Omphacite

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
CategoryPyroxene
Formula
(repeating unit)
(Ca,Na)(Mg,Fe2+,Al)Si2O6
IMA symbolOmp[1]
Strunz classification9.DA.20
Dana classification65.01.03b.01
(clinopyroxene)
Crystal systemMonoclinic
Crystal classPrismatic (2/m)
(same H-M symbol)
Space groupP2/n or C2/c
Unit cella = 9.66, b = 8.81,
c = 5.22 [Å]; β = 106.56°; Z = 4
Identification
ColorGreen to dark green; colorless to pale green in thin section
Crystal habitRarely in rough crystals; anhedral, granular to massive
TwinningSingle and polysynthetic twinning common on {100}
CleavageGood on {110}, {110} ^ {110} ≈87°; parting on {100}
FractureUneven to conchoidal
TenacityBrittle
Mohs scale hardness5–6
LusterVitreous to silky
StreakGreenish white
DiaphaneityTranslucent
Specific gravity3.16–3.43
Optical propertiesBiaxial (+)
Refractive indexnα = 1.662 – 1.701 nβ = 1.670 – 1.712 nγ = 1.685 – 1.723
Birefringenceδ = 0.023
PleochroismWeak; X = colorless; Y = very pale green; Z = very pale green, blue-green
2V angleMeasured: 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.

  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ 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
  3. ^ Handbook of Mineralogy
  4. ^ Mindat.org
  5. ^ Webmineral data
  6. ^ Hao, Ming; Pierotti, Caroline E.; Tkachev, Sergey; Prakapenka, Vitali; Zhang, Jin S. (2019). "The single-crystal elastic properties of the jadeite-diopside solid solution and their implications for the composition-dependent seismic properties of eclogite". American Mineralogist. 104 (7): 1016–1021. Bibcode:2019AmMin.104.1016H. doi:10.2138/am-2019-6990. ISSN 0003-004X. S2CID 195790171.
  7. ^ Fleet, M. E.; Herzberg, C. T.; Bancroft, G. M.; Aldridge, L. P. (1978). "Omphacite studies; I, The P2/n-->C2/c transformation". American Mineralogist. 63: 1100–1106.