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Mineral

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

   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 (organic compounds are
   excluded). The study of minerals is called mineralogy.
   An assortment of minerals. Photo from US Geological Survey.
   An assortment of minerals. Photo from US Geological Survey.

Mineral definition and classification

   To be classified as a "true" mineral, a substance must be a solid and
   have a crystal structure. It must also be an inorganic,
   naturally-occurring, homogeneous substance with a defined chemical
   composition. 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 (NaAlSi[3]O[8]) to
   calcium-rich anorthite (CaAl[2]Si[2]O[8]) 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 just over 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."

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.) 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. 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 are left over, after
   the desired mineral has been separated from the ore, are referred to as
   tailings).

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,
   time-consuming, and even risk damaging the sample.

   Physical properties commonly used are :
     * 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.
     * 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. The minerals
       that define the scale are given below:

    1. - talc
    2. - gypsum
    3. - calcite
    4. - fluorite
    5. - apatite
    6. - orthoclase feldspar
    7. - quartz
    8. - topaz or beryl
    9. - corundum
   10. - diamond

     * Luster indicates the way a mineral's surface interacts with light
       and can range from dull to glassy (vitreous).
          + Metallic -high reflectivity like metal, e.g. galena
          + Sub-metallic -slightly less than metallic reflectivity, e.g.
            magnetite
          + Vitreous -the lustre of a broken glass, e.g. quartz
          + Pearly -a very soft light shown by some layer silicates, e.g.
            talc
          + Silky -a soft light shown by fibrous materials, e.g. gypsum
          + Dull/earthy -shown by finely crystallized minerals, e.g. the
            kidney ore variety of hematite

     * Colour 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).
     * Streak refers to the colour of the powder a mineral leaves after
       rubbing it on an unglazed porcelain streak plate.
     * 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, e.g. a chonchoidal fracture is a smooth
       fracture with concentric ridges of the type shown by glass.
     * 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 rock-forming minerals, have a specific
       gravity of 2.5 - 3.5, a few are noticeably more or less dense, e.g.
       several sulphide 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), 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.

Silicate class

   The 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
   (CO[3])^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, SO[4]^2-. 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),
   celestite (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

   The 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 AO[4] 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).

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