oxide mineral From Wikipedia, the free encyclopedia
Corundum is a mineral. It is very hard, with a Mohs hardness of 9 (diamond is 10). It is usually clear. Its chemical formula is aluminium oxide, Al2O3. Sometimes some of the aluminium atoms get replaced with chromium atoms. It then becomes a ruby. Other impurities can make a sapphire.
Corundum | |
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General | |
Category | Oxide mineral – Hematite group |
Formula (repeating unit) | Aluminium oxide, Al 2O 3 |
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 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 |
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 |
Tenacity | Brittle |
Mohs scale hardness | 9 (defining mineral)[1] |
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 | [2][3][4][5] |
Major varieties | |
Sapphire | Any color except red |
Ruby | Red |
Emery | Black granular corundum intimately mixed with magnetite, hematite, or hercynite |
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