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Corundum

2007 Schools Wikipedia Selection. Related subjects: Mineralogy

                                  Corundum
                                  General
   Category            Mineral
   Chemical formula    aluminium oxide, Al[2]O[3]
                               Identification
   Colour              Brown to grey, less often red, blue, white, yellow.
   Crystal habit       Steep bipyramidal, tabular, prismatic, rhombohedral
                       crystals, massive or granular
   Crystal system      Trigonal Hexagonal Scalenohedral bar32/m
   Twinning            Polysynthetic twinning common
   Cleavage            None - parting in 3 directions
   Fracture            Conchoidal to uneven
   Mohs Scale hardness 9
   Luster              Adamantine to vitreous
   Refractive index    nω=1.768 - 1.772 nε=1.760 - 1.763, Biref 0.009
   Pleochroism         None
   Streak              White
   Specific gravity    3.95-4.1
   Fusibility          Infusible
   Solubility          Insoluble
                              Major varieties
   Sapphire            Any colour except red
   Ruby                Red
   Emery               Granular

   Corundum (from Tamil kurundam) is a crystalline form of aluminium oxide
   and one of the rock-forming minerals. It is naturally clear, but can
   have different colors when impurities are present. Transparent
   specimens are used as gems, called ruby if red, while all other colors
   are called sapphire. In addition to its hardness, corundum is unusual
   for its high density of 4.02 g/cm³ which is very high for a transparent
   mineral composed of the low atomic mass elements aluminium and oxygen.

   Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs),
   it is commonly used as an abrasive in machining, from huge machines to
   sandpaper. Some emery is a mix of corundum and other substances, and
   the mix is less abrasive, with a lower average Mohs hardness near 8.0.

   Corundum occurs as a mineral in mica schist, gneiss, and some marbles
   in metamorphic terranes. It also occurs in low silica igneous syenite
   and nepheline syenite intrusives. Other occurrences are as masses
   adjacent to ultramafic intrusives, associated with lamprophyre dikes
   and as large crystals in pegmatites. Because of its hardness and
   resistance to weathering, it commonly occurs as a detrital mineral in
   stream and beach sands.

   Corundum for abrasives is mined in Zimbabwe, Russia, and India.
   Historically it was mined from deposits associated with dunites in
   North Carolina and from a nepheline syenite in Craigmont, Ontario.
   Emery grade corundum is found on the Greek island of Naxos and near
   Peekskill, New York. Abrasive corundum is synthetically manufactured
   from bauxite.

Synthetic Corundum

   Gem-quality synthetic corundum is usually produced by the flame-fusion
   method (also called Verneuil process). This allows the production of
   large quantities of sapphire, rubies, and other corundum gems. It is
   also possible to grow gem-quality synthetic corundum by flux-growth and
   hydrothermal synthesis. Because of the simplicity of the methods
   involved in corundum synthesis, large quantities of these crystals
   became available on the market causing a significant reduction of price
   in recent years. Apart from ornamental use, synthetic corundum is also
   used to produce mechanical parts (tubes, rods, bearings, and other
   machined parts), watch crystals, and lasers.
   Retrieved from " http://en.wikipedia.org/wiki/Corundum"
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