Omphacite

Omphacite
Omphacite
General
Category Pyroxene
Chemical formula (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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Look at other dictionaries:

  • Omphacite — Catégorie IX : silicates[1] Omphacite (en vert) entrant dans la composition d une éclogite …   Wikipédia en Français

  • omphacite — ● omphacite nom féminin (grec omphaks, akos, raisin vert) Silicate naturel de la famille des pyroxènes, monoclinique, de couleur verte, caractéristique des éclogites …   Encyclopédie Universelle

  • omphacite — /om feuh suyt /, n. Mineral. a pale green variety of pyroxene similar to olivine, found in eclogite. [1820 30; < G Omphazit < Gk omphakítes green stone, equiv. to omphak (s. of ómphax) unripe grape + ites ITE1] * * * …   Universalium

  • omphacite — noun any of a range of green, monoclinic pyroxene minerals found in eclogites and similar rocks; they are solid solutions of jadeite and diopside …   Wiktionary

  • omphacite — om·pha·cite …   English syllables

  • omphacite — ˈäm(p)fəˌsīt noun ( s) Etymology: German omphazit, from Greek omphakitēs green stone, from omphak , omphax unripe grape + itēs ite : a mineral consisting of a grass green granular or foliated pyroxene found in the rock eclogite …   Useful english dictionary

  • Eclogite — Éclogite Éclogite de Norvège Une éclogite (du grec eklogê, choix) est une roche métamorphique qui s est formée dans le faciès éclogitique, c est à dire ayant subi les conditions de pression et température de ce champ. Historiquement, Eskola a… …   Wikipédia en Français

  • Éclogite — de Norvège Une éclogite (du grec eklogê, choix) est une roche métamorphique qui s est formée dans le faciès éclogitique, c est à dire ayant subi les conditions de pression et température de ce champ. Historiquement, Eskola a défini les faciès… …   Wikipédia en Français

  • Éclogites — Éclogite Éclogite de Norvège Une éclogite (du grec eklogê, choix) est une roche métamorphique qui s est formée dans le faciès éclogitique, c est à dire ayant subi les conditions de pression et température de ce champ. Historiquement, Eskola a… …   Wikipédia en Français

  • pyroxene — pyroxenic /puy rok sen ik/, adj. /puy rok seen, peuh , puy rok seen /, n. any of a very common group of minerals of many varieties, silicates of magnesium, iron, calcium, and other elements, occurring as important constituents of many kinds of… …   Universalium

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