Pyroxenite

Pyroxenite is an ultramaficigneousrock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenes have the general formula XY(Si,Al)₂O₆, where X represents ions of calcium (Ca), sodium (Na), iron (Fe(II)) or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron (Fe(II) or Fe(III)). Pyroxenes share a common structure consisting of single chains of silica tetrahedra; those that crystallize in the monoclinic system are known as clinopyroxenes and those that crystallize in the orthorhombic system are known as orthopyroxenes. Accordingly, pyroxenites are classified into clinopyroxenites and orthopyroxenites. The websterites contain both types of pyroxenes (see diagram below). Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.

Most pyroxenites are derived from upper mantle, either as solid blocks and fragments, or more commonly as crystals accumulated from magmas that formed in the mantle. They commonly occur alongside peridotites. Pyroxenites are essentially of igneous origin, though some pyroxenites are included in the metamorphicLewisian complex of Scotland where the pyroxene-rich rocks result from the type of contact metamorphism known as pyroxene-hornfels facies, have siliceous sediment or basaltic protoliths, and are respectively metapelites and .