Coarse-grained, heavy, igneous rock that contains at least 10percnt olivine, other iron- and magnesium-rich minerals (generally pyroxenes), and not more than 10percnt feldspar. Peridotite is the ultimate source of all chromium ore and naturally occurring diamonds, and of nearly all chrysotile asbestos. Nearly all peridotite is more or less altered to serpentine; in warm, humid climates peridotite and serpentine have weathered to soils and related deposits that, though now worked on a relatively small scale, are potential sources of iron, nickel, cobalt, and chromium.
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Peridotite is the dominant rock of the upper part of the Earth's mantle. The compositions of peridotite nodules found in certain basalts and diamond pipes (kimberlites) are of special interest, because they provide samples of the Earth's mantle roots of continents brought up from depths from about 30 km or so to depths at least as great as about 200 km. Some of the nodules preserve isotope ratios of osmium and other elements that record processes over three billion years ago, and so they are of special interest to paleogeologists because they provide clues to the composition of the Earth's early mantle and the complexities of the processes that were involved.
Pyroxenites are related ultramafic rocks, which are composed largely of orthopyroxene and/or clinopyroxene; minerals that may be present in lesser abundance include olivine, garnet, plagioclase, amphibole, and spinel.
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.
Many peridotite occurrences have characteristic textures. For example, peridotites with well-formed olivine crystals occur mainly as layers in gabbroic complexes. "Alpine" peridotites generally have irregular crystals that occur as more or less serpentinized lenses bounded by faults in belts of folded mountains such as the Alpines, the Pacific coast ranges, and in the Appalachian piedmont. Peridotite nodules with irregular equigranular textures are often found in alkaline basalts and in kimberlite pipes. Some peridotites rich in amphibole have a concentric layered structure and form parts of plutons called Alaskan-type zoned ultramafic complexes.
Mantle peridotites are sampled as alpine-type massifs in collisional mountain ranges or as xenoliths in basalt or kimberlite. In all cases these rocks are pyrometamorphic (that is, metamorphosed in the presence of molten rock) and represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of the ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts.
Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. Some peridotite forms by precipitation and collection of cumulate olivine and pyroxene 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.
Mantle lherzolites may be the principal source rock for basaltic magmas, whereas mantle harzburgites probably form both from the crystalline residue left after basaltic magma migrates out of lherzolite and from a crystalline accumulation of early solidification products of some basaltic magmas within the mantle.
Eclogite, a rock similar to basalt in composition, is composed primarily of sodic clinopyroxene and 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 that has been hydrated at low temperatures forms serpentine, 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 commonly associated with peridotites are the world's major ores of chromium.