corundum

 

(kərŭn'dəm) , mineral, aluminum oxide, Al₂O₃. The clear varieties are used as gems and the opaque as abrasive materials. Corundum occurs in crystals of the hexagonal system and in masses. It is transparent to opaque and has a vitreous to adamantine luster. The transparent gem varieties are colorless, pink, red, blue, green, yellow, and violet; the common varieties are blue-gray to brown. Emery is a common corundum, used as an abrasive and distinguished by its impurities of magnetite and hematite. The chief corundum gems are the ruby (red) and the sapphire (blue). Yellow, pink, green, and white stones are also called yellow, pink, green, and white sapphires. Corundum gems are also made synthetically. The chief sources of natural corundum are Myanmar, Sri Lanka, India, Thailand, Republic of South Africa, Tanzania, and the United States (North Carolina, Georgia, and Montana). Most of the emery is mined in Naxos and the other Cyclades and in Asia Minor.

Corundum
General
Category	Mineral
Chemical formula	aluminium oxide, Al2O3
Identification
Color	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 color 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

In 1837 Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a little chromium as a pigment. In 1847 Edelman made white sapphire 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 red heat. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process. ^[1]

The Verneuil process actually 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. It can scratch almost every other mineral, and when it does, its streak is white on the other mineral.

References

1. ^ Bahadur: a Handbook of Precious Stones (1943). Retrieved on 2007-08-19.

• Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300-302 ISBN 0-471-80580-7
• Mindat
• Webmineral data
• Mineral galleries

See Also

• Padparadscha
• Ruby
• Sapphire

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