An extremely hard mineral, aluminum oxide, Al2O3, sometimes containing iron, magnesia, or silica, that occurs in gem varieties such as ruby and sapphire and in a common black, brown, or blue form used chiefly in abrasives.
[Tamil kuruntam.]
corundum
Dictionary:
co·run·dum (kə-rŭn'dəm)  |
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A mineral with the ideal composition Al2O3. It is one of a large group of isostructural compounds including hematite (Fe2O3) and ilmenite (FeTiO3), all of which crystallize in the hexagonal crystal system, trigonal subsystem. Corundum has the high hardness of 9 on Mohs scale and is therefore commonly used as an abrasive, either alone or in the form of the rock called emery, which consists principally of the minerals corundum and magnetite. Crystals occurring in igneous rocks usually have an elongated barrellike shape, while crystals from metamorphic rocks are generally tabular. The specific gravity is approximately 3.98. See also Hematite; Ilmenite.
Pure corundum is transparent and colorless, but most specimens contain some transition elements substituting for aluminum, resulting in the presence of color. Substitution of chromium results in a deep red color; such red corundum is known as ruby. The term “sapphire” is used in both a restricted sense for the “cornflower blue” variety containing iron and titanium, and in a general sense for gem-quality corundums of any color other than red. Star ruby and star sapphire contain tiny needies of the mineral rutile. See also Ruby; Sapphire.
Corundum occurs as a rock-forming mineral in both metamorphic and igneous rocks, but only in those which are relatively poor in silica, and never in association with free silica. Igneous rocks which most commonly contain corundum include syenites, nepheline syenites, and syenite pegmatites. Both contact and regionally metamorphosed silica-poor rocks may contain corundum. See also
| Britannica Concise Encyclopedia: corundum |
For more information on corundum, visit Britannica.com.
| Architecture: corundum |
A hard, abrasive mineral, principally aluminum oxide, applied to a surface to make it non-slippery; for example, on the walking surface of a ramp.
| Columbia Encyclopedia: corundum |
corundum (kərŭn'dəm) , mineral, aluminum oxide, Al2O3. The clear varieties are used as gems and the opaque as abrasive materials. Corundum occurs in crystals of the hexagonal system and in masses. It is transparent to opaque and has a vitreous to adamantine luster. The transparent gem varieties are colorless, pink, red, blue, green, yellow, and violet; the common varieties are blue-gray to brown. Emery is a common corundum, used as an abrasive and distinguished by its impurities of magnetite and hematite. The chief corundum gems are the ruby (red) and the sapphire (blue). Yellow, pink, green, and white stones are also called yellow, pink, green, and white sapphires. Corundum gems are also made synthetically. The chief sources of natural corundum are Myanmar, Sri Lanka, India, Thailand, Republic of South Africa, Tanzania, and the United States (North Carolina, Georgia, and Montana). Most of the emery is mined in Naxos and the other Cyclades and in Asia Minor.
| Wikipedia: Corundum |
| Corundum | |
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| General | |
|---|---|
| Category | Mineral |
| Chemical formula | Aluminium oxide, Al2O3 |
| Identification | |
| Color | Brown to gray, less often red, blue, white,or yellow. |
| Crystal habit | Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular |
| Crystal system | Trigonal (Hexagonal Scalenohedral) Symbol (-3 2/m) Space Group: R-3c |
| Twinning | Polysynthetic twinning common |
| Cleavage | None - parting in 3 directions |
| Fracture | Conchoidal to uneven |
| Mohs Scale hardness | 9 |
| Luster | Adamantine to vitreous |
| Streak | White |
| Specific gravity | 3.95-4.1 |
| Refractive index | nω=1.768 - 1.772 nε=1.760 - 1.763, Birefringence 0.008 |
| Pleochroism | None |
| Melting point | 2044 °C |
| Fusibility | Infusible |
| Solubility | Insoluble |
| Major varieties | |
| Sapphire | Any color except red |
| Ruby | Red |
| Emery | Granular |
Corundum is a crystalline form of aluminium oxide (α-Al2O3) and is one of the rock-forming minerals. It is one of the naturally clear transparent materials, but can have different colors when impurities are present. Transparent specimens are used as gems, called ruby if red, while all other colors are called sapphire.
The name corundum probably derived from the Sanskrit "kuruvinda" meaning "ruby" [1]. In India a pinkish-orange sapphire is called padparadscha and a pink light-red sapphire is called patmaraga.
Due to corundum's hardness (pure corundum is defined to have 9.0 Mohs), it can scratch almost every other mineral, leaving behind a streak of white on the other mineral. It is commonly used as an abrasive, on everything from sandpaper to large machines used in machining metals, plastics and wood. Some emery is a mix of corundum and other substances, and the mix is less abrasive, with a lower average Mohs hardness near 8.0.
In addition to its hardness, corundum is unusual for its high density of 4.02 g/cm³, which is very high for a transparent mineral composed of the low atomic mass elements aluminium and oxygen.
Corundum occurs as a mineral in mica schist, gneiss, and some marbles in metamorphic terranes. It also occurs in low silica igneous syenite and nepheline syenite intrusives. Other occurrences are as masses adjacent to ultramafic intrusives, associated with lamprophyre dikes and as large crystals in pegmatites. Because of its hardness and resistance to weathering, it commonly occurs as a detrital mineral in stream and beach sands. The largest documented single crystal of corundum measured ~65x40x40 cm3.[2]
Corundum for abrasives is mined in Zimbabwe, Russia, and India. Historically it was mined from deposits associated with dunites in North Carolina and from a nepheline syenite in Craigmont, Ontario. Emery grade corundum is found on the Greek island of Naxos and near Peekskill, New York. Abrasive corundum is synthetically manufactured from bauxite.
Synthetic corundum
In 1837 Gaudin made the first synthetic rubies by fusing alumina at a high temperature with a small amount of chromium as a pigment. In 1847 Ebelmen made white sapphire by fusing 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 2000°C. In 1903 Verneuil announced he could produce synthetic rubies on a commercial scale using this flame fusion process.[3]
The Verneuil process allows the production of flawless single-crystal sapphires, rubies and other corundum 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 (tubes, rods, bearings, and other machined parts), scratch-resistant watch crystals and windows for optical equipment, spacecraft as well as for lasers.
References
- ^ Webminerals: Corundum
- ^ P. C. Rickwood (1981). "The largest crystals". American Mineralogist 66: 885-907. http://www.minsocam.org/ammin/AM66/AM66_885.pdf.
- ^ "Bahadur: a Handbook of Precious Stones". 1943. http://www.farlang.com/gemstones/bahadur_handbook_of_precious_stones/page_067. Retrieved on 2007-08-19.
- Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., Wiley, pp. 300-302 ISBN 0-471-80580-7
- Mindat
- Mineral galleries
See also
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 | Dictionary. The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2007. Published by Houghton Mifflin Company. All rights reserved. Read more |
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Mentioned in
- Asterism
- adamantine spar (mineralogy)
- emery rock (petrology)
- oriental topaz (mineralogy)
- orange sapphire (mineralogy)
- ruby (mineralogy)
- Smeriglio (family name)
- byon (geology)
- raglanite (petrology)
- emery
- emery
- wheel stone
- Triboluminescence
- alumina (inorganic chemistry)
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