clinopyroxenes

Clinopyroxenes are a group of monoclinic pyroxene minerals that form a major, color-variable component of many igneous and metamorphic rocks. They represent the monoclinic counterpart to orthopyroxenes and occur as part of large solid-solution series with varying calcium, magnesium, iron, sodium, and aluminum contents. The general composition is often written as XY(Si,Al)2O6, with X and Y representing large cation sites that accommodate Ca and/or Na, and Mg, Fe, and Al.

The most common endmembers are diopside CaMgSi2O6 and hedenbergite CaFeSi2O6, while augite represents a complex sodic-calcium pyroxene with Mg-Fe-Al and Si-Al substitutions. Other notable members include jadeite NaAlSi2O6, aegirine NaFeSi2O6, and the high-pressure omphacite, a jadeite-diopside solid solution found in eclogites. Clinopyroxenes exhibit a monoclinic crystal system (space group C2/m) and typically occur as prismatic to blocky crystals.

Key physical properties include two good cleavages intersecting at about 87–93 degrees, a hardness around 5–6 on the Mohs scale, and a specific gravity near 3.2–3.6. Color ranges from green to brown or black, and they commonly display weak pleochroism. Diagnostic features arise from their crystal structure and chemical substitutions, especially aluminum in the tetrahedral and octahedral sites.

Geologically, clinopyroxenes are essential constituents of basaltic and dioritic rocks and are also important in metamorphic rocks formed at high temperatures. They are used in geothermobarometry and as indicators of magma composition and pressure–temperature history. They can be distinguished from orthopyroxenes by their monoclinic symmetry, two cleavages near 90 degrees, and characteristic solid-solution behavior.