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Corundum

Corundum
Several corundum crystals.jpg
General
Category Oxide mineral – Hematite group
Formula
(repeating unit)
Aluminium oxide, Al
2
O
3
Strunz classification 4.CB.05
Dana classification 4.3.1.1
Crystal system Trigonal
Crystal class Hexagonal scalenohedral (3m)
H-M symbol: (3 2/m)
Space group R3c
Unit cell a = 4.75 Å, c = 12.982 Å; Z = 6
Identification
Color Colorless, gray, brown; 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
Tenacity Brittle
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 light
References
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 (Al
2
O
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 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 Kurundam which originates from the Sanskrit word Kuruvinda meaning ruby.

Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), 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 Mohs hardness of 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.


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Wikipedia

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