Omphacite
Encyclopedia : O : OM : OMP : Omphacite
Omphacite is a member of the pyroxene group of silicate minerals with formula: (Ca,Na)(Mg,Fe2+,Al)Si2O6. It is a variably deep to pale green or nearly colorless variety of augite and represents an intermediate composition between the calcium rich augite and the sodium rich jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group is distinct from augite and jadeite. 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 metamorphic rocks. It also occurs in kimberlites and ophiolites. It occurs in association with garnet, quartz, kyanite, and corundum in high temperature eclogite facies and with hornblende, scapolite, and epidote, in the low temperature-high pressure blueschist facies.
It was first described in 1915 in the Münchberg Metamorphic complex, Franconia, Bavaria, Germany. It is also found in Marin and Sonoma counties of California. The name omphacite derives from the 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 0471805807
- [Webmineral data]
- [Mindat with location data]
- [Mineral data (PDF)]
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