Editorial Feature

Omphacite – Occurrence, Properties and Distribution

Omphacite was discovered in 1815 at the Ecolgite outcrops. It is named after the Greek word for an unripe grape, in allusion to its green color. It is the primary constituent of the rock eclogite. It belongs to the pyroxene group.

Properties of Omphacite

The key physical properties of Omphacite are as below:

  • Cell Data
    • Space Group = C2/c or P2/n
    • a = 9.45-9.68
    • b = 8.57-8.90
    • c = 5.23-5.28
    • Z = 4

  • Crystal Data
    • Monoclinic
    • Point Group – 2/m
    • Crystals – rarely in rough crystals, to 5cm; anhedral, granular to massive
      Twinning: single and polysynthetic twinning on {100}, common
    • X-ray powder pattern: Hareidland, Sunmore, Norway
      2.976 (100), 1.402 (80), 2.127 (70), 2.118 (70), 2.019 (70), 4.4 (50), 3.19 (50)

  • Chemical Composition
    Elements Content 1 Content 2
    SiO2 54.08 55.25
    TiO2 0.14 0.23
    Al2O3 9.20 16.17
    Fe2O3 0.83 1.64
    Cr2O3 - 0.04
    FeO 2.18 1.36
    MnO Trace 0.04
    MgO 11.51 7.23
    CaO 17.50 12.34
    Na2O 4.20 6.19
    K2O 0.04 0.11
    H2O+ 0.46 -

  • Optical Properties
    • Translucent
    • Lustre – vitreous to silky
    • Color - green to dark green, colorless to pale green in thin section
    • Optical Class – Biaxial (+)

  • Estimated Properties
    Electron density Bulk density (electron density) = 3.32 g/cm3
    note: Specific gravity of Omphacite = 3.34 g/cm3
    Photoelectric PEOmphacite = 3.51 barns/electron
    U = PEOmphacite x ρelectron density = 11.66 barns/cm3
    Fermion index Fermion index = 0.02
    Boson index = 0.98
    Radioactivity
     
    GRapi = 0 (Gamma Ray American Petroleum Institute Units)
    Omphacite is not radioactive

How to Identify Omphacite

The color of omphacite varies from green to dark green, and from colorless to pale green in the thin sections. It is brittle by nature, having a hardness of 5 to 6. Its lustre is vitreous to silky.

Global Distribution

Omphacite is a mineral that is found in a number of locations. Some of the main locations are below:

  • Germany – from Hof, Bavaria
  • Austria – from Saualpe and Gertrusk, Carinthia
  • Norway – from More, Vanelvsdalen, Naustdal and Sunmore
  • Russia – from Yakutia diamond pipes
  • South Africa – from Kimberly mines, Cape Province
  • Japan – from Ohmi district and Gumma Prefecture
  • USA – from Sonoma Co. California; Tiburon Peninsula

Occurrence of Omphacite and Useful Mineral Association

Omphacite is a major component of the eclogite rock, hence is found in kimberlite pipes, from some ophiolites, and glaucophane-bearing bluechist facies rocks.

Some of the minerals with which omphacite is associated are garnet, hornblende, scapolite, epidote, galucophane, and kyanite.

References

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