Pyroxenite is an ultramafic plutonic igneous rock. Ultramafic means that more than 90% of the rock is composed of magnesium- and iron-rich minerals like pyroxenes, amphiboles, and olivine. In pyroxenite the dominant mafic mineral is a pyroxene. Pyroxenite may contain up to 40% olivine. More than that means that the rock is peridotite. If felsic minerals like feldspars constitute more than 10% of the rock, it is melanocratic gabbro. Hornblendite is a similar igneous rock, but it is composed of amphiboles instead of pyroxenes.

Pyroxenite may be almost monomineralic pyroxene rock. Hullvann, Norway. Width of sample 8 cm.

Pyroxenite is uncommon at the surface because it occurs predominantly deep in the crust or in the mantle and it is susceptible to both metasomatic alteration and weathering. It usually occurs together in the same igneous complex with peridotite. Pyroxenite mostly forms by accumulation of pyroxene crystals in the mafic-ultramafic igneous intrusions. It forms variously shaped igneous bodies like sills, layers, dikes, etc.

Pyroxenite is a common rock in the lower part of the oceanic crust, where it is associated with depleted peridotite. “Depleted” means that peridotite has gone through partial melting to yield basaltic melt, which forms the upper part of the oceanic crust (sheeted dikes and pillow lavas). Depleted peridotite is rich in orthopyroxene and associated pyroxenites are usually either or olivine ortho pyroxenites. Pyroxenite may occur as xenoliths in volcanic rocks.

Metamorphic rocks with a pyroxenite composition should be named differently. These rocks are usually high-grade granulites. The rock name pyroxenite should be reserved to true igneous rocks to avoid unwanted confusion and ambiguity.

Pyroxenite and peridotite may host economically important metals like chromium, nickel, platinum, iridium, osmium, etc. Serpentine and talc are mined from metamorphosed ultramafic rocks.