Corundum is a crystalline form of aluminium oxide typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is also a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure. Corundum has two primary gem varieties: ruby and sapphire. Rubies are red due to the presence of chromium, and sapphires exhibit a range of colors depending on what transition metal is present. A rare type of sapphire, padparadscha sapphire, is pink-orange. The name "corundum" is derived from the Tamil-Dravidian word kurundam . Because of corundum's hardness, it can scratch almost every other mineral. It is commonly used as an abrasive on everything from sandpaper to large tools 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 has a density of, which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.
Geology and occurrence
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low-silica igneoussyenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyredikes and as large crystals in pegmatites. It commonly occurs as a detrital mineral in stream and beach sands because of its hardness and resistance to weathering. The largest documented single crystal of corundum measured about, and weighed. The record has since been surpassed by certain synthetic boules. Corundum for abrasives is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with dunites in North Carolina, US and from a nepheline syenite in Craigmont, Ontario. Emery-grade corundum is found on the Greek island of Naxos and near Peekskill, New York, US. Abrasive corundum is synthetically manufactured from bauxite. Four corundum axes dating back to 2500 BCE from the Liangzhou culture have been discovered in China.
Synthetic corundum
In 1837, Marc Antoine Gaudin made the first synthetic rubies by reacting alumina at a high temperature with a small amount of chromium as a pigment.
In 1877 Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing barium fluoride| and aluminium oxide| with a little chromium at temperatures above.
In 1903, Verneuil announced that he could produce synthetic rubies on a commercial scale using this flame fusion process.
The Verneuil process allows the production of flawless single-crystal sapphire and ruby gems of much larger size than normally found in nature. It is also possible to grow gem-quality synthetic corundum by flux-growth and hydrothermal synthesis. Because of the simplicity of the methods involved in corundum synthesis, large quantities of these crystals have become available on the market causing a significant reduction of price in recent years. Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts, scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft, and laser components. For example, the KAGRA gravitational wave detector's main mirrors are sapphires, and Advanced LIGO considered sapphire mirrors.
Structure and physical properties
Corundum crystallizes with trigonal symmetry in the space group hexagonal crystal family#Crystal classes| and has the lattice parameters and at standard conditions. The unit cell contains six formula units. The toughness of corundum is sensitive to surface roughness and crystallographic orientation. It may be 6–7 MPa·m for synthetic crystals, and around 4 MPa·m for natural. In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the gaps between the octahedra are occupied by aluminium ions.
