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Mineral
Minerals are natural compounds formed through geological processes. The term
"mineral" encompasses not only the material's chemical composition, but also the
mineral's structure. Minerals range in composition from pure elements and simple
salts to very complex silicates with thousands of known forms. The study of
minerals is called mineralogy.
Mineral definition and classification
To be classified as a "true" mineral, a substance must be a solid and have a
crystalline structure. It must also be a naturally occurring, homogeneous
substance with a defined chemical composition. Traditional definitions excluded
organically derived material. However, the International Mineralogical
Association in 1995 adopted a new definition:
a mineral is an element or chemical compound that is normally crystalline and
that has been formed as a result of geological processes.[1]
The modern classifications include an organic class - in both the new Dana and
the Strunz classification schemes.[2][3]
The chemical composition may vary between end members of a mineral system. For
example the plagioclase feldspars comprise a continuous series from sodium-rich
albite (NaAlSi3O8) to calcium-rich anorthite (CaAl2Si2O8) with four recognized
intermediate compositions between. Mineral-like substances that don't strictly
meet the definition are sometimes classified as mineraloids. Other
natural-occurring substances are nonminerals. Industrial minerals is a market
term and refers to commercially valuable mined materials (see also Minerals and
Rocks section below).
A crystal structure is the orderly geometric spatial arrangement of atoms in the
internal structure of a mineral. There are 14 basic crystal lattice arrangements
of atoms in three dimensions, and these are referred to as the 14 "Bravais
lattices". Each of these lattices can be classified into one of the six crystal
systems, and all crystal structures currently recognized fit in one Bravais
lattice and one crystal system. This crystal structure is based on regular
internal atomic or ionic arrangement that is often expressed in the geometric
form that the crystal takes. Even when the mineral grains are too small to see
or are irregularly shaped, the underlying crystal structure is always periodic,
and can be determined by X-ray diffraction.
Chemistry and crystal structure together define a mineral. In fact, two or more
minerals may have the same chemical composition, but differ in crystal structure
(these are known as polymorphs). For example, pyrite and marcasite are both iron
sulfide, but their arrangement of atoms differs. Similarly, some minerals have
different chemical compositions, but the same crystal structure: for example,
halite (made from sodium and chlorine), galena (made from lead and sulfur) and
periclase (made from magnesium and oxygen) all share the same cubic crystal
structure.
Crystal structure greatly influences a mineral's physical properties. For
example, though diamond and graphite have the same composition (both are pure
carbon), graphite is very soft, while diamond is the hardest of all known
minerals. This happens because the carbon atoms in graphite are arranged into
sheets which can slide easily past each other, while the carbon atoms in diamond
form a strong, interlocking three-dimensional network.
There are currently more than 4,000 known minerals, according to the
International Mineralogical Association, which is responsible for the approval
of and naming of new mineral species found in nature. Of these, perhaps 150 can
be called "common," 50 are "occasional," and the rest are "rare" to "extremely
rare."
Differences between minerals and rocks
A mineral is a naturally occurring, inorganic solid with a definite chemical
composition and a crystalline structure. A rock is an aggregate of one or more
minerals. (A rock may also include organic remains and mineraloids.) Some rocks
are predominantly composed of just one mineral. For example, limestone is a
sedimentary rock composed almost entirely of the mineral calcite. Other rocks
contain many minerals, and the specific minerals in a rock can vary widely. Some
minerals, like quartz, mica or feldspar are common, while others have been found
in only one or two locations worldwide. The vast majority of the rocks of the
Earth's crust consist of quartz, feldspar, mica, chlorite, kaolin, calcite,
epidote, olivine, augite, hornblende, magnetite, hematite, limonite and a few
other minerals.[4] Over half of the mineral species known are so rare that they
have only been found in a handful of samples, and many are known from only one
or two small grains.
Commercially valuable minerals and rocks are referred to as industrial minerals.
Rocks from which minerals are mined for economic purposes are referred to as
ores (the rocks and minerals that remain, after the desired mineral has been
separated from the ore, are referred to as tailings).
Mineral composition of rocks
A main determining factor in the formation of minerals in a rock mass is the
chemical composition of the mass, for a certain mineral can be formed only when
the necessary elements are present in the rock. Calcite is most common in
limestones, as these consist essentially of carbonate of lime; quartz in
sandstones and in certain igneous rocks which contain a high percentage of
silica.
Other factors are of equal importance in determining the natural association or
paragenesis of rock-making minerals, principally the mode of origin of the rock
and the stages through which it has passed in attaining its present condition.
Two rock masses may have very much the same bulk composition and yet consist of
entirely different assemblages of minerals. The tendency is always for those
compounds to be formed which are stable under the conditions under which the
rock mass originated. A granite arises by the consolidation of a molten magma (a
fused rock mass) at high temperatures and great pressures and its component
minerals are such as are formed in such circumstances. Exposed to moisture,
carbonic acid and other subaerial agents at the ordinary temperatures of the
earth's surface, some of these original minerals, such as quartz and white mica
are permanent and remain unaffected; others "weather" or decay and are replaced
by new combinations. The felspar passes into kaolin, muscovite and quartz, and
if any black mica (biotite) has been present it yields chlorite, apidote, rutile
and other substances. These changes are accompanied by disintegration, and the
rock falls into a loose, incoherent, earthy mass which may be regarded as a sand
or soil. The materials thus formed may be washed away and deposited as a
sandstone or grit. The structure of the original rock is now replaced by a new
one; the mineralogical constitution is profoundly altered; but the bulk chemical
composition may not be very different. The sedimentary rock may again undergo a
metamorphosis. If penetrated by igneous rocks it may be recrystallized or, if
subjected to enormous pressures with heat and movement, such as attend the
building of folded mountain chains, it may be converted into a gneiss not very
different in mineralogical composition though radically different in structure
to the granite which was its original state.[4]
Physical properties of minerals
Classifying minerals can range from simple to very difficult. A mineral can be
identified by several physical properties, some of them being sufficient for
full identification without equivocation. In other cases, minerals can only be
classified by more complex chemical or X-ray diffraction analysis; these
methods, however, can be costly and time-consuming.
Physical properties commonly used are:[5]
Crystal structure and habit: See the above discussion of crystal structure. A
mineral may show good crystal habit or form, or it may be massive, granular or
compact with only microscopically visible crystals.
Talc
Rough diamond.Hardness: the physical hardness of a mineral is usually measured
according to the Mohs scale. This scale is relative and goes from 1 to 10.
Minerals with a given Mohs hardness can scratch the surface of any mineral that
has a lower hardness than itself.
Moh's Hardness scale:[6]
Talc Mg3Si4O10(OH)2
Gypsum CaSO4•2H2O
Calcite CaCO3
Fluorite CaF2
Apatite Ca5(PO4)3(OH,Cl,F)
Orthoclase KAlSi3O8
Quartz SiO2
Topaz Al2SiO4(OH,F)2
Corundum Al2O3
Diamond C (pure carbon)
Luster indicates the way a mineral's surface interacts with light and can range
from dull to glassy (vitreous).
Metallic -high reflectivity like metal: galena and pyrite
Sub-metallic -slightly less than metallic reflectivity: magnetite
Non-metallic lusters:
Adamantine - brilliant, the luster of diamond also cerussite and anglesite
Vitreous -the luster of a broken glass: quartz
Pearly - iridescent and pearl-like: talc and apophyllite
Resinous - the luster of resin: sphalerite and sulfur
Silky - a soft light shown by fibrous materials: gypsum and chrysotile
Dull/earthy -shown by finely crystallized minerals: the kidney ore variety of
hematite
Color indicates the appearance of the mineral in reflected light or transmitted
light for translucent minerals (i.e. what it looks like to the naked eye).
Irridescence - the play of colors due to surface or internal interference.
Labradorite exhibits internal irridescence whereas hematite and sphalerite often
show the surface effect.
Streak refers to the color of the powder a mineral leaves after rubbing it on an
unglazed porcelain streak plate. Note that this is not always the same color as
the original mineral.
Cleavage describes the way a mineral may split apart along various planes. In
thin section, cleavage is visible as thin parallel lines across a mineral.
Fracture describes how a mineral breaks when broken contrary to its natural
cleavage planes.
Chonchoidal fracture is a smooth curved fracture with concentric ridges of the
type shown by glass.
Hackley is jagged fracture with sharp edges.
Fibrous
Irregular
Specific gravity relates the mineral mass to the mass of an equal volume of
water, namely the density of the material. While most minerals, including all
the common rock-forming minerals, have a specific gravity of 2.5 - 3.5, a few
are noticeably more or less dense, e.g. several sulfide minerals have high
specific gravity compared to the common rock-forming minerals.
Other properties: fluorescence (response to ultraviolet light), magnetism,
radioactivity, tenacity (response to mechanical induced changes of shape or
form), piezoelectricity and reactivity to dilute acids.
Chemical properties of minerals
Minerals may be classified according to chemical composition. They are here
categorized by anion group. The list below is in approximate order of their
abundance in the Earth's crust. The list follows the Dana classification
system.[5][7]
Silicate class
quartzThe largest group of minerals by far are the silicates (most rocks are
>95% silicates), which are composed largely of silicon and oxygen, with the
addition of ions such as aluminium, magnesium, iron, and calcium. Some important
rock-forming silicates include the feldspars, quartz, olivines, pyroxenes,
amphiboles, garnets, and micas.
Carbonate class
The carbonate minerals consist of those minerals containing the anion (CO3)2-
and include calcite and aragonite (both calcium carbonate), dolomite
(magnesium/calcium carbonate) and siderite (iron carbonate). Carbonates are
commonly deposited in marine settings when the shells of dead planktonic life
settle and accumulate on the sea floor. Carbonates are also found in evaporitic
settings (e.g. the Great Salt Lake, Utah) and also in karst regions, where the
dissolution and reprecipitation of carbonates leads to the formation of caves,
stalactites and stalagmites. The carbonate class also includes the nitrate and
borate minerals.
Sulfate class
Sulfates all contain the sulfate anion, SO42-. Sulfates commonly form in
evaporitic settings where highly saline waters slowly evaporate, allowing the
formation of both sulfates and halides at the water-sediment interface. Sulfates
also occur in hydrothermal vein systems as gangue minerals along with sulfide
ore minerals. Another occurrence is as secondary oxidation products of original
sulfide minerals. Common sulfates include anhydrite (calcium sulfate), celestine
(strontium sulfate), barite (barium sulfate), and gypsum (hydrated calcium
sulfate). The sulfate class also includes the chromate, molybdate, selenate,
sulfite, tellurate, and tungstate minerals.
Halide class
HaliteThe halides are the group of minerals forming the natural salts and
include fluorite (calcium fluoride), halite (sodium chloride), sylvite
(potassium chloride), and sal ammoniac (ammonium chloride). Halides, like
sulfates, are commonly found in evaporitic settings such as playa lakes and
landlocked seas such as the Dead Sea and Great Salt Lake. The halide class
includes the fluoride, chloride, and iodide minerals.
Oxide class
Oxides are extremely important in mining as they form many of the ores from
which valuable metals can be extracted. They also carry the best record of
changes in the Earth's magnetic field. They commonly occur as precipitates close
to the Earth's surface, oxidation products of other minerals in the near surface
weathering zone, and as accessory minerals in igneous rocks of the crust and
mantle. Common oxides include hematite (iron oxide), magnetite (iron oxide),
chromite (iron chromium oxide), spinel (magnesium aluminium oxide - a common
component of the mantle), ilmenite (iron titanium oxide), rutile (titanium
dioxide), and ice (hydrogen oxide). The oxide class includes the oxide and the
hydroxide minerals.
Sulfide class
Many sulfide minerals are economically important as metal ores. Common sulfides
include pyrite (iron sulfide - commonly known as fools' gold), chalcopyrite
(copper iron sulfide), pentlandite (nickel iron sulfide), and galena (lead
sulfide). The sulfide class also includes the selenides, the tellurides, the
arsenides, the antimonides, the bismuthinides, and the sulfosalts (sulfur and a
second anion such as arsenic).
Phosphate class
The phosphate mineral group actually includes any mineral with a tetrahedral
unit AO4 where A can be phosphorus, antimony, arsenic or vanadium. By far the
most common phosphate is apatite which is an important biological mineral found
in teeth and bones of many animals. The phosphate class includes the phosphate,
arsenate, vanadate, and antimonate minerals.
Element class
The Elemental group includes metals and intermetallic elements (gold, silver,
copper), semi-metals and non-metals (antimony, bismuth, graphite, sulfur). This
group also includes natural alloys, such as electrum (a natural alloy of gold
and silver), phosphides, silicides, nitrides and carbides (which are usually
only found naturally in a few rare meteorites).
Organic class
The organic mineral class includes biogenic substances in which geological
processes have been a part of the genesis or origin of the existing compound.[1]
Minerals of the organic class include various oxalates, mellitates, citrates,
cyanates, acetates, formates, hydrocarbons and other miscellaneous species.[2]
Examples include whewellite, moolooite, mellite, fichtelite, carpathite,
evenkite and abelsonite.
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