Orthorhombic pyroxene

(mineralogy) A member of the mineral series enstatite-orthoferrosilite, crystallizing in the orthorhombic system, space group Pbca.

A group of minerals having the general chemical formula XYSi₂O₆, in which the Y site contains Fe or Mg and the X site contains Fe, Mg, Mn, or a small amount of Ca (up to about 3%). The end members of this solid solution series are enstatite (Mg₂Si₂O₆) and ferrosilite (Fe₂Si₂O₆). Names used for intermediate members of the series are enstatite, bronzite, hypersthene, and orthoferrosilite.

Many of the physical and optical properties of orthopyroxene are strongly dependent upon composition, and especially upon the Fe-Mg ratio. In hand specimens, orthopyroxene can be distinguished from amphibole by its characteristic 88° cleavage angles, and from augite by color—augite is typically green to black, while orthopyroxene is more commonly brown, especially on slightly weathered surfaces.

Orthopyroxene is a widespread mineral in metamorphic rocks. It is characteristic of granulite facies metamorphism, in both mafic and feldspathic gneisses. Orthopyroxene occurs in many basalts and gabbros, particularly those of tholeiitic composition, and many meteorites, but is notably absent from most alkaline igneous rocks. The greatest abundance of orthopyroxene is in ultramafic rocks, especially those in large layered intrusions. See also Pyroxene.