Corundum
Corundum is a crystalline form of aluminium oxide (Al2O3) with a hardness of 9 on the Mohs scale, making it one of the hardest minerals known to mankind. Here are some key points about corundum:
Physical Properties
- Color: Can occur in various colors including red, blue, green, yellow, brown, pink, purple, orange, and black. When colorless, it is often referred to as sapphire or ruby, depending on its color.
- Crystal System: Trigonal (rhombohedral).
- Transparency: Can be transparent to opaque.
- Luster: Adamantine to vitreous when polished.
- Cleavage: None, but it has a conchoidal fracture.
Varieties
- Ruby: Red corundum, the color due to the presence of chromium.
- Sapphire: All other colors of corundum, with blue being the most famous due to trace amounts of iron and titanium.
Formation and Occurrence
Corundum forms in metamorphic rocks, particularly in regions where there has been contact metamorphism or where pegmatites have intruded into limestone or dolomite. It can also occur in igneous rocks like syenite and nepheline syenite.
Historical Uses
- Since ancient times, corundum has been used as a gemstone, with rubies and sapphires being particularly prized for their beauty and durability.
- It has been used as an abrasive due to its hardness, with emery being a form of corundum used for this purpose.
Modern Applications
- Abrasives: Due to its hardness, corundum is used in sandpapers, grinding wheels, and cutting tools.
- Jewelry: As gemstones, especially when cut and polished.
- Refractory Material: High melting point makes it useful in refractories for furnaces.
- Electronics: Sapphire, a form of corundum, is used in the production of LEDs and as a substrate for electronic circuits.
Notable Deposits
- Myanmar (Burma) is famous for its ruby deposits.
- Kashmir, India, known for its blue sapphires.
- Sri Lanka, Thailand, Australia, and Madagascar are also significant sources.
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