Chapter 5.  Minerals

So . . .
Everything is made of matter, anything which occupies space and has mass; it occurs in several phases, solid, liquid, gas, and plasma. all matter is made of substances called elements, substances which can't be decomposed by normal processes.  Elements are 'kinds of stuff'; a quantity of matter all made up of the same kind of atom.  Currently 112 are known (newest one synthesized Feb. 1996); of these 92 are naturally occurring; the rest have been formed in atom smashers or nuclear explosions and aren't known to occur in nature.
Each element has its own symbol, usually a Latin or Greek abbreviation:
-often only a single letter, e.g. C, H, O, N
-sometimes two letters, e.g. Mg, Cl, Ca
-others Na (Natrium), K (Kalmium), Cu (Cuprum), Fe (Ferrum), Pb (Plumbum)

Symbols of elements and ions you will know:

H, C, N, O, F, Na, Mg, Al, Si, P, S, Cl, K, Ca, Mn, Fe, Cu, Zn, Ag, Sn, Au, Hg, Pb, U, OH-, (CO3)2-

A compound is a substance made up of atoms of two or more elements, and  can be broken apart by normal means. A molecule is made up of two or more atoms of any kind.  All things are made up of atoms which can't be broken down by normal processes; indeed, the word is derived from the Greek for 'indivisible'.

  • made up of a nucleus (composed of protons and neutrons) and electrons.
  • by definition, protons and neutrons weigh 1 (atomic mass unit or dalton); electrons weigh about 1/2000 AMU.
  • the difference between elements is the number of protons, also known was the atomic number, and designated by a subscript before the symbol:  e.g., 1=H, 2=He, 92=U
The alchemists thought, h-m-m-m . . . . 82Pb    minus 3 protons    = 79Au
  • atoms are electrically neutral, i.e., the number of protons = number of electrons; however, the number of neutrons can vary;
  • the atomic mass is the 'weight' of all the parts which have mass, (protons = 1, neutrons = 1, electrons = negligible); and is designated by a superscript before the symbol; different atoms of the same element by differing in # of neutrons, are isotopes. Many are stable, but some are unstable, or radioactive. These decay, or decompose, into more stable forms by giving off either particulate or energy. Isotopes generally act the same in chemical reactions, including in our bodies.

  • 1 2 3
e.g., 1H, 2H, 3H        or            232U, 235U, 238U

ATOMIC STRUCTURE

Electrons spin around the nucleus in orbitals, which make up electron shells. It is convenient, although not accurate to use the planetary model. Different shells can hold different numbers of electrons:
1st  can hold  2 electrons
2nd = 8 electrons
3rd = 8/18 electrons

Minerals
(Required minerals form KC's drawers: 1-12, 14-15, 17-18, 21-23, 25, 27-31, 35, 37, 41-43, 45-47, 49-50) 

The study of minerals is mineralogy.  A mineral is defined as a

(1) naturally occurring
(2) inorganic
(3) solid with an
(4) ordered atomic arrangement and a
(5) chemical composition which is fixed or varies only within well-defined limits.
About 3,000 or so are known with ~50 new ones discovered per year; many of these are rare, some known from only one site on earth. Far fewer of these are typically found in rocks; these are known as the rock-forming minerals.
.
More detail on the definition:
    1. naturally occurring : not synthetic; commercial diamonds are excluded.
    2. inorganic-from non-living processes: pearls and coal are excluded.
    3. solid-so, ice is a mineral, but water isn't, nor is petroleum. A mineral may be composed of a single element (e.g., graphite) but most are compounds.
    4. orderly atomic arrangement -atoms are arranged in some geometric pattern (its crystal structure), which is a function of the size of the atoms, bond types, and ionic charges.
  1. it is the most stable arrangement of the atoms in a solid form.
  2. Steno (1600's) found that the same material always showed the same characteristic form (same angle of faces, e.g.); e.g. halite, always 90 .
  3. but, how do you explain polymorphs like graphite and diamond? These have different crystal structures for the same chemical composition.

  4. -diamond is bonded in 3-D with strong covalent bonds (has density of 3.3 g/cm3; graphite, for the most part is bonded in 2-D (has a density of 2.1 g/cm3). These differences reflect the geological conditions present at the time of formation.
      • Examples of other polymorphs:
        1. sillimanite-andalusite-kyanite.
        2. calcite-aragonite
        3. silicon dioxide -quartz (Qz) has at least 6 polymorphs:
        chalcedony, flint, chert, agate, and onyx are microcrystalline quartz.
5) chemical composition which is fixed or varies only within well-defined limits: may have a somewhat variable composition due to solid solution (some elements/ion of similar size and charge substitute for each other rather freely).
e.g., Mg2+/Fe2+; if one is present, often the other is, too, to some extent.
e.g., Ca2+/Na+1 if charges can be balanced elsewhere.
Also, impurities, or trace elements (<0.1%) may be present.
Some things aren't minerals, but are mineraloids: opal and coal, e.g., lack mineral structure; also, amber and volcanic glass. 

Methods of identification:
1) X-ray diffraction:  passing an X-ray source through the substance and exposing a photographic film produces (not a pattern of individual atoms) but indicates the lattice structure of the mineral.
2) electron probe-gives off X-rays of a characteristic wavelength; X-ray intensity is proportional to concentration of an element.
3) optical determination-uses polarizing microscope to reveal distinct optical properties of a mineral; e.g., calcite shows double refraction.
4) physical properties:
a) crystal habit-shape that a crystal will assume if allowed to grow unobstructed; this is not diagnostic, but a clue; e.g., galena, fluorite, and halite are all cubic; garnet is 12 or 24 sided; quartz is hexagonal.  In rock, minerals are often obstructed and can't assume their diagnostic shape.
b) cleavage -tendency of a mineral to break along planar surfaces corresponding to zones of weakness; this reflects its atomic structure and bonding.  Examples:

    • mica forms sheet, giving a platy cleavage (in one plane)
    • halite forms cubes, giving a cubic cleavage (in three planes)
    • calcite forms rhombs, giving rhombohedral cleavage (in three planes, but not at right angles; give a 'slanted' look.
    • diamond-shows many planes; knowledge of these is exploited by gem-cutters to produce many facets for the glittering effect.
    • cleavage may be described in five classes, including poor, fair, good, perfect/eminent. Fluorite, diamond and mica have perfect cleavage; their shining faces are the result of smooth cleavage surfaces.
    • does not necessarily reflect the crystal habit, but both reflect the atomic structure; therefore, the cleavage plane doesn't necessarily equal the crystal face.
    • fracture - is the lack of cleavage; is described by terms such as: fibrous (looks like split wood), uneven, splintery, hackly, and conchoidal (Qz and obsidian)
c) hardness - ability to resist scratching
-reflects the relative strength of bonding
-scratch a fresh surface: weathered surfaces are weaker [what's harder, iron (Fe) or rust (FeO2)?]).
-uses Moh's Hardness Scale, derived by Friederich Mohs in 1822:
Hardness Index Mineral Common items
10 diamond (hardest known)
9 corundum (emery; used for bearings)
8 topaz
7 quartz 7, streak plate
6 orthoclase (feldspar) 5.5, plate glass
5, steel nail
5 apatite
4 fluorite
3 calcite 3.1, copper penny
2 gypsum 2.2, fingernail
1 talc

Here's a mnemonic to help remember the minerals Moh's Hardness Scale (from low to high):

T he G irls C an F lirt A nd O ther Q ueer T hings C an D o)
This scale is not linear. Diamond is much harder than corundum.

d) specific gravity (S.G. or G.) compares the mass of a given volume of mineral to the mass of an equal volume of water.  It is a function of the weight of atoms involved and the density of their packing. So, S.G. of diamond is 3.5, and graphite is 2.2.  Usually we just 'heft' a couple of things of the same size and call them 'high' or 'low'-examples: 
 

Water at 4oC 1.0000 . . .
NaCl 2.16
Cu 8.9
Pyrite/Fool's Gold 5.0
Gold 19.3

e) color - may be unreliable, but can be used.  For example, malachite is always green, but others may be variable, especially due to impurities.  Quartz comes in many varieties:
 

Colored Varieties of Quartz
Mineral Contaminant
Color, Name
nothing clear, rock quartz
Ti pink, rose quartz
fluid water white, milky quartz
radiation black, smoky quartz
heat treated amethyst? yellow, citrine
green, aventurine
purple, amethyst
included rutile rods blue, opalescent
     
    Colored Varieties of Corundum
    Mineral Contaminant
    Color, Name
    nothing grey, corundum
    probably Fe or Ti blue, sapphire
    chromium red. ruby
f) Streak is the color of powder left when streaked on unglazed plate.  It is not necessarily same as external color of mineral.  For example, pyrite - streak color is the same as mineral color, but cinnabar - gives quite different red streak.  There are limitations:  streak color isn't unique and minerals harder than streak plate (7) can't be done. 

g) Luster is the quality and intensity of the reflection of light from a mineral's surface.  It is described by such terms as metallic, pearly, vitreous (glassy), silky, earthy, greasy (coated with an oily substance), admantine (like a diamond).  Be sure you can distinguish metallic from non-metallic

h) other phenomena which are interesting, but of limited use for easy identification.

    1. thermoluminescence/pyroelectricity - some minerals emit visible light when heated; and increase in temperature increases the light (can even be used as a crude thermometer).
    2. triboluminescence - emit visible light when crushed/scratched; e.g., wintergreen lifesavers.
    3. piezoelectricity - electrical charge produced by applied pressure; e.g., quartz. Under pressure, electrons flow to one end of the crystal. It works in reverse, too! If electrical current is applied to a piezoelectric crystal, it bends! With alternating current a vibration/oscillation is caused.  A digital quartz watch-a quartz crystal cut to a thickness so it oscillates at 100,000/sec (accurate to one in 10 billion); this generates electrical impulses which control the watch; the whole thing is run by a 1.5 V battery.  Sulfur and topaz develop a charge when rubbed; tourmaline develops a charge when heated.
    4. fluorescence in ultraviolet light: absorbs UV --> reflects longer wavelength light; usually the result of impurities, so not all samples will fluoresce (or phosphoresce); e.g., calcite, fluorite.
    5. phosphorescence - light emission continues after external stimulus is removed.
    6. magnetism - e.g., magnetite; one type, lodestone, used in early compasses; another example is pyrrhotite.
    7. taste - some are poisonous! Examples of diagnostic taste: halite and KCl.
    8. effervescence with dilute HCl; e.g., CaCO3 (calcite), dolomite (weakly).
    9. radioactivity - detect w/Geiger counter.
    10. flexibility - e.g., mica bends, springs back; chlorite bends, stays bent.
    11. double refraction - e.g., calcite (only?)
    12. feel - talc and graphite feel greasy or slippery; finger pressure is enough to break bonds and make atoms or layers slip.
    13. odor - e.g., sulfides smell like rotten eggs; arsenic minerals smell like garlic.
Use a combination of these. For example, there are many green minerals, but how many with the combination of green, conchoidal, glassy, hardness of 6.5-7.5, . . . ?

Common Minerals

The most common minerals in the crust are
Oxygen ~47%
Silicon ~28
Aluminum ~8
Iron ~5
Calcium ~4
Sodium ~3
Potassium ~3
Magnesium ~2
Others ~1.5

Mineral groups:
1) native elements- occur in single elements; e.g., Au, Ag, S, Cu, C (diamond and graphite).

2) halides- compounds of cations (Na+, K+) and halogen anions (Cl-, I-, Br-, F-); 

e.g., NaCl, CaF2 (fluorite).

3) oxides- compounds of cations (Al+3, Fe+2, Fe+3, Sn+2) and O-2.

H2O ice, Fe2O3 hematite, Fe3O4 magnetite, Al2O3 corundum, SnO2 cassiterite

4) sulfides- compounds of cations (Cu+, Pb2+, Zn2+) and S2-.

PbS galena; FeS2 pyrite; ZnS spalerite; CuFeS2 chalcopyrite; HgS cinnabar
5) carbonates- second most common group; compounds of cations (Ca2+, Mg2+) and complex ions (CO32-, SO42-); effervesce in HCl. azurite, malachite, CaCO3, calcite CaMg(CO3)2 dolomite

6) sulfates