Omphacite
3280954
206688479
2008-04-19T13:48:01Z
Rhanyeia
4370225
wikify
'''Omphacite''' is a member of the [[pyroxene]] group of [[silicate minerals]] with formula: ([[calcium|Ca]],[[sodium|Na]])([[magnesium|Mg]],[[iron|Fe]]<sup>2+</sup>,[[aluminum|Al]])[[silicon|Si]]<sub>2</sub>[[oxygen|O]]<sub>6</sub>. It is a variably deep to pale green or nearly colorless variety of [[pyroxene]]. Omphacite compositions are intermediate between calcium-rich [[augite]] and sodium-rich [[jadeite]]. It crystallizes in the [[monoclinic]] [[Crystal system|system]] with prismatic, typically [[Crystal twinning|twinned]] forms, though usually anhedral. Its [[space group]] (P2/n) is distinct from that of augite and jadeite (C2/c). It exhibits the typical near 90° pyroxene cleavage. It is brittle with [[specific gravity]] of 3.29 to 3.39 and a [[Mohs hardness]] of 5 to 6.
It is a major mineral component of [[eclogite]] along with [[pyrope]] garnet and also occurs in [[blueschist]] [[facies]] and UHP (ultrahigh-pressure) [[metamorphic rocks]]. It also occurs in eclogite xenoliths from [[kimberlite]] as well as in crustal rocks metamorphosed at high pressures. Associated minerals in eclogites include [[garnet]], [[quartz]] or [[coesite]], [[rutile]], [[kyanite]], [[phengite]], and [[lawsonite]]. Minerals such as [[glaucophane]], [[lawsonite]], [[titanite]], and [[epidote]] occur with omphacite in [[blueschist]] facies metamorphic rocks. The name "jade," usually referring to rocks made of [[jadeite]], is sometimes also applied to rocks consisting entirely of omphacite.
The name "omphacite" has been applied to compositions that contain between 20% and 80% jadeite. The stability of intermediate compositions between [[augite]] and [[jadeite]] is not well-understood, but miscibility gaps appear to be present at temperatures below 300°C to 400°C, and perhaps at higher temperatures. Pairs of pyroxenes -- both augite plus omphacite and omphacite plus jadeite -- appear to have existed in equilibrium at low temperatures, as discussed by Tsujimori et al. (2005).
It was first described in 1915 in the Münchberg Metamorphic complex, Franconia, [[Bavaria]], [[Germany]]. The name ''omphacite'' derives from the [[Greek language|Greek]] ''omphax'' or ''unripe grape'' for the typical green color.
==References==
*Hurlbut, Cornelius S.; Klein, Cornelis, 1985, ''Manual of Mineralogy'', 20th ed., pp. 398 - 405, John Wiley and Sons, New York ISBN 0-471-80580-7
*Tatsuki Tsujimori, Juhn G. Liou, and Robert G. Coleman, 2005, ''Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala.'' American Mineralogist, v. 90, pp. 836-842.
*[http://webmineral.com/data/Omphacite.shtml Webmineral data]
*[http://www.mindat.org/show.php?id=2991&ld=1&pho= Mindat with location data]
*[http://rruff.geo.arizona.edu/doclib/hom/omphacite.pdf Mineral data (PDF)]
[[Category:Inosilicates]]
[[Category:Calcium minerals]]
[[Category:Sodium minerals]]
[[Category:Magnesium minerals]]
[[Category:Iron minerals]]
[[Category:Aluminium minerals]]
{{mineral-stub}}
[[fr:Omphacite]]
[[ja:オンファス輝石]]
[[nl:Omfaciet]]
[[pl:Omfacyt]]
[[pt:Onfacite]]