Peridotite

According to a 2008 study, peridotite may potentially be used in a low-cost, safe and permanent method of capturing and storing atmospheric CO2 as part of climate change-related greenhouse gas sequestration. It was already known that peridotite reacts with CO2 to form a solid carbonate-like limestone or marble mineral; and the study concluded that this process can be sped up a million times or more by simple drilling and hydraulic fracturing to allow injection of the CO2 into the subsurface peridotite formation. Peridotite is named for the gemstone peridot, a glassy green gem mined in Asia and Arizona (Peridot Cove). Some peridotite is mined for ornamental stone. Peridotite that has been hydrated at low temperatures forms serpentinite, which may include chrysotile asbestos (a form of serpentine) and talc. Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of the platinum used in the world today is mined from the Bushveld Igneous Complex in South Africa and the Great Dyke of Zimbabwe. The chromite bands found in peridotites are the world's major source of chromium.

Peridotite have two primary modes of origin, as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas; these magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites.

Olivine is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe)2SiO4; the pyroxenes are chain silicates having the variable formula (Ca,Na,Fe,Mg)(Cr,Al,Fe,Mg,Mn,Ti,V)Si2O6 comprising a large number of different minerals. Magnesium-rich olivine forms a large proportion of peridotite, and so magnesium content is high. Layered igneous complexes have much more varied compositions, depending on the fractions of pyroxenes, chromite, plagioclase, and amphibole.

Characterized by high Cr and Ni concentrations.

Peridotite is a source of many gemstones, including diamonds. It is also a source of chromium and platinum ore, popularly used metal alloys, as well as talc which is commonly used in baby powder, cosmetics, and chalk. Peridotite may also be a good source of other minerals and elements, including iron, nickel, and cobalt.

Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine is converted to the higher-pressure mineral wadsleyite. Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust; the crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into the mantle in subduction zones. However, pieces can be emplaced into or overthrust on continental crust by a process called obduction, rather than carried down into the mantle; the emplacement may occur during orogenies, as during collisions of one continent with another or with an island arc. The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites; typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro, pillow basalt, diabase sill-and-dike complexes, and red chert. Other masses of peridotite have been emplaced into mountain belts as solid masses but do not appear to be related to ophiolites, and they have been called "orogenic peridotite massifs" and "alpine peridotites." Peridotites also occur as fragments (xenoliths) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite. Certain volcanic rocks, sometimes called komatiites, are so rich in olivine and pyroxene that they also can be termed peridotite. Small pieces of peridotite have even been found in lunar breccias. The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite, a process in which the pyroxenes and olivines are converted to green serpentine. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in the serpentine group, chrysotile, is a type of asbestos.