kimberlite

/kim"beuhr luyt'/, n.

Petrol. a variety of micaceous peridotite, low in silica content and high in magnesium content, in which diamonds are formed.

[1885-90; named after KIMBERLEY, South Africa; see -ITE¹]

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or blue ground

Dark, heavy, often fragmented igneous rock that may contain diamonds in the rock matrix.

Kimberlite is a mica peridotite, an ultrabasic rock type with a complex and often highly altered mineral composition. It occurs in the Kimberley district of South Africa and the Kimberley and Lake Argyle regions of Australia, as well as near Ithaca, N.Y.

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▪ rock

also called  blue ground 

      a dark-coloured, heavy, often altered and brecciated (fragmented), intrusive igneous rock that contains the only diamonds (diamond) known to occur in the rock matrix where they originally formed. It has a porphyritic texture, with large, often rounded crystals (phenocrysts) surrounded by a fine-grained matrix (groundmass). It is a mica peridotite, and its most abundant mineral constituent is olivine. Other abundant minerals include phlogopite mica; chromium- and pyrope-rich garnet phenocrysts; chrome-bearing diopside, particularly in the African kimberlite; ilmenite; serpentine; calcite; and magnetite.

      Kimberlite occurs in the uplifted centres of continental platforms. In the Kimberley district, South Africa, it forms pipes (funnels, more or less oval in cross section, that become narrower with increasing depth) and, occasionally, dikes. Other occurrences include the dikes at Ithaca, N.Y., the Kimberley and Lake Argyle regions of Australia, and the lavas in the Iswisi Hills, Tanzania.

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