corundum

Corundum. Credit: Mineral Information Institute

A translucent to transparent crystalline (rhombohedral) form of aluminum oxide (alpha-Al₂O₃) which comes in many different hues. It is found in igneous and metamorphic rocks, occurring as pyramidal or prsmatic crystals in the rhombohefral class and as granular masses. The corundum structure has close-packed oxygen with aluminum in octahedral holes. Mineral varieties of corundum include ruby, sapphire, and emery. Artificial corundum, or beta-alumina, is made by calcining bauxite (see calcination).

Corundum is the hardest natural substance known after diamond and is also chemically inert and resistant to corrosion. It is used extensively as an abrasive and in bearings (for example, in motors and watches). Hardness 9, relative density 4.

Related category

• GEOLOGY AND PLANETARY SCIENCE

Also on this site:

Encyclopedia of Alternative Energy & Sustainable Living
Encyclopedia of History

BACK TO TOP