Orthopyroxenites

Orthopyroxenite is an ultramafic igneous rock in which orthopyroxene is the dominant mineral. The orthopyroxene is typically enstatite (MgSiO3) or ferrosilite (FeSiO3), and it may constitute most of the rock’s modal composition. Accessory minerals commonly present include olivine, clinopyroxene, chromite or spinel, magnetite, and sulfides; feldspars are absent or negligible.

Textures in orthopyroxenite are usually granular, but cumulate variants are common in layered mafic–ultramafic intrusions, where orthopyroxene forms extensive, coalesced crystals during fractional crystallization. In some settings orthopyroxenite can occur as nodules in basaltic rocks or as mantle-derived xenoliths brought to the surface by magmas.

Orthopyroxenite is one member of the pyroxenite group, distinguished by the dominance of orthopyroxene over clinopyroxene. In contrast, clinopyroxenite is dominated by clinopyroxene, and rocks with substantial amounts of both pyroxenes may be described as pyroxenites with varying modal proportions. Orthopyroxenite commonly occurs alongside other ultramafic lithologies such as dunite (olivine-rich) and harzburgite (olivine and orthopyroxene) in mantle-derived sequences and in layered intrusions.

Formation and significance: orthopyroxenite commonly forms as cumulate material during crystallization of ultramafic magmas in layered intrusions or as a mantle-derived assemblage preserved in xenoliths. Studying orthopyroxenite provides insights into crystallization processes, mantle composition, and the evolution of ultramafic rocks.