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peridotite(pĕr'ēdō`tīt): see olivineolivine
, an iron-magnesium silicate mineral, (Mg,Fe)2SiO4, crystallizing in the orthorhombic system. It is a common constituent of magnesium-rich, silica-poor igneous rocks; metamorphism of some high magnesium sediments also can form olivine.
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an ultrabasic intrusive rock that is chiefly composed of olivine and pyroxene and sometimes contains hornblende. The olivine and pyroxene contents range from 70 to 30 percent and 30 to 70 percent, respectively; peridotite also contains 40 to 46 percent SiO2 and 34 to 46 percent MgO. Among the accessory minerals found in peridotite are magnetite, ilmen-ite, pyrrhotite, chromite, spinel, and garnet. In addition, peridotite sometimes contains platinum and certain nickel minerals.
Peridotite’s color is dark, usually green or greenish gray. When it is highly serpentinized with olivine and pyroxenes, it alters to serpentinite. Peridotite in combination with orthorhombic pyroxene is called saxonite, or harzburgite; with monoclinic pyroxene, wehrlite; and with both rhombic and monoclinic pyroxene, websterite, or lhertzolite. Peridotite in association with other ultrabasic and with basic rocks forms extensive belts and zones that are characterized by deposits of commercial minerals, including chromite, platinum ores, silicate nickel ores, chrysotile asbestos, and talc.