Omphacite
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| Omphacite | |
|---|---|
 | |
| General | |
| Category | Pyroxene |
| Formula | (Ca,Na)(Mg,Fe2+,Al)Si2O6 |
| IMA symbol | Omp[1] |
| 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 | [2][3][4][5] |
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.[6] 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.[7] 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.
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.[9] The major mineral components of eclogite include omphacite, garnet and high-pressure silica phases (coesite and stishovite).[8] 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.[8][10] Considering the cold geotherm of subducted slabs, omphacite can be stable even in deeper mantle.
It also occurs in blueschist facies and ultrahigh-pressure metamorphic rocks.[11] It is also found in eclogite xenoliths from kimberlite as well as in crustal rocks metamorphosed at high pressures.[12] Associated minerals in eclogites except the major minerals include 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.
Chemical composition
Omphacite is the solid solution of Fe-bearing diopside (CaMgSi2O6) and jadeite (NaAlSi2O6). Depending on how much the coupled substitution of (Na, Al)-(Mg-Fe, Ca) happens, the chemical composition of omphacite varies continuously from pure diopside to pure jadeite.[6] Due to the relatively small radius of (Na, Al) atoms, the unit cell volume linearly decreases as jadeite component increases.[13] In addition, the coupled substitution also stiffens the crystals. The bulk and shear modulus linearly increases as jadeite component increases.[6]
