pyroxenite

Pyroxenite is an ultramafic to mafic igneous rock in which pyroxene minerals make up the overwhelming majority of the mineralogy. In typical pyroxenites feldspar is absent or present only as a trace, and the rock often has a dense, dark appearance.

Mineralogy is dominated by pyroxenes: clinopyroxene (diopside-augite series) and orthopyroxene (enstatite, ferrosilite). Olivine may occur as a minor constituent in some varieties, yielding websterite when accompanied by substantial olivine. Accessory minerals can include chromian spinel, apatite, ilmenite, and sulfides.

Subtypes include orthopyroxenite (mostly orthopyroxene), clinopyroxenite (mostly clinopyroxene), and websterite (pyroxenes with significant olivine). When iron-rich bronzitic orthopyroxene dominates, the rock may be described as bronzite pyroxenite; terminology reflects pyroxene composition and texture rather than a single uniform category.

Formation and occurrence: pyroxenites commonly form as cumulates in magma chambers of layered intrusions, by fractional crystallization of basaltic magmas. They also occur as mantle-derived rocks in ophiolites and can be produced by high-temperature metamorphism of other ultramafic rocks, yielding metapyroxenites.

Significance: as pyroxene-rich cumulates, pyroxenites provide information on mantle composition, magmatic differentiation, and crystallization history of basaltic magmas. They are used in studies of tectonic settings such as mid-ocean ridges, subduction zones, and layered intrusions.