An important orthorhombic pyroxene (orthopyroxene, or opx) Mg₂Si₂O₆ and end-member of an isomorphous series, the other end-member being orthoferrosilite (Fs) Fe₂Si₂O₆; this series forms the base of a compositional equilateral triangle whose third apex is wollastonite (CaSiO₃) and into which the common pyroxene minerals (augite, diopside, pigeonite) can be plotted, as well as olivine; sp. gr. 3.2 (En) to 3.96 (Fs); hardness 5.5; colourless or greyish-white, with a green tinge, or brownish-green; vitreous lustre; crystals are prismatic or tabular, but normally forming very irregular grains; cleavage good prismatic {110}, poor {010}, {100}; occurs in magnesium-rich magmatic rocks (e.g. peridotites, gabbros, norites, and basalts) and in contact metasomatic zones associated with these rocks; and in partial melt envelopes around very high-temperature basic igneous intrusions.