|Category||Oxide mineral – Hematite group|
|Aluminium oxide, Al|
|Crystal class||Hexagonal scalenohedral (3m)|
H-M symbol: (3 2/m)
|Space group||R3c (No. 167)|
|Unit cell||a = 4.75 Å, c = 12.982 Å; Z = 6|
|Color||Colorless, gray, golden-brown, brown; purple, pink to red, orange, yellow, green, blue, violet; may be color zoned, asteriated mainly grey and brown|
|Crystal habit||Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular|
|Twinning||Polysynthetic twinning common|
|Cleavage||None – parting in 3 directions|
|Fracture||Conchoidal to uneven|
|Mohs scale hardness||9 (defining mineral)|
|Luster||Adamantine to vitreous|
|Diaphaneity||Transparent, translucent to opaque|
|Optical properties||Uniaxial (−)|
|Refractive index||nω = 1.767–1.772 |
nε = 1.759–1.763
|Melting point||2,044 °C (3,711 °F)|
|Alters to||May alter to mica on surfaces causing a decrease in hardness|
|Other characteristics||May fluoresce or phosphoresce under UV light|
|Sapphire||Any color except red|
|Emery||Black granular corundum intimately mixed with magnetite, hematite, or hercynite|
Corundum is a crystalline form of aluminium oxide (Al
3) typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange.
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. Emery's blackish hue is caused by impurities of hematite, spinel, or magnetite.
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. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about 65 cm × 40 cm × 40 cm (26 in × 16 in × 16 in), and weighed 152 kg (335 lb). The record has since been surpassed by certain synthetic boules.
Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US, and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite.
Four corundum axes dating to 2500 BC from the Liangzhou culture have been discovered in China.
The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. 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 have become available on the market at a fraction of the cost of natural stones.
Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components. For example, the KAGRA gravitational wave detector's main mirrors are 23 kg (50 lb) sapphires, and Advanced LIGO considered 40 kg (88 lb) sapphire mirrors. Corundum has also found use in the development of ceramic armour thanks to its high hardiness.
In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions. The absence of aluminium ions from one of the three sites breaks the symmetry of the hexagonal close packing, reducing the space group symmetry to R3c and the crystal class to trigonal. The structure of corundum is sometimes described as a pseudohexagonal structure.
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