Lecture Notes And Figures: Minerals

Geology 12:

Minerals Lecture Notes And Figures

#1 - General Information:

• More than 2000 different kinds of minerals have been identified and named. • Minerals are the most common visible substance on earth. • Rock-forming minerals: main minerals observed in rocks • Industrial minerals: main minerals used to manufacture physical materials (ex: copper from malachite) • Biochemical minerals: manufactured by organisms (ex: clams/mollusks use to construct shells)

The solid earth (the lithosphere) is composed of minerals. Minerals: • are vital as natural resources • are active in geological processes associated with earthquakes and volcanoes • provide clues to how rocks are formed • and record the history of our planet.

#2 - Definition of a Mineral:

To be considered a mineral, a substance must possess the following five characteristics:
 1. Naturally Occurring (not made in the laboratory) 2. Inorganic (not living material) 3. Solid 4. A Regular Internal Crystal Structure* - defined by an orderly arrangement of its atoms. 5. A Definite Chemical Composition** - the presence of certain elements in certain proportions…can only vary within specified limits.

*A Regular Internal Crystal Structure: • Solids in which atoms are arranged in regular repeating patterns of three dimensional arrangement. • Materials with crystalline structure form crystals • Few minerals like limonite (rust) and opal do not have crystalline structure (mineraloids) • Most mineral crystals occur in rocks-aggregates of one of more minerals

**A Definite Chemical Composition • Minerals consist either of one element or are compounds of two or more elements. • One defining characteristic of minerals is the presence of certain elements in certain proportions.

Solid Solution:
 • 2 or more ions of similar size and charge may substitute more or less interchangeably for each other in the same mineral structure. • These substitutions will result in slightly different forms of the same minerals. • So instead of a definite chemical composition we say they have a definite compositional range of elements. • Example - Magnesium (Mg 2+) and Iron (Fe 2+) ions are nearly the same size so many minerals are able to interchange them freely!

Wollastonite = CaSiO3 Orthopyroxene = FeSiO4 Rhodonite = MnSiO3

Plagioclase Feldspar group (solid solution)

Polymorphs: • Minerals having same composition but distinctively different crystal structures. • For Example – – diamond and graphite are chemically the same. – both pure carbon (C). – but physically different due to differences in their crystalline structures.

Rocks: • A ROCK is a solid, cohesive aggregate of one or more minerals or mineral materials. • A rock consists of many individual mineral grains, not necessarily all of the same mineral, or of mineral grains plus glass, all firmly held together in a solid mass. • A beach sand consists of many mineral grains but they fall apart when handled, so sand is not a rock • Creation of rock from sediments = Lithification.

#3 - Mineral Identification &
 Mineral Properties:

Absolute Identification: • Minerals can be only identified absolutely by x-ray analysis and chemical tests DONE IN LAB! • The x-ray analysis determines the structure of the mineral and the chemical tests determine the composition of the mineral. • Structure and Composition are the FUNDAMENTAL DEFINING marks of a given mineral. • Both structure and composition affect certain physical properties. • It is through the proper use of these properties that minerals can reliably be identified in the field.

Seven Commonly Used Properties to Identify Minerals in the Field: 1) color and clarity 2) crystal form 3) luster 4) streak 5) cleavage 6) fracture 7) hardness

1) Colour and Clarity: • Colour is the first physical property we notice. • Usually a poor guide to identification since many minerals share the same colour, and impurities also effect colour! • Crystals with many colours can be assumed to be different minerals. • Minerals in variety of colours are given a variety of names. • Minerals may have different coloured interior than surface. • Unusual colouring may make mineral unusually valuable.

• The same mineral may vary in colour due to presence of small amounts of chemical impurities… • Ex) the following are all Quartz

Amethyst

Citrine

Agate

Pink Quartz

Tiger's Eye

Rock Quartz

• Mineral crystals may vary in clarity of colour (or lack of it) • May be :
 transparent - clear
 translucent - foggy
 opaque - impervious to light (like metals)

2) Crystal Form: • The characteristic crystal form (or combination of forms) of a mineral is that mineral’s habit. • Crystals form because they are the most stable arrangement of atoms in a solid. • Perfect crystal forms can only develop if a mineral crystal is unrestricted as it grows (rare)

• It is more common for crystals to crowd together as they grow, resulting in a network of inter-grown crystals that do not exhibit their crystal form! • Even if crystal forms develop they may be cryptocrystalline / microcrystalline - too small to see. • If form of crystal is visible then it can be used as a distinctive property for identification, but not usually possible. • Ex) table salt crystallizes in cubes.

• Some minerals can form massive crystal habit shapeless, no external crystal shape. https://en.m.wikipedia.org/wiki/Crystal_habit

3) Lustre:

• A description of how mineral surfaces reflect light. • • • • • • • • • •

Metallic (M): bright and shiny like metals Nonmetallic (NM): lustre unlike that of metal Adamantine - very gemmy crystals (diamond) Dull - just a non-reflective surface of any kind Earthy - the look of dirt or dried mud Fibrous - the look of fibres Greasy - the look of grease Gumdrop - the look a sucked on hard candy Pitchy - the look of tar Resinous - the look of resins such as dried glue or chewing gum

• Silky - the look of silk, similar to fibrous but more compact

Pearly - the look of a pearl

• Vitreous - the most common lustre, it simply means the look of glass

•Waxy

- the look of wax

4) Streak: • Colour of powder left after scraping sample across surface of porcelain streak plate. • Often called a mineral’s true colour because streak of certain mineral is usually same regardless of variety/ clarity. • It is a minerals colour in powdered form. • However, like colour it is not unique because many minerals share same streak colour.

• Two minerals that have similar outward color may have different colours when powdered. • For instance, the minerals hematite and galena

can be confused because both have a grey color. • However, hematite's streak is blood-red, while galena's streak is lead grey.

• When a mineral breaks it does so either by fracturing or by cleaving.

5) Cleavage:

• Crystal cleavage is a smooth break producing what appears to be a flat crystal face or crystal form… careful not to confuse them! • Tested by striking with hammer. • The same mineral will always have the same cleavage. • Cleavage surfaces occur as sets of parallel surfaces of weak chemical bonding between repeating parallel layers of atoms in a crystal. • Each set of parallel cleavage planes has an orientation relative to crystalline structure and is referred to as cleavage direction.

Biotite (black) has one good cleavage!

So does muscovite (clear)!

Biotite and muscovite are both MICAS.

Easy To Confuse Cleavage With Crystal

Form!

6) Fracture: • Also tested by hitting with a hammer! • Mineral breaks along curved surfaces, irregular, with no well-defined cleavage planes… i.e. no noticeable cleavage! • Described as uneven, conchoidal, splintery, hackly, etc.

• A very common fracture type is conchoidal fraction. • This is a smoothly curved fracture that is familiar to broken glass.

Obsidian

7) Hardness: • The ability to resist scratching! • Based on Internal Structure.

•Minerals with small atoms, packed tightly together with strong covalent bonds throughout tend to be the hardest minerals. •The softest minerals have metallic bonds or even weaker van der Waals bonds as important components of their structure. •Hardness is generally consistent because the chemistry of minerals is generally consistent.

• Measured using Mohs Hardness Scale. • Created by German mineralogist Friedrich Mohs • 10 minerals arranged in order of hardness • Talc is softest and has hardness of 1 • Diamond is hardest at 10


Other properties: 1. 2. 3. 4. 5. 6.

Tenacity Reaction with acid Magnetism Specific gravity (SG) Florescence Double Refraction

1) Tenacity: • Manner in which a substance resists breakage. • Brittle: shatters like glass • Malleable: like modeling clay/gold, hammered, bent to new shapes • Elastic/flexible: bends but returns to original shape • Sectile: can be carved with knife

2) Reaction to Acid: • Cool, dilute (1-3%) hydrochloric acid (HCL) applied from a dropper is a common acid test. • All carbonate minerals will fizz or effervesce. • Undiluted vinegar also useful for the test

3) Magnetism: • Magnetism is the ability of some minerals to behave like a magnet, particularly those that contain a large amount of iron. • Includes some minerals such as magnetite • Magnetism is somewhat of an unreliable property as not all specimens may demonstrate it. • the lack of magnetism should not generally exclude typically magnetic minerals. • A compass needle is a good test device for testing magnetism as is a magnet on a string that might sway near the specimen.

• magnetite

4) Specific Gravity (SG): • Specific gravity is the same number as density but without the units! • Hefting is easy way to judge… • Holding equal sized pieces of different mineral in either hand and feel the weight. • Heavier = higher SG

Lighter = Lower SG

• Gold and Galena and Sphalerite have unusually high SG • Gold can easily be distinguished from "fool's gold" by specific gravity alone

5) Fluorescence: • Some minerals glow in the dark under a black light (U.V. light) • Due to excitation of electrons

6) Double Refraction: • Light passing through the mineral is split into two rays. • Causes an optical "doubling" effect. • Common way to determine Calcite from halite or gypsum!

•Calcite •Calcite has the optical property of double refraction. •In the photos above, the same sample of calcite is used. •It is rotated over a thin dark line. •Examine how the appearance of the line changes in the different orientations of calcite.