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 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 [1][2][3][4]