Corundum is frequently used as an abrasive and is world famous as the mineral of rubies and sapphires
Made Famous by Rubies and Sapphires
Most people are familiar with corundum; however, very few people know it by its mineral name. A gemstone-quality specimen of corundum with a deep red color is known as a “ruby”. A gemstone-quality corundum of with a blue color is called a “sapphire”.
Rubies and sapphires are famous throughout the world, but most people do not know that they are color varieties of the same mineral, corundum.
Properties and Occurrence of Corundum
Corundum is an exceptionally hard and tough form of aluminum oxide (Al2O3). As a primary mineral it is found in igneous rocks such as syenite, nepheline syenite and pegmatite. It is also found inmetamorphic rocks in locations where aluminous shales or bauxites have been exposed to contact metamorphism. Schist, gneiss andmarble produced by regional metamorphism will sometimes contain corundum.
Corundum’s toughness, high hardness and chemical resistance enable it to persist in sediments long after other minerals have been destroyed. It thereby becomes concentrated in alluvial deposits. These deposits are sources of rubies and sapphires in several parts of the world. Notable deposits of alluvial rubies and sapphires occur in: Burma, Cambodia, Sri Lanka, India, Afghanistan, Montana and other areas.
Hardness and Use as an Abrasive
Corundum is very hard. It serves as the index mineral with a hardness of nine on the Mohs Hardness Scale. It is the third hardest mineral known, with diamond and moissanite being the only minerals with a greater hardness.
This high hardness makes corundum especially useful as an abrasive. Crushed corundum is screened to produce uniformly-sized grits and powders. These are used as grinding media and used to manufacture polishing compounds, sand papers, grinding wheels and cutting tools.
The costs of manufacturing abrasives have been declining through innovation. Synthetic corundum is increasingly being used as an abrasive instead of natural corundum. Some of it is manufactured from calcined bauxite which yields Al2O3 with the same Mohs Scale 9 hardness as natural corundum.
Emery stone is a granular metamorphic or igneous rock that is rich in corundum. It is a mixture of oxide minerals, typically corundum,magnetite, spinel and/or hematite. It is the most common form of natural corundum used to manufacture abrasives.
The use of corundum as an abrasive has declined in the last few decades. It is being replaced by manufactured abrasives such assilicon carbide. Silicon carbide has a Mohs hardness of 9 to 9.5. It is inexpensive and often performs better than natural abrasives made from corundum or emery.