May 13 2014
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