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Coarse grained ultramafic igneous rock type

Peridotite (/US/'/p//ɛ//r//ɪ//d//oʊ/,/t//aɪ//t//, //p//ə/'/r//ɪ//d//ə// / PERRihdohtyte, pəRIDə ) is a dense, coarse grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole. Peridotite is the dominant rock of the upper part of Earth's mantle. The compositions of peridotite nodules found in certain basalts are of special interest along with diamond pipes (kimberlite), because they provide samples of Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of the nodules preserve isotope ratios of osmium and other elements that record processes that occurred when Earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of Earth's mantle and the complexities of the processes that occurred. The word peridotite comes from the gemstone peridot, which consists of pale green olivine. Classic peridotite is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite. While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red.

Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89. In other words, of the total content of iron plus magnesium, 89 mol% is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite. Olivine is the essential mineral found in all peridotites. It is a magnesium orthosilicate containing some iron with the variable formula |(Mg,Fe)2SiO4. The magnesium rich olivine of peridotites is typically olive green in color. Pyroxenes are chain silicates having the variable formula |(Ca,Na,Fe^{II},Mg)(Cr,Al,Fe^{III},Mg,Mn,Ti,V)Si2O6 comprising a large group of different minerals. These are divided into orthopyroxenes (with an orthorhombic crystal structure) and clinopyroxenes (with a monoclinic crystal structure). This distinction is important in the classification of pyroxene peridotites

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. Alpine peridotite or orogenic peridotite massif is an older term for an ophiolite emplaced in a mountain belt during a continent continent plate collision. 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. Although kimberlite is a variant of peridotite, kimberlite is also considered as brecciated volcanic material as well, Such xenoliths originate from depths of up to nearly or more. The volcanic equivalent of peridotites are komatiites, which were mostly erupted early in the Earth's history and are rare in rocks younger than Archean in age. Small pieces of peridotite have 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. 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.

Most peridotite is green in color due to its high olivine content. However, peridotites can range in color from greenish gray to nearly black to pale yellowish green. Peridotite weathers to form a distinctive brown crust in subaerial exposures and to a deep orange color in submarine exposures. Peridotites can take on a massive form or may be in layers on a variety of size scales. Layered peridotites may form the base layers of layered intrusions. These are characterized by cumulate textures, characterized by a fabric of coarse (>5mm) interlocking euhedral (well formed) crystals in a groundmass of finer crystals formed from liquid magma trapped in the cumulate. Many show poikilitic texture in which crystallization of this liquid has produced crystals that overgrow and enclose the original cumulus crystals (called chadrocrysts). Another texture is a well annealed texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and deformation twinning of olivine grains, is common in peridotites because of the deformation associated with their tectonic mode of emplacement.

Peridotites 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. Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor). Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. They form from mantle derived magmas, such as those of basalt composition. Peridotites associated with Alaskan type ultramafic complexes are cumulates that probably formed in the root zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows.

Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface. Eclogite, a rock similar to basalt in composition, is composed primarily of omphacite (sodic clinopyroxene) and pyrope rich garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction.

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 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 originally mined on St. John's Island in the Red Sea and now mined on the San Carlos Apache Indian Reservation in Arizona. Peridotite that has been hydrated at low temperatures is the protolith for 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.