omphacite

{{infobox mineral | name = Omphacite | category = Pyroxene | boxwidth = | boxbgcolor = #7a5e59 | boxtextcolor = #FFFFFF | image = Eclogite Norway.jpg | imagesize = | alt = | caption = 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). | formula = (Ca,Na)(Mg,Fe2+,Al)Si2O6 | IMAsymbol = Omp | molweight = | strunz = 9.DA.20 | dana = 65.01.03b.01 (clinopyroxene) | system = Monoclinic | class = Prismatic (2/m) (same H-M symbol) | symmetry = P2/n or C2/c | unit cell = a = 9.66, b = 8.81, c = 5.22 [Å]; β = 106.56°; Z = 4 | color = Green to dark green; colorless to pale green in thin section | colour = | habit = Rarely in rough crystals; anhedral, granular to massive | twinning = Single and polysynthetic twinning common on {100} | cleavage = Good on {110}, {110} ^ {10} ≈87°; parting on {100} | fracture = Uneven to conchoidal | tenacity = Brittle | mohs = 5–6 | luster = Vitreous to silky | streak = Greenish white | diaphaneity = Translucent | gravity = 3.16–3.43 | density = | polish = | opticalprop = Biaxial (+) | refractive = 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 = Measured: 58° to 83°, Calculated: 74° to 88° | dispersion = | extinction = | length fast/slow = | fluorescence= | absorption = | melt = | fusibility = | diagnostic = | solubility = | other = | alteration = | 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.

== Formation and occurrence == alt=|left|thumb|330x330px|Phase diagram of slab crust in the Earth's upper mantle from 200 to 500 km depth. Omphacite general dissolves into garnet as depth increases. Omphacite can stable up to ~500 km depth. Omphacite is the dominant phase in the subducted oceanic crust in the Earth's upper mantle. The Mid-Ocean Ridge Basalt, which makes up oceanic crust, goes through ultrahigh-pressure metamorphic process and transforms to eclogite at depth ~60 km in the subduction zones. The major mineral components of eclogite include omphacite, garnet and high-pressure silica phases (coesite and stishovite). As depth increases, the omphacite in eclogite gradually transforms to majoritic garnet. Omphacite is stable up to 500 km depth in the Earth's interior. Considering the cold geotherm of subducted slabs, omphacite can be stable even in deeper mantle.