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Peridotite

Any of a group of igneous rocks composed mainly of olivine and various pyroxenes and having a granite-like texture. Peridotite is a dense, coarse-grained rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic and ultrabasic, as the rock contains less than 45% silica and is high in magnesium with appreciable iron.

A rock consisting of more than 90% of millimeter-to-centimeter-sized crystals of olivine, pyroxene, and hornblende, with more than 40% olivine. Other minerals are mainly plagioclase, chromite, and garnet. Much of the volume of the Earth's mantle probably is peridotite.

Peridotites have three principal modes of occurrence corresponding approximately to their textures:

(1) Peridotites with well-formed olivine crystals occur mainly as layers in gabbroic complexes.

(2) Peridotite nodules in alkaline basalts and diamond pipes generally have equigranular textures, but some have irregular grains.

(3) Peridotite also occurs on the walls of rifts in the deep sea floor and as hills on the sea floor, some of which reach the surface.

Peridotite is an igneous rock with a coarse crystalline texture. It is an intrusive rock, meaning it crystallized from magma emplaced in a surrounding rock. The fresh surface has coarse light to dark green crystals and the major minerals are olivine, clinopyroxene, and orthopyroxene. The high iron and magnesium content make peridotite an ultramafic rock. The weathered surface is brown to red and frequently very rough, due to differential weathering of the minerals. The Rogue River-Siskiyou National Forest has many slopes covered with boulders and cobbles of peridotite.

Peridotite can form very red soils due to the high iron content. These soils include laterites, very leached soils that form in tropical climates.

Peridotite consists of rock from the earth’s mantle, which is usually 5-8 kilometers below the ocean floor (see cross section of earth). The peridotite in the National Forest gives us an exciting chance to see what mantle rocks are like. Peridotite is thought to be the primary material making up the upper mantle. Partial melting of peridotite produces basaltic liquid similar to that erupted at mid ocean ridges.

Peridotites are rich in magnesium, reflecting the high proportions of magnesium-rich olivine. The compositions of peridotites from layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chromite, plagioclase, and amphibole.