peridotite 

Large sections of the Earth's mantle are thought to be peridotite, a 
plutonic rock composed mainly of coarse grains of olivine (a variety 
of which is called peridot, from which the rock's name is derived). 
Peridotites may also contain substantial amounts of pyroxene, 
hornblende, and mica, and accessory minerals include apatite, 
chromite, garnet, magnetite, and pyrrhotite. Peridotites are often 
partially or largely altered to serpentinite. Some are of economic 
value, providing ores of chromium and asbestos (pyroxene peridotites) 
and diamonds (mica peridotites). Peridotites occur at the bases of 
layered igneous complexes, probably as the result of gravitational 
settling of dense crystals that formed early. Some have intruded other 
bodies as crystal mush. WILLIAM D. ROMEY 

Bibliography: Hyndman, D. W., Petrology of Igneous and Metamorphic 
Rocks, ed. by Jack L. Farnsworth (1972); Williams, Howel, et al., 
Petrography (1955).