Corundum
| Corundum | |
|---|---|
 Corundum
|
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| General | |
| Category | Mineral |
| Chemical formula | aluminium oxide, Al2O3 |
| Identification | |
| Colour | Brown to grey, less often red, blue, white, yellow. |
| Crystal habit | Steep bipyramidal, tabular, prismaticl, 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 colour except red |
| Ruby | Red |
| Emery | Granular |
Corundum is the crystalline form of aluminium oxide and one of the rock-forming minerals. Corundum 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. The word corundum comes from the Tamil kurundam.
The oxygen atoms in corundum are arranged in a hexagonal close-packing, with the smaller aluminium atoms occupying 2/3 of the octahedral gaps. The coordination of the atoms are thus 6:4, compared to 4:2 for quartz, which accounts for its greater hardness despite the Al-O bonds being less covalent. 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 (typically 9.0), it is commonly used as an abrasive in machining, from huge machines to sandpaper. Emery is an impure and less abrasive variety, with a Mohs hardness of 8.0.
Corundum occurs as an accessory 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 deposites 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