Corundum
General
Category	Oxide mineral – Hematite group
Chemical formula	Aluminium oxide, Al2O3
Strunz classification	04.CB.05
Dana classification	4.3.1.1
Crystal symmetry	Trigonal (32/m)
Unit cell	a = 4.75 Å, c = 12.982 Å; Z=6
Identification
Color	Colorless, gray, brown; pink to pigeon-blood-red, orange, yellow, green, blue to cornflower blue, violet; may be color zoned, asteriated mainly grey and brown
Crystal habit	Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular
Crystal system	Trigonal (Hexagonal Scalenohedral) Symbol (32/m) Space group: R3c
Twinning	Polysynthetic twinning common
Cleavage	None – parting in 3 directions
Fracture	Conchoidal to uneven
Mohs scale hardness	9 (defining mineral)
Luster	Adamantine to vitreous
Streak	White
Diaphaneity	Transparent, translucent to opaque
Specific gravity	3.95–4.10
Optical properties	Uniaxial (–)
Refractive index	nω = 1.767–1.772 nε = 1.759–1.763
Pleochroism	None
Melting point	2044 °C
Fusibility	Infusible
Solubility	Insoluble
Alters to	May alter to mica on surfaces causing a decrease in hardness
Other characteristics	May fluoresce or phosphoresce under UV
References	[1][2][3][4]
Major varieties
Sapphire	Any color except red
Ruby	Red
Emery	Black granular corundum intimately mixed with magnetite, hematite, or hercynite

Corundum is a crystalline form of aluminium oxide (Al₂O₃) with traces of iron, titanium and chromium.^[1] It is a rock-forming mineral. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red and padparadscha if pink-orange. All other colors are called sapphire, e.g., "green sapphire" for a green specimen.

The name "corundum" is derived from the Tamil word குருந்தம் "kuruntam" meaning "ruby", and related to Sanskrit "kuruvinda".^[3]

Because of corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics, and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with an average hardness near 8.0.

In addition to its hardness, corundum is unusual for its density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.^[5]

Geology and occurrence[link]

Corundum from Brazil, size about 2×3 cm.

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.^[4] It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering.^[4] The largest documented single crystal of corundum measured about 65×40×40 cm.^[6]

Corundum for abrasives is mined in Zimbabwe, Russia, Sri Lanka and India. Historically it was mined from deposits associated with dunites in North Carolina, USA and from a nepheline syenite in Craigmont, Ontario.^[4] Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York, USA. Abrasive corundum is synthetically manufactured from bauxite.^[4]

Corundum should not be confused with the similarly named carborundum, silicon carbide.

Synthetic corundum[link]

In 1837, Marc Antoine Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment.^[7] In 1847, Ebelmen made white sapphires by fusing alumina in boric acid. In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with a little chromium at temperatures above 2,000 °C (3,632 °F). In 1903, Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.^[8]

Crystal structure of corundum

The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum 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 causing a significant reduction of price in recent years. 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 from the UV to IR), and laser components.

References[link]

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