Mineral



A mineral is a naturally occurring substance formed through geological processes that has a characteristic chemical composition, a highly ordered atomic structure and specific physical properties. A mineralogy.  

Mineral definition and classification

To be classified as a "true" mineral, a substance must be a 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.[2]

The modern classifications include an organic class - in both the new Dana and the Strunz classification schemes.[3][4]

The Industrial minerals is a market term and refers to commercially valuable mined materials (see also Minerals and Rocks section below).

A 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, oxygen) all share the same cubic crystal structure.

Crystal structure greatly influences a mineral's physical properties. For example, though 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 limonite and a few other minerals.[5] 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 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 silica.

Other factors are of equal importance in determining the natural association or paragenesis of rock-forming 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 gneiss not very different in mineralogical composition though radically different in structure to the granite which was its original state.[5]

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:[1]

  • 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.

   

  • 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]
  1. Talc Mg3Si4O10(OH)2
  2. Gypsum CaSO4·2H2O
  3. Calcite CaCO3
  4. Fluorite CaF2
  5. Apatite Ca5(PO4)3(OH,Cl,F)
  6. Orthoclase KAlSi3O8
  7. Quartz SiO2
  8. Topaz Al2SiO4(OH,F)2
  9. Corundum Al2O3
  10. 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: pyrite
    • Sub-metallic -slightly less than metallic reflectivity: magnetite
    • Non-metallic lusters:
      • Adamantine - brilliant, the luster of anglesite
      • Vitreous -the luster of a broken glass: quartz
      • Pearly - iridescent and pearl-like: talc and apophyllite
      • Resinous - the luster of resin: 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).
    • Iridescence - the play of colors due to surface or internal interference. Labradorite exhibits internal iridescence 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 sections, 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: 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.[1][7]

Silicate class

  The largest group of minerals by far are the micas.

Carbonate class

The borate minerals.

Sulfate class

molybdate, selenate, sulfite, tellurate, and tungstate minerals.

Halide class

  The iodide minerals.

Oxide class

hydroxide minerals.

Sulfide class

Many sulfosalts (sulfur and a second anion such as arsenic).

Phosphate class

The 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.[2] Minerals of the organic class include various oxalates, mellitates, citrates, cyanates, acetates, formates, hydrocarbons and other miscellaneous species.[3] Examples include abelsonite.

See also

  • Minerals.net
  • mindat.org Mindat database
  • Webmineral.com
  • Mineral atlas with properties, photos

References

  1. ^ a b c Klein, Cornelis and Cornelius Hurlbut, Jr. (1985) Manual of Mineralogy, Wiley, 20th ed., ISBN 0-471-80580-7
  2. ^ a b http://www.minsocam.org/msa/ima/ima98(04).pdf Ernest H. Nickel, 1995, The definition of a mineral, The Canadian Mineralogist, vol. 33, pp. 689 - 690
  3. ^ a b http://www.mindat.org/dana.php?a=50 Dana Classification 8th edition - ORGANIC COMPOUNDS
  4. ^ http://www.mindat.org/strunz.php?a=9 Strunz Classification - Organic Compounds
  5. ^ a b This article incorporates text from the Encyclopædia Britannica Eleventh Edition article "Petrology", a publication now in the public domain.
  6. ^ http://volcanoes.usgs.gov/Products/Pglossary/mineral.html USGS Photo glossary of volcano terms
  7. ^ http://www.minerals.net/mineral/sort-met.hod/dana/dana.htm Dana classification - Minerals.net
 
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