Pyroxenite

Pyroxenite is a coarse-grained ultramafic igneous rock made up of more than about 90% pyroxene, with minor olivine, hornblende, biotite, or chromite. It is very dark, dense, and heavy, reflecting its iron- and magnesium-rich composition.

Pyroxenites are subdivided by their dominant pyroxene: orthopyroxenites (orthopyroxene), clinopyroxenites (clinopyroxene), and websterites (both). They commonly occur as layers, dikes, or veins within larger mafic and ultramafic complexes.

Though not a household rock, pyroxenite is geologically important as a building block of the Earth's mantle and lower crust.

Pyroxenite forms by the accumulation of dense pyroxene crystals that settle out of cooling mafic magma, a process called crystal settling, within layered intrusions such as the Bushveld Complex in South Africa and the Stillwater Complex in Montana.

It also forms in the upper mantle and at the base of the crust, and can occur as cumulate layers alongside peridotite, dunite, and gabbro. Some pyroxenites originate as reaction products where mantle peridotite interacts with infiltrating melts.

Pyroxenite is recognizable as a very dark green-to-black, dense, coarsely crystalline rock that feels notably heavy. The interlocking pyroxene crystals often show good cleavage and a glassy-to-dull luster, with pyroxene faces meeting at roughly 90-degree cleavage angles.

Hardness is about 5-6. Distinguish from peridotite by the dominance of dark pyroxene over green olivine, and from gabbro by the near-absence of light-colored plagioclase feldspar. From basalt it is told apart by its coarse, visible crystals.

Pyroxenite-rich layered intrusions are major sources of chromium, platinum-group metals, and nickel, making them economically valuable host rocks. The rock itself is also crushed for durable aggregate and road stone.

It has no gemstone or metaphysical tradition, but it is a key sample for scientists studying mantle composition, magma differentiation, and ore-forming processes.