Corundum is a form of aluminium oxide () typically containing traces of iron, titanium, vanadium, and chromium.[ 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.]
The name "corundum" is derived from the Tamil language-Dravidian word kurundam (ruby-sapphire) (appearing in Sanskrit as kuruvinda).
Because of corundum's hardness (pure corundum is defined to have 9.0 on the Mohs scale), it can scratch almost all other minerals. Emery, a variety of corundum with no value as a gemstone, is commonly used as an abrasive on sandpaper and on large tools used in machining metals, plastics, and wood. It is a black granular form of corundum, in which the mineral is intimately mixed with magnetite, hematite, or hercynite.[
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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 in metamorphic . It also occurs in low-silica igneous syenite and nepheline syenite intrusive rock. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in .[ 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 .]
Corundum for is mined in Zimbabwe, Pakistan, Afghanistan, Russia, Sri Lanka, and India. Historically it was mined from deposits associated with 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.][
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Four corundum axes dating to 2500 BC from the Liangzhu culture and Sanxingcun culture (the latter of which is located in Jintan District) have been discovered in China.
Synthetic corundum
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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 colourant.
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In 1847, J. J. Ebelmen made white synthetic sapphires by reacting Aluminium oxide in boric acid.
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In 1877, Frenic and Freil made crystal corundum from which small stones could be cut. Frimy and Auguste Verneuil manufactured artificial ruby by fusing and with a little chromium at temperatures above .
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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 at a fraction of the cost of natural stones.
Synthetic corundum has a lower environmental impact than natural corundum by avoiding destructive mining and conserving resources.
Apart from ornamental uses, synthetic corundum is also used to produce mechanical parts (tubes, rods, bearings, and other machined parts), scratch-resistant optics, scratch-resistant watch crystals, instrument windows for satellites and spacecraft (because of its transparency in the ultraviolet to infrared range), and laser components. For example, the KAGRA gravitational wave detector's main mirrors are sapphires,[Defense World.Net, Russia’s Armored Steel-Comparable Ceramic Plate Clears Tests, 5 September 2020, Retrieved 29 December 2020]
Structure and physical properties
Corundum crystallizes with trigonal symmetry in the space group 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
In the lattice of corundum, the oxygen atoms form a slightly distorted hexagonal close packing, in which two-thirds of the octahedral sites between the oxygen ions are occupied by aluminium ions.
The Young's modulus of corundum (sapphire) has been reported by many different sources with values varying between 300 and 500 GPa, but a commonly cited value used for calculations is 345 GPa.
Single crystal corundum fibers have potential applications in high temperature composites, and the Young's modulus is highly dependent on the crystallographic orientation along the fiber axis. The fiber exhibits a max modulus of 461 GPa when the crystallographic c-axis 0001 is aligned with the fiber axis, and minimum moduli ~373 GPa when a direction 45° away from the c-axis is aligned with the fiber axis.
The hardness of corundum measured by indentation at low loads of 1-2 N has been reported as 22-23 GPa
Polycrystalline corundum formed through sintering and treated with a hot isostatic press process can achieve grain sizes in the range of 0.55-0.7 μm, and has been measured to have four-point bending strength between 600 and 700 MPa and three-point bending strength between 750 and 900 MPa.
Structure type
Because of its prevalence, corundum has also become the name of a major structure type ( corundum type) found in various Binary compound and .
See also
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Aluminium oxynitride
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Gemstone
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Spinel – natural and synthetic mineral often mistaken for corundum
