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Omphacite

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Title: Omphacite  
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Subject: Pyroxene, Metamorphic facies, WikiProject Rocks and minerals/Worklist, Augite, Inosilicates
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Omphacite

Omphacite
General
Category Pyroxene
Formula
(repeating unit)
(Ca,Na)(Mg,Fe2+,Al)Si2O6
Strunz classification 09.DA.20
Dana classification 65.01.03b.01 clinopyroxene
Crystal symmetry Monoclinic 2/m
Unit cell a = 9.66 Å, b = 8.81 Å, 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
Crystal system Monoclinic - Prismatic
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]

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.

The name "omphacite" has been applied to compositions that contain between 20% and 80% jadeite. The stability of intermediate compositions between augite and jadeite is not well-understood, but miscibility gaps appear to be present at temperatures below 300 °C to 400 °C, and perhaps at higher temperatures. Pairs of pyroxenes -- both augite plus omphacite and omphacite plus jadeite -- appear to have existed in equilibrium at low temperatures.[5]

It was first described in 1815 in the Münchberg Metamorphic complex, Franconia, Bavaria, Germany. The name omphacite derives from the Greek omphax or unripe grape for the typical green color.

References

  1. ^ Hurlbut, Cornelius S.; Klein, Cornelis, 1985, Manual of Mineralogy, 20th ed., pp. 398 - 405, John Wiley and Sons, New York ISBN 0-471-80580-7
  2. ^ Handbook of Mineralogy
  3. ^ Mindat.org
  4. ^ Webmineral data
  5. ^ Tatsuki Tsujimori, Juhn G. Liou, and Robert G. Coleman, 2005, Coexisting retrograde jadeite and omphacite in a jadeite-bearing lawsonite eclogite from the Motagua Fault Zone, Guatemala. American Mineralogist, v. 90, pp. 836-842
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