Omphacite - (Ca,Na)(Mg,Fe2+,Al)Si2O6

It was first described in 1815 in the Münchberg Metamorphic complex, Franconia, Bavaria, Germany. The name omphacite derives from the Greek omphax or unripe grape for the typical green color. Omphacite is a member of the pyroxene group of silicate minerals with formula: (Ca,Na)(Mg,Fe2+,Al)Si2O6. Omphacite is a clinopyroxene solid solution of jadeite (25%-75%), augite (25%-75%), and aegirine (0%-25%).

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.

Optical properties:

• Form: Usually as coarse anhedral crystals.
• Color: Colorless to pale green or green and also deep green.
• Relief: Moderate .
• Cleavage: typical pyroxene cleavage.
• Interference colors: II order.


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Green massive omphacite crystals, Sonoma County, California, USA. From RRUFF.




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Pale green massive omphacite crystals, Kefalos Hill, Skotini Bay, Galissas, Syros Island, Aegean Sea, Greece. From RRUFF.




Bibliography



• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.


Photo
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Omphacite (green), garnet (pink), glaucophane (blue) in an eclogite. PPL image, 2x (Field of view = 7mm)		 eclogite1(11).jpg

Omphacite (green), garnet (pink), glaucophane (blue) in an eclogite. PPL image, 2x (Field of view = 7mm)		 eclogite1(5).jpg

Omphacite (green), garnet (pink), glaucophane (blue) in an eclogite. PPL image, 2x (Field of view = 7mm)
eclogite1(6).jpg

Omphacite (green), garnet (pink), glaucophane (blue) in an eclogite. PPL image, 2x (Field of view = 7mm)		 eeclogitee(12).jpg

Omphacite (green), garnet (pink), glaucophane (blue) in an eclogite. PPL image, 10x (Field of view = 27mm)		 omphacitebohemia(13).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)
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Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)		 omphacitebohemia(15).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)		 omphacitebohemia(16).jpg

Omphacite, garnet (isotropic) in an eclogite from Bohemia. XPL image, 2x (Field of view = 7mm)
omphacitebohemia(17).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)		 omphacitebohemia(18).jpg

Omphacite, Quartz, garnet (isotropic) in an eclogite from Bohemia. XPL image, 2x (Field of view = 7mm)		 omphacitebohemia(19).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)
omphacitebohemia(20).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)		 omphacitebohemia(21).jpg

Omphacite (pale green), garnet (pink) in an eclogite from Bohemia. PPL image, 2x (Field of view = 7mm)		 omphacitebohemia(22).jpg

Omphacite, quartz, garnet (isotropic) in an eclogite from Bohemia. XPL image, 2x (Field of view = 7mm)