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 x 3 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]]