What is Omphacite?
Omphacite is a dark-green pyroxene and a diagnostic constituent of eclogite, recording high-pressure metamorphism deep in the Earth.
Mineralogy and crystal structure
Reference formula: (Ca,Na)(Mg,Fe,Al)Si2O6. Omphacite is a sodium-calcium magnesium-iron pyroxene with variable aluminium. The COD model is a representative monoclinic composition; zoning and exsolution can make natural grains chemically complex. Omphacite is classified with silicates and has a monoclinic crystal system. Typical hardness is 5–6 on the Mohs scale and specific gravity is approximately 3.2–3.6.
Colour, lustre and habit
Typical colours include dark green, green, black, brown. The usual granular, prismatic, massive habit and vitreous, pearly lustre help describe a specimen, but colour alone is not diagnostic.
Formation and notable localities
Omphacite forms in or is associated with high-pressure metamorphism of basaltic rocks, eclogite bodies and some mantle-derived xenoliths. Important documented localities include Norway, Austria, Germany, Italy, the Czech Republic, Japan and the United States.
The green fingerprint of eclogite
Omphacite is a mineralogical pressure gauge. Alongside garnet, its green grains preserve the journey of basalt into the deep metamorphic roots of mountain belts.
Collecting and care
Protect edges from abrasion and keep the rock association intact; thin sections are often more diagnostic than hand specimens.
