ONE OF THE MOST ECONOMICALLY important rocks, peridotite is the ultimate source of all chromium ore, and its variety kimberlite is the source of all naturally occurring diamonds. Fresh peridotite contains olivine, pyroxene, and chromite. Peridotite that has been altered by weathering becomes serpentinite, containing chrysotile, other serpentines, and talc. Formerly, peridotite was a major source of magnesite, an important economic mineral. It is also the major source of chrysotile asbestos, now of declining economic importance.
An intrusive igneous rock, peridotite is coarse-grained, dark-colored, and dense. It contains at least 40 per cent olivine. It is related to the rock pyroxenite in that they form in similar environments and both are olivine-pyroxene rocks. Pyroxenite contains at least 60 per cent pyroxene. Another related rock is dunite, a yellowish-green, intrusive igneous rock composed almost entirely of olivine. Like peridotite, dunite is an important source of the chromium ore chromite. Peridotite is found interlayered with iron- and magnesia-rich rocks in the lower parts of layered, igneous rock bodies, where its denser crystals formed first through selective crystallization and then settled to the bottom of still-fluid or semi-solid crystallizing mushes. Within peridotite bodies, layers of chromite (the principal ore of chromium), an early crystallite and the first to settle, can reach several yards thick. Peridotite is also found in mountain belts as irregular, olivine-rich masses, sometimes with related gabbro, and in dikes (vertical or steeply inclined sheet-like bodies, which often cut existing rock structure). Garnet is often present as an accessory mineral, and weathered peridotites are commonly a source of nickel. Peridotite’s other important occurrence is in volcanic pipes as kimberlite.
- Rock type Ultramafic, plutonic, igneous
- Major minerals Olivine, pyroxene
- Minor minerals Garnet, chromite
- Color Dark green to black
- Texture Coarse
Peridotite is a major structural component of the Earth. It is the major component of the asthenosphere, the upper portion of the mantle upon which the plates rest. At depths of 40–125 miles (70–200km), the mantle rocks are thought to exist at temperatures slightly above their melting point, and a small percentage may be molten. This gives the mantle the plasticity for continental movement. The basaltic magmas of the oceanic crust are generated in the asthenosphere, their compositions determined to an extent by the mineral make-up of the peridotite.
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