The branch of geology that deals with the
classification and properties of minerals. It is closely related
to petrology, the branch of geology that deals with the occurrence,
origin, and history of rocks. Minerals are the basic
building blocks of rocks, soil, and sand. Most beaches are
made of the mineral quartz, which is very resistant to weathering
and erosion by the waves. Most minerals, like quartz or
mica, are abundant and common, although some minerals
like diamonds, rubies, sapphires, gold, and silver are rare and
very valuable. Minerals contain information about the chemical
and physical conditions in the regions of the Earth that
they formed in. They can often help discriminate which tectonic
environment a given rock formed in, and they can tell
us information about the inaccessible portions of Earth. For
example, mineral equilibrium studies on small inclusions in
diamonds show that they must form below a depth of 90
miles (145 km). Economies of whole nations are based on
exploitation of mineral wealth; for instance, South Africa is
such a rich nation because of its abundant gold and diamond
mineral resources.
The two most important characteristics of minerals are
their composition and structure. The composition of minerals
describes the kinds of chemical elements present and
their proportions, whereas the structure of minerals
describes the way in which the atoms of the chemical elements
are packed together.
We know of 3,000 minerals, most made out of the eight
most common mineral-forming elements. These eight elements
make up greater than 98 percent of the mass of the continental
crust. Most of the other 133 scarce elements do not occur
by themselves, but occur with other elements in compounds
by ionic substitution. For example, olivine may contain trace
amounts of Cu, Ni, Co, Mn, and other elements.
The two elements oxygen and silicon make up more than
75 percent of the crust, with oxygen alone forming nearly half
of the mass of the continental crust. Oxygen forms a simple
anion (O–2), and silicon forms a simple cation (Si+4). Silicon
and oxygen combine together to form a very stable complex
anion that is the most important building block for minerals—
the silicate anion (SiO4)–4. Minerals that contain this anion are
known as the silicate minerals, and they are the most common
naturally occurring inorganic compounds in the solar system.
The other, less common, building blocks of minerals (anions)
are oxides (O–2), sulfides (S–2), chlorides (Cl–1), carbonates
(CO3)–2, sulfates (SO4)–2, and phosphates (PO4)–3.
The Rock-Forming Minerals
Approximately 20 minerals are so common that they account
for greater than 95 percent of all the minerals in the continental
and oceanic crust; these are called the rock-forming minerals.
Most rock-forming minerals are silicates, and they have
some common features in the way their atoms are arranged.
The Silicate Tetrahedron
The silicate anion is made of four large oxygen atoms and
one small silicon atom that pack themselves together to occupy
the smallest possible space. This shape, with big oxygen
atoms at four corners of the structure, and the silicon atom at
the center, is known as the silicate tetrahedron. Each silicate
tetrahedron has four unsatisfied negative charges (Si has a
charge of +4, whereas each oxygen has a charge of –2). To
make a stable compound the silicate tetrahedron must therefore
combine to neutralize this extra charge, which can happen
in one of two ways:
1. Oxygen can form bonds with cations (positively charged
ions). For instance, Mg+2 has a charge of +2, and by combining
with Mg+2, the silicate tetrahedron makes a mineral
called olivine (Mg2)SiO4.
2. Two adjacent tetrahedra can share an oxygen atom,
making a complex anion with the formula (Si2O7)–6. This
process commonly forms long chains, so that the charge
is balanced except at the ends of the structure. This process
of linking silicate tetrahedra into large anion groups
is called polymerization. It is the most common way to
build minerals, but in making the various possible combinations
of tetrahedra, one rule must be followed, that
is, tetrahedra can only be linked at their apices.
Olivine is one of the most important minerals on Earth,
forming much of the oceanic crust and upper mantle. It has
the formula (Mg,Fe)2SiO4 and forms the gem peridot.
Garnet is made of isolated silicate tetrahedra packed
together without polymerizing with other tetrahedra. There
are many different kinds of garnets, with almandine being
one of the more common, deep red varieties that forms a
common gemstone. Ionic substitution is common, with garnet
having the chemical formula A3B2(SiO4)3, where:
A = Mg+2
Fe+2
Ca+2
Mn+2
B = Al+3
Fe+3
Pyroxene and amphibole both contain continuous
chains of silicate tetrahedra. Pyroxenes are built from a
polymerized chain of single tetrahedra, whereas amphiboles
are built in double chains or linked rings. In both of these
structures, the chains are bound together by cations such as
Ca, Mg, and Fe, which satisfy the negative charges of the
polymerized tetrahedra. Pyroxenes are common minerals in
the oceanic crust and mantle, and they also occur in the
continental crust. Amphiboles are common in metamorphic
rocks. They have a complicated chemical formula and can
hold large varieties of cations in their crystal structure.
Clays, micas, and chlorites are all closely related to sheet
silicates, made of polymerized sheets of tetrahedra. By sharing
three oxygens with adjacent tetrahedra, there is only a
single unbalanced oxygen in each tetrahedra on which is typically
balanced by Al+3 cations, which occupy spaces between
the sheets. The sheet structure is why micas are easy to peel
apart on cellophane-like surfaces.
Quartz, one of the most common minerals, also has one
of the most common polymerizations. It has all of its charges
satisfied by sharing all of its oxygen in a three-dimensional
network. Quartz typically has six-sided crystals and has
many other different forms and colors.
Feldspars are the most common minerals in the Earth’s
crust. They account for 60 percent of all minerals in the continental
crust, and 75 percent of the volume. Feldspars are also
common in the oceanic crust. Like quartz, feldspars also have
a structure formed by polymerization of all the oxygen atoms,
and some of the silicon atoms are replaced by Al+3. There are
many different kinds of feldspar minerals, formed by different
cations added to the structure. For instance, potassium
feldspar has the formula K(Si3Al)O8, albite has the formula
Na(Si3Al)O8, and anorthite has the formula Ca(Si2Al2)O8.
There is a complete range of chemical compositions of
feldspars possible between the albite and anorthite varieties.
These feldspar minerals are known as the plagioclase feldspars.
Silicates are the most abundant rock-forming minerals,
but other types do occur in sufficient quantities to call them
rock-forming minerals. Oxides use the oxygen anion and
include ore minerals such as chromium, uranium, tin, and
magnetite (FeO4). Sulfides are minerals such as copper, lead,
zinc, cobalt, mercury, and silver that use the sulfur anion. For
instance, FeS2 is the formula for pyrite, commonly known as
fool’s gold. The carbonates calcite, aragonite, dolomite are
formed with the complex carbonate anion (CO3)–2. Phosphates
are formed using the complex anion (PO4)–3. An
example is the mineral apatite, used for fertilizers, and the
same substance as our teeth and bones are made from. Sulfate
minerals are formed using the complex sulfate ion
(SO4)–2. Gypsum and anhydrite are sulfate minerals formed
by evaporation of saltwater, commonly used to make plaster.
The Properties of Minerals
Minerals have specific properties determined by their chemistry
and crystal structure. Certain properties are characteristic
of certain minerals, and we can identify minerals by
learning these properties. The most common properties are
crystal form, color, hardness, luster, cleavage, specific gravity,
and taste.
When a mineral grows freely, it forms a characteristic
geometric solid bounded by geometrically arranged plane surfaces
(this is the crystal form). This symmetry is an external
expression of the symmetric internal arrangement of atoms,
such as in repeating tetrahedron arrays. Individual crystals of
the same mineral may look somewhat different because the
relative sizes of individual faces may vary, but the angle
between faces is constant and diagnostic for each mineral.
Every mineral has a characteristic crystal form. Some
minerals have such distinctive forms that they can be readily
identified without measuring angles between crystal faces.
For instance, pyrite is recognized as interlocking growth of
cubes, whereas asbestos forms long silky fibers. These distinctive
characteristics are known as growth habit.
Cleavage is the tendency of a mineral to break in preferred
directions along bright reflective planar surfaces. The
planar surface along which cleavage occurs is deterred by
external structure; cleavage occurs along planes where the
bands between the atoms are relatively weak.
Luster is the quality and intensity of light reflected from
a mineral. Typical lusters include metallic (like a polished
metal), vitreous (like a polished glass), resinous (like resin),
pearly (like a pearl), and greasy (oily).
Color is not reliable for identification of minerals, since
it is typically determined by ionic substitution. For instance,
sapphires and rubies are both varieties of the mineral corundum,
with different types of ionic substitution. However, the
color of the streak a mineral leaves on a porcelain plate is
often diagnostic for opaque minerals with metallic lusters.
The density of a mineral is a measure of mass per unit
volume (g/cm3). Density describes “how heavy the mineral
feels.” Specific gravity is an indirect measure of density; it is
the ratio of the weight of a substance to the weight of an
equal volume of water (specific gravity has no units because
it is a ratio).
Hardness is a measure of the mineral’s relative resistance
to scratching. Hardness is governed by the strength of bonds
between atoms and is very distinctive and useful for mineral
identification. A mineral’s hardness can be determined by the
ease with which one mineral can scratch another. For
instance, talc (used for talcum powder) is the softest mineral,
whereas diamond is the hardest mineral. Hardness is commonly
measured using Moh’s Hardness Scale.
See also PETROLOGY.
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