Chapter 5. Minerals
So . . .
Everything is made of matter, anything which occupies space
and has mass; it occurs in several phases, solid,
and plasma. all matter is made of substances called
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'.
The alchemists thought, h-m-m-m . . . . 82Pb
minus 3 protons = 79Au
made up of a nucleus (composed of protons and neutrons)
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
e.g., 1H, 2H, 3H
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
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
(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
(1) naturally occurring
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
(3) solid with an
(4) ordered atomic arrangement and a
(5) chemical composition which is fixed or varies only within well-defined
More detail on the definition:
naturally occurring : not synthetic; commercial diamonds are excluded.
inorganic-from non-living processes: pearls and coal are excluded.
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
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.
it is the most stable arrangement of the atoms in a solid form.
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 .
but, how do you explain polymorphs like graphite and diamond? These
have different crystal structures for the same chemical composition.
-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:
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
silicon dioxide -quartz (Qz) has at least 6 polymorphs:
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:
c) hardness - ability to resist scratching
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,
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
fracture - is the lack of cleavage; is described by terms such as:
(looks like split wood), uneven, splintery,
and conchoidal (Qz and obsidian)
-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:
||diamond (hardest known)
||corundum (emery; used for bearings)
||7, streak plate
||5.5, plate glass
5, steel nail
||3.1, copper penny
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
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'
|Water at 4oC
||1.0000 . . .
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
||clear, rock quartz
||pink, rose quartz
||white, milky quartz
||black, smoky quartz
|heat treated amethyst?
|included rutile rods
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
Colored Varieties of Corundum
|probably Fe or Ti
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.
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, . . . ?
thermoluminescence/pyroelectricity - some minerals emit visible
light when heated; and increase in temperature increases the light (can
even be used as a crude thermometer).
triboluminescence - emit visible light when crushed/scratched; e.g.,
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.
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.
phosphorescence - light emission continues after external stimulus
magnetism - e.g., magnetite; one type, lodestone, used in early
compasses; another example is pyrrhotite.
taste - some are poisonous! Examples of diagnostic taste: halite
effervescence with dilute HCl; e.g., CaCO3 (calcite), dolomite (weakly).
radioactivity - detect w/Geiger counter.
flexibility - e.g., mica bends, springs back; chlorite bends, stays
double refraction - e.g., calcite (only?)
feel - talc and graphite feel greasy or slippery; finger pressure
is enough to break bonds and make atoms or layers slip.
odor - e.g., sulfides smell like rotten eggs; arsenic minerals smell
The most common minerals in the crust are
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