Peridotite

Peridotite is a dense, coarse grained ultramafic rock, consisting mainly of the minerals olivine and pyroxene. Peridotite is also a group of mantle derived igneous rocks. They all are ultramafic or ultrabasic meaning they contain less than 45% silica and are high in iron and magnesium.

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Members of the peridotite family include (see the classification diagram below):

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:Dunite - predominately composed of olivine, with minor pyroxene and chromite.

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:Harzburgite - composed of olivine, orthopyroxene (90%), and minor spinel.

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:Werlite - composed of olivine, clinopyroxene (90%), and lesser spinel.

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:Lherzolite - composed of olivine, enstatite, diopside, and minor chromite and/or pyrope garnet.

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Pyroxenites are also ultramafic rocks which are composed largely of orthopyroxene and/or clinopyroxene, with lesser amounts of olivine, garnet, and spinel.

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These peridotite rocks are usually subducted back into the mantle in subduction zones. However, they can be emplaced into or overthrust on continental crust during continental collisions (orogenies) or island arc collisions by the process of obduction. The occurrences of these peridotites along with other mafic gabbros and basalts within continental crust are referred to as ophiolites. Peridotites also occur as xenoliths in some basalts and in kimberlite pipes.

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A related rock type, eclogite, may be mostly metamorphosed peridotite from deep within the subduction environment.

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The rocks of the peridotite family are uncommon at the surface and are highly unstable. Many, if not most, surface outcrops have been highly altered by retrograde metamorphism to serpentinites in which the pyroxenes and olivines have been converted to green serpentine along with amphibole minerals. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures.

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Komatiites are the rare volcanic equivalent of peridotite.

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Ultramafic: Ultramafic rocks are igneous rocks with very low silica content (less than 45%) and are composed of usually greater than 90% mafic minerals (dark colored, high magnesium and iron content). Ultramafic rocks are typical of the Earth's mantle....

Rock: Rock may refer to:...

Olivine: The mineral olivine is a magnesium iron silicate with the formula (Mg,Fe)2SiO4 in which the ratio of magnesium and iron varies between the two endmembers of the series: forsterite (Mg-rich) and fayalite (Fe-rich). It gives its name to the group of minerals with a related structure (the olivine group...