peridotite A coarse-grained, ultrabasic, igneous rock consisting of essential magnesium-rich olivine accompanied by lesser amounts of other ferromagnesian minerals, e.g. orthopyroxene (enstatite-bronzite), clinopyroxene (chromium-diopside), and chromite, with or without pyrope garnet. Peridotites are found in large, layered, ultrabasic, intrusive bodies, in ophiolite complexes, and as xenoliths brought up in alkali basalts and kimberlites. As well as forming by crystal fractionation processes in igneous bodies, their mineral chemistry and presence as xenoliths in kimberlites suggest that much of the Earth's mantle, and therefore of the mass of the Earth, is peridotite. Meteorites are also composed largely of peridotite, suggesting that peridotites are probably the commonest rock in the solar system.

Peridotite

Peridotite is a dark-colored, coarse-grained igneous rock believed by many scientists to be the primary rock of the exterior of Earth's mantle. The rock typically forms in volcanic pipes and is forced to the surface from great depths during a volcanic eruption.

Peridotite consists of a dense iron and magnesia mineral called olivine , as well as pyroxenes and a small amount of feldspar . It is a pistachio-green color when fresh, but weathering creates iron oxides that turn it a medium brown. The iron and magnesia-rich rock is the most common host for naturally occurring diamonds. South African diamonds are obtained from a mica-rich form of peridotite called kimberlite. Periodite is also a source of valuable ores and minerals including chromite, platinum, nickel, and precious garnet. In rare instances, individual olivine crystals in periodite are large enough and pure enough to be designated a gem. The resulting gem is a semi-precious mineral called peridot.

Periodite is found worldwide, but particularly in New Zealand.

See also Gemstones; Igneous rocks; Plagioclase

peridotite Heavy igneous rock of coarse texture composed of olivine and pyroxene with small flecks of mica or hornblende. It alters readily into serpentine.