Corundum 5974 222128720 2008-06-27T18:17:08Z Vuo 143648 /* Synthetic corundum */ {{Infobox mineral | name = Corundum | category = | boxwidth = | boxbgcolor = | image = Corundum_USGOV.jpg | caption = | formula = [[Aluminium oxide]], Al<sub>2</sub>O<sub>3</sub> | color = Brown to gray, less often red, blue, white,or yellow. | habit = Steep bipyramidal, tabular, prismatic, rhombohedral crystals, massive or granular | system = [[Trigonal]] Hexagonal Scalenohedral bar32/m | twinning = Polysynthetic twinning common | cleavage = None - parting in 3 directions | fracture = Conchoidal to uneven | mohs = 9 | luster = Adamantine to vitreous | refractive = nω=1.768 - 1.772 nε=1.760 - 1.763, Biref 0.009 | opticalprop = | pleochroism = None | streak = White | gravity = 3.95-4.1 | melt = | fusibility = Infusible | diagnostic = | solubility = Insoluble | other = | var1 = [[Sapphire]] | var1text = Any color except red | var2 = [[Ruby]] | var2text = Red | var3 = [[Emery (mineral)|Emery]] | var3text = Granular }} '''Corundum''' (from [[Tamil language|Tamil]] ''kurundam'' குருந்தம் or ''kuruvindam'' குருவிந்தம்) is a [[crystal]]line form of [[aluminium oxide]] and one of the [[rock (geology)|rock]]-forming [[mineral]]s. It is naturally clear, but can have different colors when impurities are present. [[Transparency (optics)|Transparent]] specimens are used as [[Gemstone|gem]]s, called [[ruby]] if [[red]], while all other colors are called [[sapphire]]. A pinkish-orange sapphire is called [[padparadscha]]. Due to corundum's hardness (pure corundum is defined to have 9.0 [[Mohs scale of mineral hardness|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 (mineral)|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]]. [[Image:Corindon azulEZ.jpg|thumb|left|180px|Corundum from [[Brazil]], about 2&nbsp;x&nbsp;3&nbsp;cm of size]] Corundum occurs as a mineral in mica [[schist]], [[gneiss]], and some [[marble]]s in [[Metamorphic rocks|metamorphic terranes]]. It also occurs in low silica [[igneous]] [[syenite]] and [[nepheline syenite]] [[intrusive]]s. Other occurrences are as masses adjacent to [[ultramafic]] intrusives, associated with [[lamprophyre]] [[Dike (geology)|dikes]] and as large crystals in [[pegmatite]]s. Because of its hardness and resistance to weathering, it commonly occurs as a [[detrital]] mineral in stream and beach sands. Corundum for abrasives is mined in [[Zimbabwe]], [[Russia]], and [[India]]. Historically it was mined from deposits associated with [[dunite]]s in [[North Carolina]] and from a [[nepheline]] [[syenite]] in [[Craigmont, Ontario]]. [[Emery (mineral)|Emery]] grade corundum is found on the [[Greece|Greek]] island of [[Naxos (Crete)|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 little [[chromium]] as a pigment. In [[1847]] Edelman made white sapphire by fusing [[Aluminum oxide|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 BaF<sub>2</sub> and Al<sub>2</sub>O<sub>3</sub> 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. <ref>{{cite web | url = http://www.farlang.com/gemstones/bahadur_handbook_of_precious_stones/page_067 | title = Bahadur: a Handbook of Precious Stones | date = 1943 | accessdate = 2007-08-19 }}</ref> The [[Verneuil process]] allows the production of flawless single-crystal [[sapphire]], [[ruby|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#Overview|watch crystals]] and windows for optical equipment, spacecraft as well as for [[lasers]]. ==References== {{reflist}} * Hurlbut, Cornelius S.; Klein, Cornelis, [[1985]], ''Manual of Mineralogy'', 20th ed., Wiley, pp. 300-302 ISBN 0-471-80580-7 * [http://www.mindat.org/min-1136.html Mindat] * [http://webmineral.com/data/Corundum.shtml Webmineral data] * [http://www.galleries.com/minerals/oxides/corundum/corundum.htm Mineral galleries] ==See also== {{Commons|Category:Corundum|Corundum}} *[[Padparadscha]] *[[Ruby]] *[[Sapphire]] [[Category:Aluminium minerals]] [[Category:Oxide minerals]] [[Category:Superhard materials]] [[Category:Tamil words and phrases]] [[Category:Hematite group]] [[bs:Korund]] [[bg:Корунд]] [[cs:Korund]] [[da:Korund]] [[de:Korund]] [[et:Korund]] [[es:Corindón]] [[eo:Korundo]] [[eu:Korindoi]] [[fr:Corindon]] [[gl:Corindón]] [[hr:Korund]] [[id:Korundum]] [[it:Corindone]] [[he:קורונדום]] [[lv:Korunds]] [[lt:Korundas]] [[hu:Korund]] [[nl:Korund]] [[ja:コランダム]] [[no:Korund]] [[pl:Korund]] [[pt:Coríndon]] [[ro:Corindon]] [[ru:Корунд]] [[sk:Korund]] [[sl:Korund]] [[sr:Корунд]] [[fi:Korundi]] [[sv:Korund]] [[th:คอรันดัม]] [[uk:Корунд]] [[zh:剛玉]]