industry consumed about 16% of the talc used in the United States. Talc in Cosmetics and Antiperspirants Finely ground talc is used as the powder base of many cosmetic products. The tiny platelets of a talc powder readily adhere to the skin but can be washed off easily. Talc's softness allows it to be applied and removed without causing skin abrasion. Talc also has the ability to absorb oils and perspiration produced by human skin. The ability of talc to absorb moisture, absorb odor, adhere to the skin, serve as a lubricant, and produce an astringent effect in contact

with human skin make it an important ingredient in many antiperspirants. In 2011, about 7% of the talc consumed in the United States was used to make cosmetics and antiperspirant. Talc and asbestos occur naturally and may occur in close proximity in some metamorphic rocks. Studies published in the 1960s and 1970s identified health concerns about the use of talc that contains asbestos in some cosmetic products. According to the FDA, "These studies have not conclusively demonstrated such a link, or if such a link existed, what risk factors might be involved." To address these concerns, talc mining sites are now carefully selected and ores are carefully processed to avoid the presence of asbestos in talc destined for use in the cosmetics industry. Talc in Roofing Materials Talc is added to the asphaltic materials used to make roofing materials to improve their weather resistance. It is also dusted onto the surface of roll roofing and shingles to prevent sticking. In 2011, about 6% of the talc consumed in the United States was used to manufacture roofing materials. Dimension Stone A rock known as "soapstone" is a massive variety of talc with varying amounts of other minerals such as micas, chlorite, amphiboles, and pyroxenes. It is a soft rock that is easy to work, and that has caused it to be used in a wide variety of dimension stone and sculpture applications. It is used for counter tops, electrical panels, hearthstones, figurines, statuary, and many other projects. Other Uses of Talc Ground talc is used as a lubricant in applications where high temperatures are involved. It is able to survive at temperatures where oil-based lubricants would be destroyed. Talc powder is used as a carrier for insecticides and fungicides. It can easily be blown through a nozzle and readily sticks to the leaves and stems of plants. Its softness reduces wear on application equipment. Contributor: Hobart King

Titanite - Also Known as Sphene Titanite is a minor ore of titanium and a minor gemstone known as "sphene."

Titanite: A twinned crystal of titanite with adularia and clinochlore on matrix. The crystal is about one inch (2.5 centimeters) in height. From Tormiq Valley, Haramosh Mountains, Skardu District, Baltistan, Northern Areas, Pakistan. Specimen and photo by Arkenstone / www.iRocks.com.

What is Titanite? Titanite is a rare titanium mineral that occurs as an accessory mineral in granitic and calciumrich metamorphic rocks. It is a minor ore of titanium and a minor gemstone known as "sphene." Physical Properties of Titanite Chemical Classification

Color

Calcium titanium silicate. Commonly yellow, green, brown, black or gray. Rarely pink, red, or orange.

Streak

White.

Luster

Resinous to adamantine.

Diaphaneity Cleavage

Translucent to transparent. Fair to good.

Mohs Hardness

5 to 5.5

Specific Gravity

3.4 to 3.6

Diagnostic Properties

Luster, hardness, color, dispersion.

Chemical Composition Crystal System Uses

CaTiSiO5 Monoclinic. Minor ore of titanium. Minor gemstone.

Physical Properties of Titanite Titanite's diagnostic properties are its crystal habit, color, and luster. Its monoclinic crystals are often wedge-shaped or tabular-shaped. Its typical color range is yellow, green, brown, and black. Pink, orange, and red specimens are rare. Titanite has a resinous to adamantine luster that is rarely seen in other minerals. It has one of the highest dispersions of any mineral - significantly higher than diamond. Titanite is also pleochroic. Transparent specimens might show its three trichroic colors. Titanite is sometimes confused with sphalerite, especially when observing an adamantine to resinous luster. Sphalerite is softer than titanite, and often produces an odor of sulfur immediately after a streak test.

Titanite: Numerous titanite crystals on a specimen of schist. The large crystal is about 22 millimeters (one inch) in length. From Tormiq Valley, Haramosh Mountains, Skardu District, Baltistan, Northern Areas, Pakistan. Photo by Parent Gery, used here under a creative commons license.

"Titanite" or "Sphene" Before 1982, the name "sphene" was common usage for this mineral. Then the International Mineralogical Association adopted the name "titanite" and discredited "sphene." Geologists and mineralogists worldwide quickly switched to the name "titanite" and it is now in common use. The name "sphene" is rarely seen in current publications. The name "sphene" is still the dominant usage in the gem, jewelry, and lapidary industries. There, a name change can cause severe disruption in marketing gemstone and jewelry products.

Pink Titanite: Massive pink titanite from Westport, Ontario, Canada. Pink is a rare color for this mineral. Specimen is approximately 10 centimeters across.

Chemical Composition of Titanite Titanite has a chemical composition of CaTiSiO5 and sometimes contains rare earth elements such as cerium, niobium, and yttrium. It can contain other elements such as aluminum, chromium, fluorine, iron, magnesium, manganese, sodium, and zirconium. Iron has a strong influence on the color of titanite. Small amounts of iron darken the color. Yellow and green specimens have a low iron content, while brown and black specimens have a higher iron content.

Geologic Occurrence of Titanite Titanite is a rare mineral. It occurs as an accessory mineral in a few igneous rocks that include granite, granodiorite, diorite, syenite, and nepheline syenite. It is sometimes present in marble or calciumrich gneiss and schist. Its habit is often as individual grains. When abundant it is usually granular to massive. The best crystals are usually found in marble. Unlike other titanium minerals, titanite is rarely found in placer deposits. Its cleavage, parting, and a low hardness make it vulnerable to the abrasion of stream transport.

Sphene: A greenish yellow faceted sphene, back illuminated to show its very high dispersion. This 8 x 6 millimeter oval was cut from material mined in Pakistan.

Sphene the Gemstone Sphene continues to be the name used for titanite in the gem and jewelry industries. It is a minor gemstone that is popular with collectors because of its high dispersion. Sphene is one of the few minerals with a dispersion higher than diamond. The dispersion of diamond is 0.044, while the dispersion of sphene is 0.051. Specimens of sphene with high clarity can display a strong, colorful fire when light is passed through them (see accompanying image). Sphene is not commonly seen in jewelry. Its hardness of 5 to 5.5 on the Mohs scale, along with its easy cleavage and parting, make it too fragile as a ring stone. Reliable supplies of cut stones in commercial quantities have not been developed, and the jewelry-buying public is unfamiliar with the gem. For these reasons, sphene has not become a mainstream gem that is commonly available in jewelry.

Topaz A gemstone that occurs in a wide range of natural and treated colors.

Colored Topaz Crystals: A collection of topaz crystals of various natural colors - sherry, imperial, pink, and purple. Most topaz crystals are colorless. Most topaz in commercial jewelry has been heated, irradiated, or coated to improve its color. Specimens and photos by Arkenstone / www.iRocks.com

What is Topaz? Topaz is a rare silicate mineral with a chemical composition of Al2SiO4(F,OH)2. It usually forms in fractures and cavities of igneous rocks such as pegmatite and rhyolite, late in their cooling history. It is also found as water-worn pebbles in stream sediments derived from those igneous rocks. Topaz is a well-known gemstone sold in a wide variety of attractive colors. Some of these colors are natural, while others are produced by treating pale or colorless topaz with heat, radiation, or metallic coatings. Physical Properties of Topaz Chemical Classification

Silicate.

Color

Natural colors include: colorless, yellow, orange, red, pink, blue, green. Occurs in a wide range of treated colors, most often blue.

Streak

Colorless - harder than the streak plate.

Luster

Vitreous.

Diaphaneity

Translucent to transparent.

Cleavage

Perfect basal cleavage.

Mohs Hardness

8

Specific Gravity

3.4 to 3.6

Diagnostic Properties

Hardness, prismatic crystals, sometimes striated, cleavage, specific gravity.

Chemical Composition Crystal System Uses

Al2SiO4(F,OH)2

Orthorhombic. Gemstone, Mohs hardness index mineral.

Physical Properties of Topaz One of the best-known physical properties of topaz is its hardness. It has a hardness of 8 on the Mohs hardness scale. It also serves as the Mohs hardness scale index mineral for a hardness of 8. Every student who takes an introductory geology course learns about the hardness of topaz. Diamond, corundum, and chrysoberyl are the only commonly-known minerals that are harder. Most topaz is colorless or milky. Yellowish and brownish colors are also common. Natural pink, orange, red, purple, and blue topaz are rare and valuable if they are of gem quality. When allowed to grow unrestricted, topaz forms orthorhombic crystals, often with striations that parallel the long axis of the crystal. It also has a distinct basal cleavage that breaks perpendicular to the long axis of the crystal. This cleavage makes topaz a more fragile gemstone than its hardness of 8 would imply. Hardness is the resistance to being scratched, but the ability to resist breakage is a property known as tenacity. Topaz has a specific gravity that ranges between 3.4 and 3.6. This is quite high for a mineral composed of aluminum, silicon, and gaseous elements.

Blue Topaz: Faceted ovals of two colors of blue topaz that are popular today. On the left is a "Swiss Blue" topaz weighing 2.02 carats. On the right is a "London Blue" weighing 2.26 carats. Both stones were colorless topaz mined in Brazil, then irradiated and heated to produce the blue colors. Topaz with treated blue color is the most common color of topaz in commercial jewelry today.

Topaz Treatment Methods: Colorless topaz, also known as white topaz, (top left) can be irradiated and heated to produce gems with a blue color (top right). Irradiation alone can produce a pale pink color (bottom right). Coating with certain metallic oxides can produce a vivid pink color (bottom left). Most topaz in commercial jewelry today is colorless material that has been heated, irradiated, or coated to improve its color.

Use of Topaz as a Gemstone The name "topaz" and many language variants have been used for yellowish gemstones for at least two thousand years. At that time all yellowish gems were called topaz in many parts of the world. People who traded in gems did not realize that these yellowish stones were of many different kinds. Then, about two hundred years ago, people who traded in gems began to realize that these yellowish gems might be topaz, quartz, beryl, olivine, sapphire, or one of many other minerals. They also learned that topaz occurred in a wide range of colors other than yellow. Today, gemologists know the full range of colors for natural topaz.

If you visited a jewelry store fifty years ago and asked to see topaz, you would likely be shown gems that were in the color range of yellow, orange, and brown. Starting in the 1970s and 1980s, the most common color that you would be shown began to be blue. This blue color was usually produced by treatments that converted colorless topaz into a more marketable gemstone.

"Mystic" Topaz: Some topaz is heated and then coated with a metallic oxide to change its color or to produce an iridescent effect. These treatments are sold under the trade name of "mystic topaz." These materials are simply clear topaz with a coating that might not be very durable.

Topaz Treatments Today most topaz offered in mall and department store jewelry stores at low to moderate price levels has been treated in a laboratory. Colorless topaz is heated, irradiated, and coated with thin layers of metallic oxides to alter its color. Natural blue topaz is extremely rare and is usually pale blue. Almost all of the blue topaz offered in stores is colorless topaz that has been irradiated and then heated to produce a blue color. "Swiss blue" and "London blue" are trade names for two of the most common varieties of treated blue topaz seen in today's market. Natural pink to purple topaz is also extremely rare. These colors are often produced in a laboratory. The starting point is a stone cut from colorless topaz. It is first heated and then coated with a layer of metallic oxide to produce the pink color. If coated stones are worn in jewelry, over time the coating often wears thin or wears through at points on the stone where abrasion occurs. Some topaz is coated with a metallic oxide that gives the stone a multicolored iridescent luster. These stones, known as "mystic topaz," appear to change color if the observer moves the stone under a light or changes the angle of observation. These coatings are also thin and can be worn through during wear.

Topaz Mountain Rhyolite: Outcrop of stratified tuff of the Topaz Mountain Rhyolite, filling a paleovalley. These valley fills were once thought to be deposited by water, but now many of them are believed to have been deposited by ground surges of hot ash. The Topaz Mountain Rhyolite has many vuggy areas, which often contain champagne-colored topaz crystals. Located in western Utah. USGS image.

Geologic Occurrence of Topaz Topaz has a chemical composition of Al2SiO4(F,OH)2. The fluorine in its composition is a limiting factor on its formation. Fluorine gas in concentrations high enough to form minerals only occurs in a few geologic environments. Most topaz forms the veins and cavities of igneous rocks such as pegmatite and rhyolite. It forms during the late stages of magma cooling while hydrothermal activity delivers fluorine. Topaz occurs in pockets within pegmatites, in fractures that carried hydrothermal fluids, and in gas cavities within rhyolite. Precipitating in cavities, topaz sometimes develops nicely-formed crystals. These crystals can have excellent clarity and can be used as a gem material. Especially attractive crystals of topaz are popular with mineral collectors. They have the value of a mineral specimen and the value of a gem material. Topaz is also found as water-worn pebbles in stream sediments derived from the weathering of pegmatites and rhyolites. These are often produced by placer mining. Topaz is found in many locations worldwide where rocks like pegmatite and rhyolite are formed. It is only a minor mineral at these locations, and it is considered to be a rare mineral on the basis of its general abundance. Brazil is the leading source of gem-quality topaz today. Sri Lanka is another important producer. Topaz is also produced in Nigeria, Australia, Pakistan, Russia, India, Zimbabwe, Madagascar, and Namibia. In the United States, Utah named topaz as its state gemstone.

Tourmaline Earth's most colorful mineral and gem material.

Tourmaline: Six faceted tourmaline gemstones from Africa. Clockwise from top left: A blue-green oval weighing 5.5 carats; emerald-cut chrome tourmaline, 1.51 carats; green round, 1.87 carats; pink emerald cut, 1.04 carats; pink-orange emerald cut, 1.88 carats; red cushion cut, 3.34 carats. (Photos are not to scale.) Specimens and images copyright by Lapigems.

What is Tourmaline? "Tourmaline" is the name of a large group of boron silicate minerals that share a common crystal structure and similar physical properties - but vary tremendously in chemical composition. The wide range of compositions, along with trace elements and color centers, causes tourmalines to occur in more colors and color combinations than any other mineral group. Crystals of good color and clarity are often cut into beautiful gemstones. Tourmaline is such a popular gemstone that it is easy to find in jewelry stores. Nice tourmaline crystals are also valued by mineral specimen collectors. Specimens with attractive colors and habits can sell for thousands of dollars. Tourmalines commonly occur as accessory minerals in igneous and metamorphic rocks. Large crystals of tourmaline can form in cavities and fractures during hydrothermal activity. Tourmaline also exists as durable grains in sediments and sedimentary rocks.

Accessory tourmaline: A specimen of the Crabtree Pegmatite from North Carolina, showing black prismatic tourmaline and green emerald crystals in a matrix of white feldspar and quartz. The width of this view is about two inches.

Geologic Occurrence of Tourmaline Accessory Mineral

The most common occurrence of tourmaline is as an accessory mineral in igneous and metamorphic rocks. It usually occurs as millimeter-size crystals scattered through granite, pegmatite, and gneiss. In this mode of occurrence, tourmaline rarely makes up more than a few percent of the rock's volume. The mineral most often found as an accessory mineral is black schorl.

Tourmaline crystals on cleavelandite: A large mineral specimen consisting of prismatic tourmaline crystals on cleavelandite with quartz and lepidolite. The tourmaline crystals are color zoned with red tourmaline at the base that sharply transitions to bluegreen along their length. From the Pederneira Mine of Minas Gerais, Brazil. Measures 21 x 15 x 14 cm. Specimen and photo by Arkenstone / www.iRocks.com.

Crystals in Fractures Voids and Pockets

The most spectacular tourmaline crystals are formed by hydrothermal activity. They are found in pockets, voids, or fractures and range in size from tiny millimeter crystals to massive prisms weighing over 100 kilograms. A rich pocket of nice tourmaline crystals can yield mineral specimens and gem materials worth millions of dollars.

Alluvial tourmaline: About 30 carats of stream-rounded tourmaline rough from Tanzania in yellow, orange, and green colors.

Alluvial Tourmaline

With a hardness of 7 to 7 1/2, tourmaline weathered from igneous or metamorphic rocks can be a durable sediment grain. Tourmaline gem rough is mined from streams sediments in many parts of the world, often by artisanal miners. Tourmaline is often one of many minerals produced from a single mining location. Physical Properties of Tourmaline Chemical Classification

Boron silicate

Color

Usually black. Blue, green, yellow, pink, red, orange, purple, brown, and colorless. Single crystals are often zoned.

Streak

White when softer than the streak plate. Colorless when harder than the streak plate.

Luster

Vitreous

Diaphaneity Cleavage

Transparent to translucent to nearly opaque Indistinct

Mohs Hardness

7 to 7.5

Specific Gravity

2.8 to 3.3

Diagnostic Properties

Chemical Composition Crystal System

Lack of visible cleavage, prismatic crystals with rounded triangular cross-sections that are often striated, vibrant colors (Ca,Na,K,[]) (Li,Mg,Fe+2,Fe+3,Mn+2,Al,Cr+3,V+3)3(Mg,Al,Fe+3,V+3,Cr+3)6 ((Si,Al,B)6O18) (BO3)3(OH,O)3 (OH,F,O) Hexagonal

Uses

A popular gemstone and mineral specimen

Physical Properties of Tourmaline Tourmaline has a few properties that can aid in its identification. If you have a tourmaline crystal, identification should be easy. Tourmaline crystals are prismatic and often have obvious striations that parallel their long axis. They often have triangular or six-sided cross-sections with rounded edges. They are often color zoned through their cross-sections or along their length. And, tourmaline is pleochroic with the darkest color viewing down the C-axis and lighter color viewing perpendicular to the C-axis. Don't despair if your suspected tourmaline is an accessory mineral in an igneous or metamorphic rock. It often occurs in these rocks as tiny prismatic crystals. Get a hand lens and look for striations and rounded cross-sections. Tourmaline has indistinct cleavage, so any specimen with obvious cleavage is not tourmaline. Color might not be helpful. The most common tourmaline color is black, but the mineral occurs in all colors of the spectrum.

Colorful tourmaline crystals: A couple dozen small tourmaline crystals in pretty colors from Afghanistan, suitable for faceting very small stones. Some of them are bicolor and a few are oriented to show pleochroism, with the color looking down the long axis of the crystal being much darker than when looking at the crystal in side view.

Tourmaline crystals: Tourmaline crystals often have many fractures and inclusions, but these crystals exhibit wonderful clarity and very rich color. They also show the striations along the long axis of the crystals that are characteristic of tourmaline. The bluegreen cluster on the left sits atop cleavelandite with purple lepidolite, and it measures 13 cm tall. The rubellite cluster on the right measures 6.7 cm tall. Specimens and photos by Arkenstone / www.iRocks.com.

Tourmaline Chemistry Tourmaline is a complex boron silicate mineral with a generalized chemical composition of: XY3Z6(T6O18)(BO3)3V3W

Letters in the formula above represent positions in the atomic structure of tourmaline that can be occupied by ions listed below.      

X = Ca, Na, K, [vacancy] Y = Li, Mg, Fe+2, Fe+3, Mn+2, Al, Cr+3, V+3 Z = Mg, Al, Fe+3, V+3, Cr+3 T = Si, Al, B V = OH, O W = OH, F, O

The complex formula and many substituting ions produce the large number of minerals in the tourmaline group. The International Mineralogical Association has recognized 32 different tourmaline minerals based upon the chemical composition of solid solution series end members. These minerals are listed in the table below.

Tourmaline Group Minerals Mineral

Composition

Adachiite

CaFe3Al6(Si5AlO18)(BO3)3(OH)3OH

Bosiite

NaFe3(Al4Mg2)Si6O18(BO3)3(OH)3O

Chromium-dravite Chromo-alumino-povondraite

NaMg3Cr6Si6O18(BO3)3(OH)3OH NaCr3(Al4Mg2)Si6O18(BO3)3(OH)3O

Darrellhenryite

NaLiAl2Al6Si6O18(BO3)3(OH)3O

Dravite

NaMg3Al6Si6O18(BO3)3(OH)3OH

Elbaite

Na2(Li3,Al3)Al12Si12O36(BO3)6(OH)6(OH)2

Feruvite

CaFe3(MgAl5)Si6O18(BO3)3(OH)3OH

Fluor-buergerite

NaFe3Al6Si6O18(BO3)3O3F

Fluor-dravite

NaMg3Al6Si6O18(BO3)3(OH)3F

Fluor-elbaite

Na2(Li3,Al3)Al12Si12O36(BO3)6(OH)6F2

Fluor-liddicoatite

Ca(Li2Al)Al6Si6O18(BO3)3(OH)3F

Fluor-schorl

NaFe3Al6Si6O18(BO3)3(OH)3F

Fluor-tsilaisite

NaMn3Al6Si6O18(BO3)3(OH)3F

Fluor-uvite

CaMg3(Al5Mg)Si6O18(BO3)3(OH)3F

Foitite

[](Fe2Al)Al6Si6O18(BO3)3(OH)3OH

Lucchesiite Luinaite-(OH) Magnesio-foitite Maruyamaite Olenite Oxy-chromium-dravite

Ca(Fe)3Al6Si6O18(BO3)3(OH)3O (Na,[])(Fe,Mg)3Al6Si6O18(BO3)3(OH)3OH [](Mg2Al)Al6Si6O18(BO3)3(OH)3OH K(MgAl2)(Al5Mg)Si6O18(BO3)3(OH)3O NaAl3Al6Si6O18(BO3)3O3OH NaCr3(Mg2Cr4)Si6O18(BO3)3(OH)3O

Oxy-dravite

Na(Al2Mg)(Al5Mg)Si6O18(BO3)3(OH)3O

Oxy-schorl

Na(Fe2Al)Al6Si6O18(BO3)3(OH)3O

Oxy-vanadium-dravite

NaV3(V4Mg2)Si6O18(BO3)3(OH)3O

Povondraite

NaFe3(Fe4Mg2)Si6O18(BO3)3(OH)3O

Rossmanite

[](LiAl2)Al6Si6O18(BO3)3(OH)3OH

Schorl

NaFe3Al6Si6O18(BO3)3(OH)3OH

Tsilaisite

NaMn3Al6Si6O18(BO3)3(OH)3OH

Uvite

CaMg3(Al5Mg)Si6O18(BO3)3(OH)3OH

Vanadio-oxy-chromium-dravite

NaV3(Cr4Mg2)Si6O18(BO3)3(OH)3O

Vanadio-oxy-dravite

NaV3(Al4Mg2)Si6O18(BO3)3(OH)3O

Faceted tourmaline: A collection of faceted tourmalines of various colors. Some of these stones exhibit multiple colors because they were cut from color-zoned crystals. Two are pink and green bicolor stones known as "watermelon tourmaline." Bicolor and pleochroic tourmalines are favorite stones of many jewelry designers because they can be used to make especially interesting pieces of jewelry. The small round stones weigh about 0.5 carat each. The watermelon in the lower left corner weighs 0.61 carat.

Names Used for Tourmaline Gems Tourmaline is one of the most popular gemstones because it occurs in every color of the spectrum. Jewelers and gemologists use trade names for different colors of tourmaline to simplify communications with their customers. Red tourmaline is sold as "rubellite." Dark blue tourmaline is sold as "indicolite." Dark green tourmaline is sold as "chrome tourmaline." Black tourmaline is sold as "schorl." These names work much better in a jewelry store than the mineralogical names in the table above! For other tourmaline colors, the name of the color is used as an adjective. For example, "pink tourmaline" or "purple tourmaline." "Yellow tourmaline" is sometimes sold as "canary tourmaline." Sometimes gem dealers inadvertently introduce trade names that result in confusion. In 1990, spectacular electric-blue tourmaline, colored by trace amounts of copper, was found in the state of Paraiba, Brazil. The material was named "paraiba" after its locality. The beautiful gems were soon selling for over $2000 per carat, and the name "paraiba" rapidly spread through gemstone markets. People loved the gems and their exotic name. The name caused two problems in the gem trade. First, the name "paraiba" could be used when selling copper-bearing gems with the electric-blue color. It could also be used when selling any tourmaline from the state of Paraiba. "Paraiba Tourmaline" and "tourmaline from Paraiba" have different meanings to knowledgeable people. People who have heard the name "paraiba" but don't understand could be misled.

The second problem began in 2001 when electric-blue tourmaline was discovered in Nigeria. More was discovered in Mozambique in 2005. All of these tourmalines were marketed using the popular "paraiba" name. This confusion persists in the marketplace. It was slightly reduced when some sellers began using "African Paraiba" for gems mined in Nigeria and Mozambique. Some geologists use the gemological names for members of the tourmaline group. However, determining the correct mineralogical name can be impossible in the field or a classroom, so communication is done with these alternative names.

Watermelon tourmaline: A pair of rough and faceted tourmalines that exhibit a superb example of watermelon color. Both the faceted stone and the crystal have clarity problems. This is typical for bicolor tourmaline. Perfect specimens near the color transition are extremely rare. The change in conditions that caused the color change might have also disrupted the crystal growth to produce the clarity problems. From Minas Gerais, Brazil. The rough crystal measures approximately 4.2 x 1.4 x 1.1 cm, and the faceted gem measures 27.79 mm x 18.51 mm and weighs nearly 50 carats. Specimens and photos by Arkenstone / www.iRocks.com.

Tourmaline crystal cross-section: A "slice" of watermelon tourmaline which shows the pink interior, green outer layer, and triangular shape of the crystal. This specimen shows where the "watermelon" name comes from. Image © iStockphoto / Sun Chan.

Color Zoning in Tourmaline

Changing conditions during tourmaline crystal growth often result in single crystals that contain two different colors of tourmaline. The earlier color is usually overgrown by the later color. These bicolor crystals are known as "zoned crystals." In many gems, color zoning is undesirable. Most gem and jewelry buyers prefer stones that have a single, uniform face-up color. Tourmaline is an exception to this trend. Gems cut from color-zoned crystals with pleasing colors are a novelty prized by designers and collectors. Color-zoned crystals are often sawn into thin cross-sections and polished. These thin bicolor gems can be very attractive. The most popular bicolor tourmaline is "watermelon tourmaline." It has a pink interior and a green rind - just like a slice of watermelon. The closer the colors match those of a real watermelon, the more people enjoy them and the higher the price. Tourmaline crystals are also sawn and faceted to produce bicolor gems. "Watermelon" is again the most popular, but many other beautiful color combinations are cut. Zoned tourmaline crystals often have clarity problems in the color-change area. If the color combination is attractive, minor clarity problems will not kill their desirability or price. Video: Pleochroism in Tourmaline: This video demonstrates pleochroism in two short sections of tourmaline crystals. The crystal sections are lightest when viewed perpendicular to the c-axis of the crystal (the side being held by the gem tweezers) and they are darkest when viewed down the c-axis of the crystal.

Pleochroism in Tourmaline Tourmaline is a pleochroic mineral. That means its apparent color can change with different directions of observation. The color is usually darkest looking down the c-axis of the crystal (down the long axis). It is usually lightest when viewing perpendicular to the long axis of the crystal. Cutting pleochroic gem materials requires skill and knowledge. Rough must be studied and oriented to produce a gem with pleasing face-up color. A light piece of rough can be cut with the table of a stone perpendicular to the c-axis of the rough to maximize color. Dark rough can produce lighter gems if it is cut with the table plane of the stone parallel to the c-axis of the rough. Some rough can be cut to nicely display two pleochroic colors in the face-up position. Color optimization of pleochroic rough is time-consuming, requires special skills, and usually involves sacrifice. Which will produce a higher profit? A stone of premium color with a lower carat weight, or a larger stone with a less desirable color?

Turquoise Properties of a gemstone mineral that has been held in high regard for thousands of years.

Turquoise and argillite inlay pieces: A collection of Ancestral Puebloan (Anasazi) turquoise and orange argillite inlay pieces from Chaco Canyon National Historical Park in New Mexico. These pieces date from about 1020-1140 CE and show the typical materials used in the ancient Chacoan bead and inlay industry. Public domain image from the National Park Service.

What is Turquoise? Turquoise is an opaque mineral that occurs in beautiful hues of blue, blue-green, and yellow-green. It has been treasured as a gemstone for thousands of years. Isolated from one another, the ancient people of Africa, Asia, South America and North America independently made turquoise one of their preferred materials for producing gemstones, inlay, and small sculptures. Chemically, turquoise is a hydrous phosphate of copper and aluminum (CuAl6(PO4)4(OH)8·5H2). Its only important use is in the manufacture of jewelry and ornamental objects. However, in that use it is extremely popular - so popular that the English language uses the word "turquoise" as the name of a blue-green color that matches the stone. Very few minerals have a color that is so well known, so characteristic and impressive that the name of the mineral becomes so commonly used. Only three other minerals - gold, silver, and copper - have a color that is used in common language more than turquoise.

Turquoise cabochons: A diverse collection of turquoise cabochons from various locations. From left to right in the upper row: a greenish blue turquoise cabochon with black matrix from China; a teardrop-shaped, slightly greenish blue turquoise cabochon from Arizona's Sleeping Beauty Mine; and, two sky-blue turquoise cabochons with chocolate brown matrix from the Altyn-Tyube Mine in Kazakhstan. In the center row: a small sky-blue turquoise cabochon from the Kingman Mines in Arizona; and, two small round sky-blue cabochons from the Sleeping Beauty Mine of Arizona. In the bottom row: two small cabochons with black matrix from unknown mines in Nevada; a teardrop-shaped cabochon with slightly greenish blue turquoise in black matrix from the Newlanders Mine in Nevada; and, a rectangular cabochon of slightly greenish blue turquoise in reddish brown matrix from the #8 Mine in Nevada.

Turquoise Colors Blue minerals are rare, and that is why turquoise captures attention in the gemstone market. The most desirable color of turquoise is a sky blue or robin's-egg blue. Some people inappropriately describe the color as "Persian blue" after the famous high-quality material mined in the area that is now known as Iraq. Using a geographic name with a gem material should only be done when the material was mined in that locality. After blue, blue-green stones are preferred, with yellowish green material being less desirable. Departure from a nice blue color is caused by small amounts of iron substituting for aluminum in the turquoise structure. The iron imparts a green tint to the turquoise in proportion to its abundance. Turquoise, especially the more porous varieties, can discolor with exposure to prolonged sunlight, heat, cosmetics, perspiration, and body oil. Some turquoise contains inclusions of its host rock (known as matrix) that appear as black or brown spiderwebbing or patches within the material. Many cutters try to produce stones that exclude the matrix, but sometimes it is so uniformly or finely distributed through the stone that it cannot be avoided. Some people who purchase turquoise jewelry enjoy seeing the matrix within the stone, but as a general rule, turquoise with heavy matrix is less desirable. Some turquoise localities produce material with a characteristic color and appearance. For example, the Sleeping Beauty Mine is known for its light blue turquoise without matrix. Much of the turquoise from the Kingman Mine is bright blue with a spider web of black matrix. The Morenci Mine produces a lot of dark blue turquoise with pyrite in the matrix. Much of the Bisbee turquoise has a bright blue color with a chocolate brown matrix. People who know turquoise can often, but not always, correctly associate a stone with a specific mine.

Turquoise cabochons: Turquoise cabochons from many parts of the world, showing a diversity of color and matrix. Image © iStockphoto, IrisGD.

Turquoise Occurrence Turquoise is rarely found in well-formed crystals. Instead it is usually an aggregate of microcrystals. When the microcrystals are packed closely together, the turquoise has a lower porosity, greater durability, and polishes to a higher luster. This luster falls short of being "vitreous" or "glassy." Instead many people describe it as "waxy" or "subvitreous." Porous turquoise is sometimes treated by soaking it in melted wax or impregnating it with polymer plastic to improve its characteristics. Turquoise forms best in an arid climate, and that determines the geography of turquoise sources. Most of the world's turquoise rough is currently produced in the southwestern United States, China, Chile, Egypt, Iran, and Mexico. In these areas, rainfall infiltrates downward through soil and rock, dissolving small amounts of copper. When this water is later evaporated, the copper combines with aluminum and phosphorus to deposit tiny amounts of turquoise on the walls of subsurface fractures. Turquoise can also replace the rock in contact with these waters. If the replacement is complete, a solid mass of turquoise will be formed. When the replacement is less complete, the host rock will appear as a "matrix" within the turquoise. The matrix can form a "spider web," "patchy" design, or other pattern within the stone. Physical Properties of Turquoise Chemical Classification

Color

Phosphate

Sky blue (most desirable as a gemstone), blue, blue-green, yellowish green, often with brown, black or metallic matrix spider-webbing through the material

Streak

White, greenish

Luster

Waxy to subvitreous

Diaphaneity

Nearly opaque

Cleavage

Perfect

Mohs Hardness

5 to 6

Specific Gravity

2.6 to 2.9 (variable because of porosity)

Diagnostic

Color

Properties Chemical

CuAl6(PO4)4(OH)8·4H2O

Composition Crystal System

Triclinic

Uses

Decorative stone, gemstone

The Turquoise Group of Minerals The turquoise group consists of five monoclinic minerals with a similar chemical composition and structure. Included are turquoise, aheylite, chalcosiderite, faustite, and planerite. Their compositions are listed below. Turquoise Group Minerals Mineral Turquoise Aheylite

Chemical Composition CuAl6(PO4)4(OH)8·4H2O (Fe,Zn)Al6(PO4)4(OH)8·4H2O

Chalcosiderite Cu(FeAl)6(PO4)4(OH)8·4H2O Faustite

(Zn,Cu)Al6(PO4)4(OH)8·4H2O

Planerite

Al6(PO4)2(OH)8·4H2O

Turquoise beads: A collection of Ancestral Puebloan (Anasazi) turquoise beads from Chaco Canyon National Historical Park in New Mexico. These pieces date from about 1050-1100 CE and show the typical materials used in the ancient Chacoan bead and inlay industry. Public domain image from the National Park Service.

Turquoise in the United States Most of the turquoise production in the United States has been located in the arid southwest, and most of that production has been in or around deposits of copper.Arizona, New Mexico, and Nevada have all held the position of the leading turquoise-producing state. New Mexico held that position until the 1920s, Nevada held the position until the 1980s, and Arizona is currently the leading state. Significant amounts of turquoise have been produced in California, Colorado, Utah, Texas, and Arkansas. Most of the turquoise is a byproduct of copper production. The large open-pit copper mines excavate down through the shallow rock units where the turquoise is formed. When turquoise is encountered, the quantity and quality of the material is assessed, and if warranted, a temporary effort is made to recover the gem material. If the value of the turquoise is worth disrupting the mining operation, it will be mined. The mining could be done by copper company employees, but the job is often given to contract miners who are able to come in and quickly recover the turquoise.

Turquoise jewelry: Old and new turquoise and silver Navajo bracelets. Image by Silverborders, used here under a Creative Commons license.

Turquoise Jewelry and Art The earliest record of turquoise being used in jewelry or in ornaments is from Egypt. There, turquoise has been found in royal burials over 6000 years old. About 4000 years ago, miners in Persia produced a blue variety of turquoise with a "sky blue" or "robin's-egg blue" color. This material was very popular and traded through Asia and into Europe. This is the source of the term "Persian Blue" color. In North America the earliest known use of turquoise was in the Chaco Canyon area of New Mexico, where the gem was used over 2000 years ago. Ancient artists produced beads, pendants, inlay work, and small sculptures.

Rough turquoise and turquoise objects were held in high regard by Native Americans and were traded widely. This spread North American turquoise across the southwest and into South America. These early Native American jewelry designs were simple, and the turquoise was not set in metal findings. In the late 1800s, Native American artists began using coin silver to make jewelry. This work evolved into the turquoise and sterling silver style of Native American jewelry that is popular today. The demand for turquoise and turquoise jewelry rises and falls over time. In the United States there was a surge in demand that began in the 1970s and declined in the 1980s. Demand for turquoise jewelry is always highest in the southwestern states where turquoise mining and Native American artists make turquoise part of the local culture.

Synthetic turquoise cabochons: Cabochons made from synthetic turquoise produced in Russia. These stones are 7mm x 5mm ovals.

Synthetic and Imitation Turquoise A small amount of synthetic turquoise was produced by the Gilson Company in the 1980s, and some of their material was used to make jewelry. It was produced in a sky blue color, sometimes with a gray spider webbing. It was a ceramic product with a composition similar to natural turquoise. Synthetic turquoise and turquoise simulants have been produced in Russia and China since the 1970s. They are prolific producers. The material is used to make cabochons, beads, small sculptures, and many other items. A photo on this page shows some synthetic turquoise cabochons made in Russia. There are many different glass, plastic, and ceramic materials with an appearance similar to turquoise. Many of these can easily be distinguished from turquoise by testing their hardness, specific gravity, or other properties.

Turquoise rough: A specimen of rough sky blue turquoise in host rock from Mohave County, Arizona. Specimen and photo by Arkenstone / www.iRocks.com.

Look-Alike Materials Howlite and magnesite are light gray to white minerals that often have markings that resemble the spider webbing seen in some turquoise. They can be dyed a turquoise blue color that makes them look very similar to natural turquoise. These dyed stones fooled many people when they first entered the marketplace and still are mistaken for genuine turquoise by unfamiliar buyers. Dyed stones have damaged the market for genuine turquoise. They have been purchased with the thought that they were turquoise by many people and have produced uncertainty in the mind of many jewelry buyers. This causes some people to avoid turquoise jewelry. Today dyed howlite and magnesite are still used to make mass-produced beads, cabochons, tumbled stones, and other turquoise look-alike items. They are almost ubiquitous in the marketplace. The dye generally does not penetrate deeply into the material. Scratching the back of a stone with a pin will often reveal a white interior. When heavily dyed, a stone must be scratched deeply or be broken to reveal the light interior. A few minerals are sometimes confused with turquoise by people who are unfamiliar with turquoise. These minerals include: variscite, larimar, blue-green chalcedony, lapis lazuli, and chrysocolla. Simple tests will usually differentiate these materials; however, some of the tests are destructive and are usually not suitable for use on finished jewelry items. Contributor: Hobart King

Witherite Mineral Properties and Uses

Physical Properties of Witherite Chemical Classification

carbonate

Color

white, colorless, light grayish, yellowish brown

Streak

white

Luster

vitreous

Diaphaneity Cleavage

transparent to translucent fair, distinct

Mohs Hardness

3 to 3.5

Specific Gravity

4.3 to 4.4

Diagnostic Properties Chemical Composition Crystal System Uses

specific gravity, effervesces in dilute HCl BaCO3 orthorhombic barium

Witherite from Hexham, England. Specimen is approximately 4 inches (10 centimeters) across.

Wollastonite Mineral Properties and Uses

Physical Properties of Wollastonite Chemical Classification

silicate

Color

white, gray, pale green, colorless

Streak

white

Luster

vitreous

Diaphaneity Cleavage

subtransparent to translucent three directions of cleavage, perfect

Mohs Hardness

4.5 to 5.5

Specific Gravity

2.8 to 3.1

Diagnostic Properties

color, cleavage

Chemical Composition

Calcium silicate, CaSiO3

Crystal System Uses

triclinic asbestos, siding, roofing tile, ceramics

Wollastonite with garnet from Willsboro, New York. Specimen is approximately 4 inches (10 centimeters) across.

Wollastonite with garnet from Willsboro, New York. Specimen is approximately 4 inches (10 centimeters) across.

Zircon Used as a gemstone for over 2000 years. Today it is the primary ore of zirconium.

Zircon Crystal: A small cluster of nicely-formed zircon crystals which clearly demonstrate that zircon is a member of the tetragonal crystal system. The four-sided crystals are prismatic with a square cross-section and terminate with a pyramid. The largest crystal in the cluster is about 1.7 centimeters in length. Specimen from Mt. Malosa, Malawi.

What Is Zircon? Zircon is a zirconium silicate mineral with a chemical composition of ZrSiO4. It is common throughout the world as a minor constituent of igneous, metamorphic, and sedimentary rocks. Zircon is a popular gemstone that has been used for nearly 2000 years. It occurs in a wide range of colors and has a brightness and fire that rivals those of diamond. Colorless zircon is sometimes used as a lowercost alternative for diamond. Zircon should not be confused with cubic zirconia, which is a man-made material. Zircon is present in most soils and clastic sediments. Zircon-rich sediments are mined and the recovered zircon is used to produce zirconium metal and zirconium dioxide. These are used in a wide variety of manufactured products and industrial processes.

Zircon Gem Rough: Stream-rounded crystals of zircon from an alluvial deposit in Australia. Although brown zircon is not highly marketable, much of it can be altered to a rich blue color by heating in a reducing atmosphere to between 900 and 1000 degrees Celsius. These stones range from about 5 to 10 millimeters in size.

Geologic Occurrence Of Zircon Zircon is a primary accessory mineral in most granitic rocks. It is also present in gneiss and other rocks derived from the metamorphism of zircon-bearing igneous rocks. Zircon is so common and widely distributed across the rocks of Earth's surface that it could be considered to be a ubiquitous mineral. However, zircon is usually not noticed in rocks and sediments because of its very small particle size. Grains of zircon over a few millimeters in size are rare - they are usually under one millimeter in size. It is one of Earth's most common but most overlooked minerals. Zircon is highly resistant to chemical alteration and abrasion. When rock units containing zircon are weathered and their sediments are eroded, enormous numbers of tiny zircon crystals are dispersed. These can persist in soils, sediments, and sedimentary rocks for millions - even billions - of years. They can survive several cycles of uplift, weathering, erosion and deposition. Some of the largest crystals of zircon are formed in pegmatites, carbonate igneous rocks known as carbonatites, and in limestones altered by hydrothermal metamorphism. These large zircons are sometimes of high clarity and suitable for use as gemstones.

Faceted Zircon: Natural and heat-treated zircons in a range of colors that include white, champagne, blue, green, yellow, peach, rose, cognac, honey, and mocha. These stones are about 5 millimeters in size and weight approximately 7.47 carats total. The white, blue, green, yellow, peach and rose colors have been produced by heat treatment.

Physical Properties of Zircon Chemical Classification

Silicate

Color

Usually yellow, brown, or red. Also colorless, gray, blue, and green.

Streak

Colorless. Usually harder than the streak plate.

Luster

Vitreous to adamantine, sometimes oily.

Diaphaneity

Translucent to transparent

Cleavage

Imperfect

Mohs Hardness

7.5

Specific Gravity

4.6 to 4.7

Diagnostic Properties

Hardness, luster, specific gravity

Chemical Composition

ZrSiO4

Crystal System Uses

Tetragonal Ore of zirconium metal, ore of zirconium dioxide, whitening agents, white pigment, gemstones, radiometric dating.

Zircon as a Gemstone Zircon has been used as a gemstone for over 2000 years. Its very high dispersion and refractive index give it a brilliance and fire that rival those of diamond. For that reason, colorless faceted zircon has been used as both a popular and fraudulent substitute for diamond. Gemologists and many knowledgeable jewelers are able to distinguish zircon from diamond with a quick examination. To do this they look into the stone, through the table facet, and focus on the pavilion facet

junctions, with a 10x loupe. The pavilion facet junctions should appear as double-images caused by zircon's double-refraction. Diamond is singly refractive and will not show doubling of features within the stone. This same test can be used to distinguish zircon from cubic zirconia. Zircon is a popular gem because it is available in a variety of pleasing colors. Most natural zircons are yellow, red, or brown. Heating and irradiation can be used to produce colorless, blue, green, and many other zircon colors. Blue is the most popular zircon color. About 80% of the zircons sold today are blue. Although it is not as durable as diamond, zircon has good physical durability as a gem. It has a hardness of 7.5 and imperfect cleavage. That combination makes it suitable for most gemstone uses that include rings, earrings, pendants, brooches and other jewelry. Some zircon, especially gems that have been heat treated, can be brittle. The facet edges of these gems are susceptible to nicks and chipping.

Zircon Damage in Tanning and Nail Salons Most blue zircon sold in jewelry today is produced by heat treating brown zircon in a reducing atmosphere to 900 to 1000 degrees Celsius. Some people have damaged these blue zircons by exposing them to ultraviolet radiation in tanning beds or under ultraviolet lamps used to cure acrylic fingernail adhesives. The blue color can degrade to brown with just minutes of exposure. The color of some of these gems have been restored by exposure to low wattage incandescent light. To avoid ruining a nice gem, remove jewelry at tanning and nail salons. [1]

Zircon Concentrate Production: Zircon mineral concentrates are produced by mining heavy mineral sands from land- and marine-based deposits. Australia, South Africa, China, Indonesia, Mozambique, India, Ukraine, Sri Lanka, Madagascar, Brazil, Kenya and several other countries were producing zircon concentrates in 2014. [2]

Heavy Mineral Mining: Photo of the Concord heavy-mineral-sands mine in south-central Virginia. Weakly consolicated Pliocene-age sand and silt deposits here contain about 4% by weight heavy minerals. A separation plant is used to recover ilmenite, leucoxene, rutile, and zircon. [3]

Zircon Mining Zircon has been mined from stream gravels for over 2000 years. This early mining of zircon was mainly to obtain nice crystals for use as gemstones. Today, most zircon is produced by mining or dredging zircon-rich sediments. These sediments can be in beach, littoral, or alluvial deposits. Zircon has a specific gravity of 4.6 to 4.7, which is much higher than the typical detrital sediment grain that is between 2.6 and 2.8. This specific gravity difference allows zircon grains to be recovered from the sediments by mechanical separation. Specific gravity separation methods make it possible to profitably recover zircon and other heavy minerals at an ore grade of just a few percent. Zircon is often a coproduct at mining/processing operations where ilmenite and rutile are being mined for titanium. In the United States, zircon is mined in Virginia, Georgia, North Carolina and Florida. Industrial-grade zircon is mined from land- and marine-based deposits of alluvial origin in many parts of the world. Australia, South Africa, China, Indonesia, Mozambique, India, Ukraine, Sri Lanka, Madagascar, Brazil, Kenya and several other countries were important producers in 2014. These alluvial deposits contain mainly sub-millimeter grains of zircon derived from the weathering of granitic rocks. Gem-grade zircon has been produced from alluvial deposits in Sri Lanka, Cambodia, Myanmar, and Vietnam for hundreds of years. More recent gem-grade deposits are in Australia, Nigeria, and Madagascar. Deposits mined for gem-grade zircon must contain crystals at least several millimeters in size with good clarity. They are typically derived from the weathering of carbonate rocks and other rock types associated with hydrothermal activity. Some of the best gem-grade zircon crystals are mined directly from cavities in pegmatite.

Zircon, Zirconium, Zirconia and Cubic Zirconia There is much public confusion between four materials: zircon, zirconium, zirconia and cubic zirconia. Summary definitions of these terms are provided below. Zircon is a naturally occurring mineral with a chemical composition of ZrSiO4. Zirconium is a silvery white metal and a chemical element. It has an atomic number of 40 and an atomic symbol of Zr.

Zirconia is the white crystalline oxide of zirconium with a chemical composition of ZrO2. A naturally occurring, but rare, form of ZrO2 is the mineral baddeleyite. Cubic Zirconia is a synthetic gemstone with an appearance that is very similar to diamond. It sells for a tiny fraction of the cost of diamond and has historically been the most commonly used diamond simulant. All of these materials are related. Zirconium, zirconia and cubic zirconia are all produced from industrialgrade zircon.

Industrial Uses of Zircon Zircon References [1] Reversible Color Modification of Blue Zircon by Long-Wave Ultraviolet Radiation, by Nathan D. Renfro, Gems & Gemology, Volume 52, Number 3, Fall 2016. [2] Zircon and Hafnium, by George M. Bedinger, 2014 Minerals Yearbook, United States Geological Survey, August 2016. [3] Deposit Model for Heavy-Mineral Sands in Coastal Environments, by Bradley S. Van Gosen, David L. Fey, Anjana K. Shah, Philip L. Verplanck, and Todd M. Hoefen, Mineral Deposit Models for Resource Assessment, Scientific Investigations Report 2010–5070–L, United States Geological Survey, 2014. [4] Mineville, Eastern Adirondacks – Geophysical and Geologic Studies, by Anjana Shah, article on the Mineral Resources Program website of the United States Geological Survey, accessed November 2016.

Zircon sand has a low expansion coefficient and is very stable at high temperatures. It is used as a refractory material in many foundry and casting applications. One of its most common uses is in the production of ceramics. Zirconium dioxide (zirconia) is produced by heating zircon sand to a high enough temperature to break down the zircon molecule. In powdered form, zirconium dioxide is bright white, highly reflective and thermally stable. It is used as an opacifier, whitening agent, and pigment in glazes and stains used on ceramics and pottery. Yttria-stabilized zirconia is used to manufacture cubic zirconia, fiber optic components, refractory coatings, ceramics, dentures and other dental products. Zircon serves as the primary ore of zirconium metal. Zirconium is used in a variety of metal products that require a resistance to heat and corrosion. It is used to make high-performance alloys, specialty steel, lamp filaments, explosive primers, computer equipment and many electronics components.

Billion-year-old zircons: These zircon grains were hand-picked from a quartz-albite rock collected in Essex County, New York. This petrographic microscope transmitted light image reveals cracks, inclusions, and age “zones” throughout the grains. The cores and rims of the zircon grain reflect magmatic and tectonic events that occurred within the region about 1-1.15 billion years ago. [4]

Zircon and Radioactive Decay Many zircon crystals contains trace amounts of uranium and thorium. These radioactive elements were incorporated into the zircon at the time of crystallization. They convert into their decay products at a steady rate. The ratio of parent materials to daughter products can be used to estimate the time of crystallization. Using this method, the oldest mineral grains in the world are zircon crystals found in Australia. They are estimated to be about 4.4 billion years old. When radioactive elements in zircon crystals or nearby materials decay, radiation is emitted. The zircon crystal can be damaged by this radiation. Some zircon has been so damaged by exposure to this radiation that it no longer retains the clarity and optical properties of an attractive gem material. This is why some zircon is not suitable for use as a gem.

Zoisite and Clinozoisite Two very similar minerals with the same chemical composition but different crystal structures

Blue zoisite - Tanzanite: Tanzanite is the most widely known zoisite and one of the world's most popular gemstones. This violetish blue tanzanite is an exceptional faceted oval weighing 8.14 carats and measuring 14.4 x 10.5 x 7.6 millimeters in size. On the basis of its color and clarity, it would be rated in the top 1% of the tanzanite produced by TanzaniteOne Mining Ltd., the leading producer of tanzanite. Photo copyright by Richland Gemstones and used here with permission.

What are Zoisite and Clinozoisite? Zoisite and clinozoisite are minerals that form during the regional metamorphism and hydrothermal alteration ofigneous, metamorphic, and sedimentary rocks. In those environments they are found in massive form and as prismatic crystals in veins that cut schists and marbles. They are also found as crystals in pegmatites that form on the margins of igneous bodies. The two minerals are dimorphs - they share the same chemical composition but have a different crystal structure. Zoisite is the orthorhombic form of Ca2Al3(SiO4)(Si2O7)O(OH) and clinozoisite is the monoclinic form. The minerals have extremely similar physical properties and can be very difficult to tell apart in hand specimens unless the specimens are well-formed crystals. Clinozoisite forms a solid solution series with the mineral epidote in which iron can substitute for aluminum.

Zoisite: Shown above are 4 specimens of zoisite in unusual colors. Top row: pink and yellow crystals with orthorhombic crystal habit. Bottom row: (left) a parti-colored specimen with shades of green and pink in the same crystal; (right) a blue-green crystal with nice termination. Specimens and images copyright by Lapigems.

Uses of Zoisite and Clinozoisite Zoisite and clinozoisite are minerals that are usually found in small quantities. They have not been used in significant amounts by industry. Transparent and colorful specimens of both minerals have been used as gemstones. Zoisite is the mineral of some very diverse gem materials, one being the extremely popular tanzanite which was discovered in the 1960s and immediately became one of the world's most popular gems.

Thulite is a pink, opaque variety of zoisite that is often cut into cabochons or used to produce small sculptures. It can be an attractive material but is rarely seen in commercial use because the supply is limited and the public is unfamiliar with the gem.

Ruby in Zoisite: Anyolite, also known as "ruby in zoisite," is a rock composed of zoisite, with red corundum crystals (ruby) and often accented by black crystals of the hornblende, tschermakite. It is a rock that attracts attention and is cut into attractive cabochons and used to produce small sculptures. Image © iStockphoto / MarcelC.

Tanzanite Tanzanite is the most famous zoisite. It is a transparent blue zoisite that is colored by the presence of vanadium. Some blue zoisite is found naturally, but most is produced by heat-treating brown zoisite. The heat changes the oxidation state of vanadium to produce the blue color. Tanzanite is the second most popular blue stone, after sapphire. It is a rare gem only found in one small area in northern Tanzania. Thulite Thulite is an opaque pink variety of zoisite that is cut into cabochons and used to produce small sculptures. It is also a rare material, found in Norway, Namibia, Australia, North Carolina, and a few other locations. It is rarely seen in commercial use. Anyolite Anyolite is a very colorful rock composed mainly of zoisite. It is also known as "ruby in zoisite" because it is composed of green zoisite with bright red ruby crystals, sometimes accompanied by black crystals of the hornblende tschermakite. It is used to produce cabochons, tumbled stones, small sculptures and ornamental objects. Nice pieces of rough material are also sold as specimens. A material with a similar appearance, "ruby in fuchsite" is often misidentified as ruby in zoisite. Careful testing can easily differentiate these materials because the green fuchsite has a hardness of only 2 to 3, while the green zoisite has a hardness of at least 6. In addition, most specimens of ruby in fuchsite exhibit blue kyanite alteration rims around the ruby crystals, and this does not occur around ruby crystals in zoisite. Clinozoisite Gem-quality crystals of clinozoisite are sometimes cut into faceted stones. It is considered to be a "collectors" stone because it is rarely seen in jewelry.

Physical Properties of Zoisite and Clinozoisite Zoisite

Clinozoisite

Silicate

Silicate

Color

Colorless, gray, yellow, brown, pink, green, blue, and violet

Usually gray, yellow, green, or pink

Streak

White

White

Luster

Vitreous to granular, sugary

Vitreous to granular, sugary

Diaphaneity

Translucent to transparent

Translucent to transparent

Perfect in one direction

Perfect in one direction

Mohs Hardness

6.5

6.5

Specific Gravity

3.2 to 3.4

3.2 to 3.4

Diagnostic Properties

Hardness, specific gravity, striated crystals

Hardness, specific gravity, striated crystals

Chemical Composition

Ca2Al3(SiO4)(Si2O7)O(OH)

Ca2Al3(SiO4)(Si2O7)O(OH)

Orthorhombic

Monoclinic

Gemstones (tanzanite, anyolite, and thulite) and small sculptures

Gemstones

Chemical Classification

Cleavage

Crystal System Uses

Clinozoisite: Two views of the same crystal of clinozoisite from the Haramosh Mountains of Pakistan. The specimen is about 3.2 centimeters tall. Specimen and photo by Arkenstone / www.iRocks.com.

Physical Properties of Zoisite and Clinozoisite Zoisite and clinozoisite have the same chemical composition. This gives them very similar physical properties, as shown in the accompanying table. The difference between the two minerals is in their crystal structure. Zoisite is a member of the orthorhombic crystal system, and clinozoisite is monoclinic. They are difficult to tell apart in hand specimen unless well-formed crystals are present. Optical tests and x-ray diffraction are the best ways to make positive identification.

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