What are Minerals?
Solid Earth materials are made up of minerals and
locally, their corresponding liquids. A mineral can be defined as:
"a naturally occurring, inorganic, crystalline
solid, with a fixed range of chemical composition and physical properties."
Minerals are chemical compounds composed of anions (e.g.
oxygen - O-2) or anionic groups (e.g. carbonate - CO3-2)
bonded to positively charged cations in a regular geometric atomic structure
or lattice. An external expression of the atomic lattice of a mineral is
the development of crystal faces. Crystals of different mineral have characteristic
form or habit that reflects bond lengths, bond strengths, and inter-bond
angles within the atomic structure. As a result, angles between equivalent
faces of crystals for a given mineral have a constant value, as stated
by Steno’s Law. Because of this Constancy of Interfacial Angles,
crystals exhibit varied degrees of symmetry, a repetition of the
pattern of faces observed during manipulation of the crystal. Two general
types of relationships can exist within the pattern of faces:
mirror symmetry - repetition across a plane
rotational symmetry - repetition about an
axis
Minerals that have the same composition but different
crystal structures are called polymorphs (from "poly" meaning many and
"morph" meaning form). The compound SiO2 crystallizes in one
of six different atomic structures depending on the conditions of formation
(P-T); the most common form of this compound is the mineral quartz. Graphite
and diamond are also polymorphs - in this case of the element carbon (C).
In contrast, some minerals have different compositions, but similar
atomic structures and hence, crystal habits. These minerals are called
isomorphs
(from "iso" meaning constant and "morph" meaning form)
Classification of Minerals
Approximately 5000 different minerals known to exist,
but there are only about 15 to 20 that are common rock-forming minerals.Minerals
can be subdivided into eight groups on the basis of the anion or anionic
complex involved in the mineral atomic structures. These groups are:
|
Mineral Group
|
Anion or Anionic Complex
|
Representative Minerals
|
| native elements: |
-
|
sulfur, gold, silver, copper,
diamond, graphite |
| sulfides: |
S-2
|
pyrite, galena, sphalerite,
chalcopyrite |
| oxides: |
O-2
|
hematite, magnetite, chromite |
| halides: |
Cl -1, F-1
|
halite, flourite |
| sulfates |
(SO4)-2
|
anhydrite, gypsum, barite |
| carbonates |
(CO3)-2
|
calcite, dolomite |
| phosphates |
(PO4)-3
|
apatite |
| silicates |
(SiO4)-2
|
quartz, feldspar |
Silicate Minerals
Silicate minerals are the most common rock-forming minerals.
Their atomic structure is based on silica tetrahedron (SiO4-2),
in which four oxygen atoms are bond to each silicon (Si) atom. The mineral
structures are built by polymerization or sharing of oxygen between
Si atoms producing linkages of tetrahedra. This sharing of oxygen between
Si tetrahedra produces chains and other 3D Si tetrahedra structures,
which are themselves linked together through bonds between O and other
atoms (e.g., Al, Mg and Fe). Chemically, silicate minerals can be separated
into two major types:
-
ferromagnesian (iron/magnesian) - olivine, pyroxenes, amphiboles,
biotite
-
felsic (silica/aluminum) - quartz, plagioclase, potassium feldspars
Silicate minerals, however, are typically classified on the
basis
of their silica tetrahedra polymerization. The simplest silicate
mineral structures have isolated Si tetrahedra linked together through
bonds between oxygen and cations other than silicon. More complicated structures
involve tetrahedra linked together to form rings (beryl), single chains
(pyroxenes), double chains (amphiboles), sheets (micas and clay minerals),
and 3D frameworks or networks (quartz and feldspars).
Physical Properties of Minerals
Various physical properties, in addition to crystal habit, can be used
to identify different minerals. The properties include:
-
Luster is the appearance of light reflected from fresh mineral surface,
and includes two major subdivisions:
metallic
non-metallic - vitreous (glassy), waxy, pearly, earthy, resinous,
etc.
-
Cleavage is the tendancy to break along the direction of weak atomic
bonds and reflects relative strength of bonds within a mineral’s atomic
structure. Some minerals have cleavage, whereas others do not. Micas provide
a good example of the tendancy to break along direction of weak atomic
bonds. Like all sheet silicates, a single cleavage is well-developed between
the sheets.Minerals that do not have cleavage (e.g., quartz) tend to fracture
or break irregularly. Conchoidal fracture is developed where a mineral
breaks with smooth curved surfaces (like broken glass). Cleavage
may be the best means of identifying between minerals that otherwise look
identical, e.g.,
pyroxene — 90°
amphibole — 120°
-
Hardness is the resistance to scratching, and reflects differences
in the relative strength of chemical bonds in different mineral structures.
Mineral hardness is described in terms of Mohs’ Hardness Scale (from
softest to hardest):
-
Talc
-
Gypsum
-
Calcite
-
Fluorite
-
Apatite
-
Orthoclase (K-feldspar)
-
Quartz
-
Topaz
-
Corundum
-
Diamond
-
Streak is the color of the powdered mineral.
-
Specific gravity or density is a measure of the weight and packing
of atoms involved in the mineral’s atomic structure. It equals the weight
of the mineral divided by the weight of an equal volume of water.
-
Color is the least reliable property.
-
Double refraction involves generation of two images of single object
when viewed through a mineral. The two images are caused by non-uniform
velocities of light transmission in different directions (e.g., calcite).
-
Striations are parallel lines or bands across the surface of a mineral’s
crystal or cleavage face. They are caused by non-uniform growth rates or
multiple individuals intergrown in the crystal (e.g., feldspar).
-
Magnetism is a characteristic feature of magnetite.
-
Taste is a characteristic feature of rock salt.
Many mineral properties (e.g., cleavage) may be seen best when a wafer-thin
slice of the mineral (or rock) mounted on a glass slide (a thin section)
is viewed under a microscope.
Mineral Resources & Reserves
Minerals represent the non-renewable source of many commodities utilized
in our everyday lives. A resource is a naturally-occurring solid
liquid or gas that is or potentially may be extracted economically. The
term "mineral resource" describes the total amount of metal,
non-metal (industrial minerals) or fossil fuel present, whereas reserve
refers to only that which is economically extractable.
Geologists are at the forefront of resource exploration and exploitation.
U.S. per capita use of commodities is the highest in the world. Some typical
quantities are as follows:
|
Commodity
|
per capita use
|
| sand and gravel |
8,600 lb.
|
| iron and steel |
1,140 lb.
|
| crushed rock |
8,800 lb.
|
| aluminum |
56 lb.
|
| copper |
21 lb.
|
| lead |
14 lb.
|
| zinc |
12 lb.
|
| manganese |
12 lb.
|
| salt |
430 lb.
|
| clays |
500 lb.
|
| phosphate rocks |
380 lb.
|
