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Just What Is a
Mineral?
A mineral is a
naturally occurring,
usually inorganic,
homogenous solid with a
definite chemical
composition that is variable within fixed limits and has a
highly ordered atomic
arrangement.
 :
Naturally
Occurring: These words distinguish
between materials formed by natural processes and those artificially created
by man. A man-made substance is not a mineral, even if it has the same
chemical composition and structure as a non-man-made substance that is
considered to be a mineral. For example, a crystal of beryl chiseled
from the rock of a Brazilian or Columbian emerald mine is a mineral, but a
crystal of beryl synthesized in a laboratory is not.
Inorganic: Almost all minerals are
formed through inorganic processes - that is, not through the activities of
living organisms. Several materials that are formed organically have
been recognized as minerals; these biogenic minerals
include aragonite, the orthorhombic calcium carbonate that composes
pearls and the shells of
molluscs,
brachiopods, and
other invertebrate organisms, the
opal in
diatom shells, and
sulfur and
sulfide minerals
 precipitated by
bacteria.
Petroleum, coal,
amber, and
jet are not minerals -
nor are your teeth (although they are composed of material very similar to
the mineral apatite). However, if you were to be subducted under
the South American continent, and the apatite in your teeth recrystallized
as temperatures and pressures increased, your teeth could then be classified
as minerals and would be eligible for display on these pages.
Needless, to say, the organic material in the rest of your body would go
through some major changes, but ultimately your body could be reduced to
carbon in the form of the mineral graphite, the low temperature and pressure polymorph of the
mineral diamond.
Homogenous Solids: Homogeneous materials
cannot be physically subdivided into simpler chemical compounds. The
requirement that a mineral be a solid eliminates all gases and liquids.
A crystal of glacial ice (a solid) is therefore a mineral; a drop of water
or the mercury that was once mined in Terlingua, Texas (which is a liquid),
are not.
Definite Chemical Composition:
The chemical composition of a mineral can vary, but
only within fixed limits. This means that a mineral always has the
same basic "recipe", at least as far as major components or, in baking
terminology, ingredients, are concerned. The principle is much the
same as making a cake: The proportions of basic ingredients (such as
flour, sugar, butter, and baking powder) can only be varied slightly without
making it inedible, while changing the special ingredients that are typical
of one type of cake will turn it into a different type.
The
structure of a mineral determines how much variation is possible. For
instance, the mineral quartz (SiO2) has very little compositional
variation; it is essentially composed of one atom of silicon (Si) for every
two oxygen (O) atoms, and its structure makes it difficult for other sorts
of atoms to become included. Other minerals are much more variable,
and some materials, such as limonite, are so extremely variable that they
are considered to be "mineraloids",
rather than true minerals.
The chemical
formulae of minerals with variable composition tends to be very complex.
One example is tourmaline, whose formula is: (Na, Ca)(Li, Mg, Al)(Al, Fe3+,
Mn3+)6(BO3)3 (Si6O18)(OH)4.
A
detailed explanation of the the meaning of this formula and what it
tells about tourmaline's structure and composition can be found on a
separate page.
Variations in mineral
composition can produce differences in structure, and are reflected in
differences in the color, density, and other
physical and optical
properties. These differences are used to define different
"varieties" of a particular mineral. For example, red or pink
tourmaline is called rubellite, blue tourmaline is indicolite and iron-rich
and black tourmaline is schorl.
Ordered Atomic
Structure: This means that the
atoms (or ions)
in a mineral are arranged in a regular, repeated, three-dimensional array
(this is what defines a crystalline solid). Solids such as common opal
and chrysocolla that do not have an ordered atomic arrangement (referred to
as "amorphous") thus are not minerals. In between crystalline and
amorphous solids lie the "glasses" (including man-made glass and
naturally-occurring
obsidian). The atoms of these are partly ordered, usually
due to very rapid cooling from a molten state, creating what are called
"supercooled liquids". Supercooled liquids tend to flow under the
effects of gravity, but far too slowly to be visible to the human eye.
For example, each pane of the multi-storied windows in the United Nations
building in New York City (installed in the 1960s) is now much thicker at
its base than at its top.
Experts
currently recognize approximately 3,000 distinct minerals, and a few new
ones are discovered each year. These minerals are classified into
chemical groups. Minerals may also be classified by their
affiliation with certain chemical elements or compounds.
 Mineraloids
meet most, but not all, of the criteria required to be
classified as minerals. For example, opals (SiO2 .
nH2O) are composed of layers of closely packed silica
spheres, with H2O molecules and additional silica filling the
void spaces between the silica spheres. Common, or amorphous, opal is
a mineraloid because the silica spheres have highly variable sizes and thus
lack a predictable three-dimensional structure. Precious opal,
however, is composed of silica spheres of approximately the same size,
producing a predictable three-dimensional array (as shown in this
SEM image made at the Caltech mineral spectroscopy lab). Precious
opals formed through the low temperature inorganic precipitation of
colloidal silica from silica-rich waters are classed as minerals; precious
opals formed through the accumulation of siliceous tests of silica-secreting
organisms are not.
 Gemstones
are minerals, mineraloids, or non-minerals that can be
faceted, polished, or otherwise modified in shape to enhance their
appearance. In addition to being attractive due to its color, or the
ability to reflect or transmit light, a gemstone typically is fairly hard
and durable. The most valuable gemstones, referred to as "precious"
gemstones, are extremely hard (and extremely rare!). Softer, less
durable minerals usually are less valuable and are classified as
"semi-precious".
Diamonds, emeralds (beryl),
rubies, and sapphires (red and all other colors of gem-quality corundum) are
referred to as "precious" minerals; the softer, less durable jade (nephrite
and jadeite), garnet, amethyst, citrine, tiger eye, rose quartz (quartz),
peridot (olivine), tourmaline, and turquoise are called "semi-precious".
Two exceptions to this rule are precious opals and pearls, which are classed
as precious, but are soft compared to other precious stones. |
