Corundum

Corundum is a crystalline form of aluminium oxide (Al₂O₃) typically containing traces of iron, titanium, vanadium, and chromium.^[3][4] It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the presence of transition metal impurities in its crystalline structure.^[7] 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.^[7] A rare type of sapphire, padparadscha sapphire, is pink-orange.

The name "corundum" is derived from the Tamil-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).^[8][9]

Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. It is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.^[6]

In addition to its hardness, corundum has a density of 4.02 g/cm³ (251 lb/cu ft), which is unusually high for a transparent mineral composed of the low-atomic mass elements aluminium and oxygen.^[10]

Geology and occurrence

Corundum occurs as a mineral in mica

• In 1837,
  alumina at a high temperature with a small amount of chromium as a colourant.^[15]
• In 1847, J. J. Ebelmen made white synthetic sapphires by reacting alumina in boric acid.
• In 1877, Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing BaF₂ and Al₂O₃ with a little chromium at temperatures above 2,000 °C (3,630 °F).
• In 1903,
  flame fusion process.^[16]

Corundum crystallizes with trigonal symmetry in the space group R3c and has the lattice parameters a = 4.75 Å and c = 12.982 Å at standard conditions. The unit cell contains six formula units.^[4][21]

The toughness of corundum is sensitive to surface roughness^[22][23] and crystallographic orientation.^[24] It may be 6–7 MPa·m^1/2 for synthetic crystals,^[24] and around 4 MPa·m^1/2 for natural.^[25]

In the lattice of corundum, the oxygen atoms form a slightly distorted

Because of its prevalence, corundum has also become the name of a major structure type (corundum type) found in various

Wikimedia Commons has media related to Corundum.

• Aluminium oxynitride
• Gemstone
• Spinel – natural and synthetic mineral often mistaken for corundum

References