2008/9 Schools Wikipedia Selection. Related subjects: Mineralogy


Category Mineral
Chemical formula aluminium oxide, Al2O3
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.

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