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Omphacite

Omphacite
Eclogite Norway.jpg
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,Fe2+,Al)Si2O6
Strunz classification 9.DA.20
Dana classification 65.01.03b.01
(clinopyroxene)
Crystal system Monoclinic
Crystal class Prismatic (2/m)
(same H-M symbol)
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

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.


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Wikipedia

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