Peridotite is a medium-grained, dark-colored, intrusive igneous rock that contains at least 10 percent olivine, other iron- and magnesium-rich minerals (generally pyroxenes), and not more than 10 percent feldspar. It occurs in four main geologic environments: (1) interlayered with other ultramafic rocks in the lower parts of layered igneous complexes or masses; (2) in alpine-type mountain belts as irregular, olivine-rich masses, with or without related gabbro; (3) in volcanic pipes (funnels, more or less oval in cross section, that become narrower with increasing depth) as kimberlite; and (4) as dikes and irregular masses with rocks exceptionally rich in potassium and sodium. The layered complexes are believed to have been formed in place by crystal settling from a previously intruded fluid or magma. Other types seem to have ranged from fluid magmas to semisolid crystal mushes at the time of emplacement like kimberlites.

Peridotite is the source of all chromium ore and naturally occurring diamonds, and of nearly all chrysotile asbestos. It is one of the main host rocks of talc deposits and platinum metals and formerly was a major source of magnesite. Fresh dunite (olivine-rock) is used in parts of glass furnaces. Nearly all peridotite is more or less altered to serpentine (hydrous phases are present) In warm, humid climates peridotite and serpentine have weathered to soils worked on a relatively small scale for iron, nickel, cobalt, and chromium.

Based on the observed samples, peridotite with about 50 percent olivine, 30 percent pyroxene, and 15 percent garnet is considered to be the dominant rock of the upper mantle. This conclusion is supported by melting studies that demonstrate that a basaltic liquid is produced if peridotite is heated enough to melt partially. It is by partial melting of the upper mantle beneath mid-oceanic ridges that the basalt and gabbro composing the oceanic crust is produced. The effect of this partial melting is to slightly deplete the original peridotite in aluminum and iron that preferentially are concentrated into the basaltic liquid. Thus, when aluminum and iron-rich xenoliths are present in a peridotite, they are referred to as "fertile," meaning that they have not yet been significantly melted and could produce basaltic liquid upon partial melting.


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