Corundum structure
| Corundum | |
|---|---|
 |
|
| General | |
| Category | Oxide mineral – Hematite group |
|
Formula (repeating unit) |
Aluminium oxide, Al 2O 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
2O
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