Peridotite

Peridotite (US: /ˈpɛrɪdoʊˌtaɪt, pəˈrɪdə-/ PERR-ih-doh-tyte, pə-RID-ə-) is a dense, phaneritic (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.

Igneous rocks rich in magnesium and iron with a color index greater than 90 are defined as ultramafic. Ultramafic rocks may be further classified by their relative proportions of olivine, orthopyroxene, clinopyroxene, and hornblende, which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of the total volume of these four mineral families in the rock.

Peridotites are further classified as follows:

Mantle peridotite is highly enriched in magnesium, with a typical magnesium number^{[further explanation needed]} of 89. In other words^[whose?], 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 an iron-magnesium orthosilicate 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 since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is enstatite, Mg₂Si₂O₆, in which iron substitutes for some of the magnesium. The most important clinopyroxene is diopside, CaMgSi₂O₆, again with some substitution of iron for magnesium (hedenbergite, FeCaSi₂O₆). Ultramafic rock in which the fraction of pyroxenes exceeds 60% are classified as pyroxenites rather than peridotites. Pyroxenes are typically dark in color.