Omphacite

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,Fe2+,Al)Si2O6
IMA symbol	Omp
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 clinopyroxene 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 clinopyroxene. It normally appears in eclogite, which is the high-pressure metamorphic rock of basalt. Omphacite is the solid solution of Fe-bearing diopside and jadeite. It crystallizes in the monoclinic system with prismatic, typically twinned forms, though usually anhedral. Its space group can be P2/n or C2/c depending on the thermal history. 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.

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