omphacite

Omphacite is a clinopyroxene mineral that forms the jadeite-augite solid solution. It represents a spectrum of compositions between jadeite (NaAlSi2O6) and augite ((Ca,Mg,Fe)Si2O6), with aluminum and sodium substituting in the crystal structure and various cations (Ca, Mg, Fe) occurring in solid solution. In practice, omphacite typically contains a jadeite-rich component, but the exact composition can vary.

Omphacite is a characteristic mineral of high-pressure metamorphic rocks, especially eclogites, which form in subduction zones. It is commonly found together with garnet and other high-pressure minerals, and rocks containing omphacite record eclogite facies metamorphism. The mineral is usually green, sometimes with yellowish or brownish tones, and crystals are commonly elongated. It exhibits the two good cleavages typical of pyroxenes.

Physical properties include a monoclinic crystal system and a vitreous to pearly luster. It has a hardness near 6 and a specific gravity around 3.3–3.6. Omphacite displays two cleavages at angles close to 90 degrees and a typically prismatic habit. Its occurrence and composition make it an important indicator for studying subduction-related metamorphism and for geobarometry in high-pressure rocks.