Minerals

Chapter 5

What is a mineral?

Minerals are the building blocks of rocks.. All of them are solid.

They occur naturally and have a definite chemical composition

The atoms are arranged in an orderly fashion (crystal)

Inorganic (never alive)

How many minerals are there?

Well, there about 4000 known but there are a whole lot more I am sure to be found.

Minerals can be elements like lead or compounds like feldspar

Most are compounds

Elements of earth—native elements Even though there are 92 elements that are

naturally found, only 8 elements make up more than 98% of the earth’s crust.

46.6% Oxygen (O)27.7% Silica (Si)8.1% Aluminum (Al)5.0% Iron (Fe)3.6% Calcium (Ca)2.8% Sodium (Na)2.6% Potassium (K)2.1% Magnesium (Mg)

Compounds of the earth

Most minerals are compounds. Quartz is the most common chemical

compound in the earth's crust. We know it better as common beach sand. Si02

Mineral formation

Process of crystallization Growth of a solid from a gas or liquid

whose constituent atoms come together in the proper chemical proportions and crystalline arrangement

When do minerals form?

Cooling of magma (quartz)

Precipitation from saturated solution (halite)

Atoms and ions become mobile an rearrange themselves at high temperatures (pyroxene) 

The earth, the other inner planets and the moon are made of ROCKS.

Rocks in turn, are made of MINERALS

How are minerals formed?

Minerals of one type of atom - native elements (Au - gold, Ag - silver, Cu - copper, among others).

Minerals with cations combined with the large, negative ions (anions): oxygen (O - oxides), sulfur (S - sulfides), halogens (Cl, F or Br - halides) .

and anions groups made of oxygen combined with cations of valency +3 or greater (see periodic table - non-metals of groups 13-17): silicates (SiO4), carbonates (CO3) and sulfates (SO4).

Only a dozen minerals account for most of the world's rocks.

Plagioclase feldsparNaAlSi3O8 to CaAl2Si2O8

Potassium feldsparKAlSi3O8 QuartzSiO2 Pyroxene(Mg,Fe)2Si2O6 and

Ca(Mg,Fe)Si2O6 (augite)

BiotiteK(Fe,Mg)3AlSi3O10(OH)2 Hornblende(Ca,Na)2–

3(Mg,Fe,Al)5[(Si,Al)8O22](OH)2 Olivine(Fe,Mg)2SiO4 MagnetiteFe3O4 Garnet(Ca,Mg,Fe2+)3(Al,Fe3+)2Si3O12 CalciteCaCO3DolomiteCaMg(CO3)2

Physical properties of minerals

Classifying minerals can range from simple to very difficult. A mineral can be identified by several physical properties.

Sometimes minerals can only be classified by chemical or X-ray diffraction analysis; these methods, however, can be costly, time-consuming, and even risk damaging the sample.

Crystal structure

See page 99 It is the arrangement of atoms that

determines the shape…(just to repeat) Each mineral has its own unique form The angle at which crystal faces meet is

unique and can be used to identify the mineral

Crystal structure

Not all minerals form crystal faces….if the space is limited when forming there is no room for a “face” to form.

Silicates

Minerals that contain oxygen and silicon May include other metallic elements

Staurolite, Fe2Al9O A few silicates do not have metal

included----quartz 90% of all minerals are silicates

Silica tetrahedron

The basic building blocks of all silicates is the silica tetrahedron

Physical identification

Crystal structure and habit: A mineral may show good crystal habit or form, or it may be massive, granular or compact with only microscopically visible crystals.

Hardness:

Mohs scale is relative and goes from 1 to 10. Minerals with a given Mohs hardness can scratch the surface of any mineral that has a lower hardness than itself. The minerals that define the scale are given below:

1- talc 2- gypsum 3- calcite 4- fluorite 5- apatite 6- orthoclase feldspar 7- quartz 8- topaz or beryl 9- corundum 10- diamond

Mineral Properties

luster  hardness  streak  cleavage 

fusibility  specific gravity  habit  tenacity  color 

luminescence  radioactivity

magnetism

Luster

A mineral’s luster is the overall sheen of its surface – it may have the sheen of polished metal, or that of an unpolished metal that is pitted by weathering – or it may have the sheen of glass, or look dull or earthy, etc

Luster cont’d

Adamantine, having the hard, sparkly look of a diamond Glassy/Vitreous, having the look of glass; Resinous, having the look of amber – not quite glassy; Pearly, having the iridescent look of mother-of-pearl

(though usually just barely); Greasy/Oily, having the look of an oil-coated substance; Silky, having the look of silk, fine parallel fibers of mineral

– such as chrysotile "asbestos;" Dull, having a plain looking surface that is not

submetallic; Earthy, having the look of soil or clay

color

a double-edged sword in mineral identification:  There are many minerals which have distinctive colors; but there are also many which come in a variety of hues.  And the same color can be seen in several different species

Color cont’d

Opalescence, an opal-like play of light, reflections off the mineral producing flashes of color that may appear somewhat like a patch-work of different "grains" of color that aren’t really there:  Move the sample minutely and the color disappears from that spot. 

Color cont’d

Iridescence is similar to opalescence except that it is usually associated with metallic minerals and surface reflections rather than glassy minerals and sub-surface reflections:  An exception is the type of iridescence known as labradoresence

Color cont’d

Luminescence is the emission of light by a mineral other than the reflected light of the sun or a lamp – the mineral "glows" due to some other reason.

Chatoyancy

Chatoyancy is the play of light off closely packed parallel fibers or parallel inclusions in cavities.  The light reflects along lines – which may be straight or curved – giving the mineral a somewhat silky appearance.  This characteristic is seen in such minerals as "satin spar" gypsum, "tiger’s eye" (fibrous crocidolite replaced by quartz), and chrysoberyl

Fusibility

The measure of how much heat it takes to melt a mineral into a globule, or at least to melt the sharp edge of a sharp splinter and make it round over.

specific gravity

Density and Specific Gravity are not properties easily determined, requiring special equipment.

"heft" can come in handy:  The denser a mineral is, the heavier it is per given volume.  A 1 inch cube of galena is noticeable heavier in the hand than a 1 inch cube of pyrite.    With a little practice a collector can become good judging the relative weight of minerals and using that to help establish a sample’s identity.

habit

Habit is the general appearance a mineral tends to have – whether it is found as blocky crystals, long slender ones, or aggregates of some type, etc. 

Distinct crystals may be described by the following……..see website

Tenacity

refers to a mineral’s resistance to breaking, bending, or otherwise being deformed.  A mineral may be:

brittle, malleable, sectile, flexible, elastic,

Streak

Streak is simply the color of a mineral powder.  Many minerals have a different color when powdered than they do in crystal or massive forms. The color may be entirely different, or it may be a different shade.

specific gravity

Density and Specific Gravity are not properties easily determined, requiring special equipment.

"heft" can come in handy:  The denser a mineral is, the heavier it is per given volume.  A 1 inch cube of galena is noticeable heavier in the hand than a 1 inch cube of pyrite.    With a little practice a collector can become good judging the relative weight of minerals and using that to help establish a sample’s identity.

Cleavage

Cleavage refers to the way some minerals break along certain lines of weakness in their structure. 

Radioactivity

is another property that, while not too common, is found in some minerals and can be useful in identification.  Collectors who have a Geiger counter may find it useful at certain localities, particularly pegmatite – where many of the more common radioactive minerals are found.

Fracture

Minerals break in directions other than along cleavage surfaces

Conchoidal—shell like--smooth Splintery—leave jagged edge Uneven or irregular—rough surface

Chemical tests

Reaction with acid—calcite (CaCO3) fizzes and releases carbon dioxide gas

Special properties

Double refraction—causes a single object to appear as two

Simple kit

A piece of plain white paper (a blank specimen label works great.)

Your fingernails (preferable still attached to your fingers!) A copper penny (or small –½ inch – piece of copper or

short piece of heavy copper wire.) A small piece of fluorite (a broken cleavage piece is fine.) A pocket knife (NOT a Swiss Army knife – the steel in

those is harder than in most cheap pocket knives, which can throw hardness tests off.)

A small section of a steel file (a 2 or 3 inch tip from a triangular file for sharpening chain saws works fine.)

A piece of a quartz crystal (with at least one good face and a sharp point - a broken section

Simple kit cont’d

usually has a sharp point on it somewhere, it doesn’t have to be a crystal termination.)

A small piece of a beryl or topaz crystal (with at least one good face and a sharp point or edge.)

A small piece of a corundum crystal (with at least one good face and sharp point or edge.)

A "streak plate" (unglazed porcelain tile – a 2 inch square is plenty.)

A short candle stub and matches (in waterproof container) or a cigarette lighter.

A small pair of tweezers. A small magnet (a refrigerator magnet is fine, but should be a

fairly strong one.) A 10x hand lens/jeweler’s loupe. Blank specimen labels. Pens or pencils.

Mineral groups

Major silicates comprise 90% of earth minerals

Classified by ways the tetrahedral linked together see page 101

Quartz

Entirely tightly bound silica tetrahedral Glass or greasy luster Pure quartz-colorless or white Colored quartz—amethyst, rose, smoky,