Barker, Daniel S. Department of Geological Sciences, University of Texas, Austin, Texas.
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A variety of peridotite, an igneous rock containing at least 35% olivine. The full definition is unusually complex for an igneous rock, with many qualifications. Briefly, kimberlite is richer in carbon dioxide than most peridotites, and has crystals larger in diameter than 0.5 mm of olivine, garnet, clinopyroxene, phlogopite, and orthopyroxene. All of these silicate minerals have high Mg/Fe ratios in kimberlites. In addition, there can be 0.5 mm or larger crystals of magnesian ilmenite and titanium-poor chromite. The matrix (crystals less than 0.5 mm) contains olivine plus one or more of monticellite, phlogopite, perovskite, spinel, apatite, serpentine, and carbonate minerals. Melilite, leucite, kalsilite, and alkali feldspar do not crystallize from kimberlite magma. In part, the complexity of this definition is caused by the complicated processes of generation and emplacement of kimberlites and, in part, a restrictive definition is imposed as a defense against wishful thinking, because kimberlite was long thought to be the only rock that brought diamonds to the Earth's surface. However, in the 1970s other diamond-bearing rocks (lamproites) were found in western Australia and elsewhere, and in the 1980s R. H. Mitchell distinguished orangeite and kimberlite as separate hosts for diamonds in South Africa. Some eclogites have recently been found to contain diamonds also. See also: Eclogite; Olivine; Peridotite
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