Pyroxene

Pyroxene is a group of inosilicate minerals characterized by single-chain silicate (SiO3) tetrahedra that share two oxygens to form chains. These chains are linked by metal cations to create a three-dimensional framework. The crystal structure commonly yields two prominent cleavage planes that intersect near 90 degrees, helping distinguish pyroxenes from amphiboles.

The general chemical formula for pyroxenes is XY(Si,Al)2O6, where X and Y are metal cations such as Ca, Na, Mg, Fe2+, Mn, or Al3+. Aluminium and alkali or alkaline earth cations can substitute for silicon in the chain, producing extensive solid-solution series. Pyroxenes are subdivided mainly into orthopyroxenes (low-clin orientation, exemplified by enstatite MgSiO3 and ferrosilite FeSiO3) and clinopyroxenes (monoclinic, such as diopside CaMgSi2O6 and augite Ca(Mg,Fe,Al)Si2O6).

Common members include enstatite, ferrosilite, diopside, hedenbergite, augite, jadeite (NaAlSi2O6), aegirine (NaFeSi2O6), and spodumene (LiAlSi2O6). Pyroxenes form in a wide range of igneous rocks, including basalts, andesites, and gabbros, as well as in metamorphic rocks such as granulites. They record high-temperature, high- to moderately high-pressure conditions and are useful for interpreting magmatic and metamorphic histories.

Physically, pyroxenes vary in color from green to brown to black and typically have a vitreous to dull luster. They are important in petrographic analysis because their composition and zoning reveal crystallization sequences and tectonic settings.