Omphacite is a mineral that belongs to the pyroxene group, specifically the calcium-rich clinopyroxene subgroup. It is typically green in color, ranging from pale green to dark green or even black. The name "omphacite" is derived from the Greek word "omphax," meaning unripe grape, which is a reference to the green color of the mineral.
Omphacite is commonly found in metamorphic rocks, particularly those formed during high-pressure conditions in subduction zones. It occurs as a major component in eclogite, a type of metamorphic rock that results from the transformation of basalt or gabbro under extreme pressure and temperature conditions. Omphacite can also occur in some granulites, which are high-grade metamorphic rocks.
With its monoclinic crystal structure, omphacite possesses distinct cleavage planes that intersect at an approximate angle of 87 degrees. It has a high density and is classified as moderately hard on the Mohs scale, typically ranging between 5 to 6. Its chemical composition consists mainly of calcium, magnesium, iron, and aluminum, with small amounts of other elements such as sodium and titanium.
Omphacite's distinctive green color is a result of the presence of iron within its crystal structure. It often exhibits a prismatic habit, forming elongated crystals or columnar aggregates. Under polarized light, omphacite displays pleochroism, meaning it shows different colors depending on the orientation of the mineral. In thin sections, it appears as green to colorless, with variations in intensities and shades.
Due to its occurrence in eclogite, omphacite is considered an indicator mineral for high-pressure metamorphism. Its presence can provide important clues about
