Kimberlite

kimberlite: see diamond.

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kimberlite

 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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kimberlite [′kim·bər‚līt]

(petrology)

A form of mica periodite that is formed mainly of phenocrysts, olivine, phlogopite, and subordinate melilite with minor amounts of pyroxene, apatite, perovskite, and opaque oxides.

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Kimberlite 

(named after the city of Kimberley in the Republic of South Africa), an ultrabasic, brecciated igneous rock of effusive habit that fills volcanic pipes. The rock consists essentially of olivine, pyroxenes, garnet of the pyrope-almandine series, picroilmenite, and phlogopite and more rarely zircon, apatite, and other minerals that are surrounded by a fine-grained groundmass. The groundmass has usually been altered by post-volcanic processes to serpentine-carbonate with perovskite and chlorite.

Kimberlite also contains inclusions of other rocks (xenoliths) of two types: (1) plutonic ultrabasic and basic rocks (pyropeolivine ultrabasites, eulysites, and eclogites, which contain corundum, rutile, and graphite), and (2) crustal rock such as granites, crystaline schists, and sandstones. The quantity of the second rock type may be so great that it is difficult to establish the igneous nature of the kimberlite.

Kimberlite is the basic source of diamonds. The diamonds usually occur in the kimberlite itself and may also occur in the inclusions of the plutonic rock and even in the minerals of these rocks. It is thought that the kimberlite-bearing pipes formed as a result of explosions and that the pipes became filled with material that was carried up from great depths where the diamonds and pyrope formed under great pressure. The size of these pipes varies from several square meters to thousands of square meters (for example, the area of the Mwadui pipe in Tanzania is 1,068 × 1,525 m). With increasing depth the cross sections of the pipes decrease, and they become narrow dikes. In addition to volcanic pipes, kimberlites fill some cavities in the earth’s crust, forming veins, dikes, and sheet-like deposits (sills).

Kimberlite occurs, as a rule, on ancient platforms and very rarely in geosynclinal areas. More than 1,500 kimberlite bodies are known to exist, but only 8–10 percent of them are diamond-bearing. Kimberlites with a diamond content of at least 0.3–0.5 carats per cu m are considered of commercial importance. Some of the kimberlite pipes are very large producers. Thus, more than 65 million carats have been taken from the Premier Mine in the Republic of South Africa. Kimberlites are most widely distributed in the USSR (Yakut ASSR) and abroad in Africa, India, and North America.

REFERENCES

Trofimov, V. S. Osnovnye zakonomernosti razmeshcheniia i obrazovaniia almaznykh mestorozhdenii na drevnikh platformakh i ν geosinklinal’nykh oblastiakh. Moscow, 1967.
Trofimov, V. S. “O termine ’kimberlit’.” Izv. AN SSSR: Ser. geologicheskaia, 1970, no. 11.
Trofimov, V. S. “Prirodnye almazy.” Priroda, 1972, no. 3.

V. S. TROFIMOV