Omphacite

Omphacite
Picture of pieces of eclogite (type of rock) from the Western Gneiss Region in Norway. The rock contains the minerals omphacite (green), pyrope-garnet (red), quartz (milky), kyanite (blue) and some phengite (golden white).
General
Category	Pyroxene
Formula (repeating unit)	(Ca,Na)(Mg,Fe^2+,Al)Si2O6
Strunz classification	9.DA.20
Dana classification	65.01.03b.01 (clinopyroxene)
Crystal system	Monoclinic
Crystal class	Prismatic (2/m) H-M symbol (2/m)
Space group	P2/n or C2/c
Unit cell	a = 9.66, b = 8.81, c = 5.22 [Å]; β = 106.56°; Z = 4
Identification
Color	Green to dark green; colorless to pale green in thin section
Crystal habit	Rarely in rough crystals; anhedral, granular to massive
Twinning	Single and polysynthetic twinning common on {100}
Cleavage	Good on {110}, {110} ^ {110} ≈87°; parting on {100}
Fracture	Uneven to conchoidal
Tenacity	Brittle
Mohs scale hardness	5-6
Luster	Vitreous to silky
Streak	Greenish white
Diaphaneity	Translucent
Specific gravity	3.16-3.43
Optical properties	Biaxial (+)
Refractive index	nα = 1.662 - 1.701 nβ = 1.670 - 1.712 nγ = 1.685 - 1.723
Birefringence	δ = 0.023
Pleochroism	Weak; X = colorless; Y = very pale green; Z = very pale green, blue-green
2V angle	Measured: 58° to 83°, Calculated: 74° to 88°
References	[1][2][3][4]

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 pyroxene. Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite. It crystallizes in the monoclinic system with prismatic, typically 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.^[5]

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.

References

1. 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
2. Handbook of Mineralogy
3. Mindat.org
4. Webmineral data
5. 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