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Kimberlite

Pictures of Igneous Rock Types

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Kimberlite, an ultramafic volcanic rock, is quite rare but much sought after because it is the ore of diamonds. (more below)
Kimberlite

Specimen courtesy University of Kansas

Photo (c) 2013 Andrew Alden, licensed to About.com (fair use policy)
Kimberlite is a peculiar rock that erupts very rapidly from deep in the Earth's mantle, leaving behind a narrow pipe of this greenish brecciated lava. The rock is of ultramafic composition—very high in iron and magnesium—and is largely composed of olivine crystals in a groundmass consisting of various mixtures of serpentine, carbonate minerals, diopside and phlogopite. Diamonds and many other ultra-high pressure minerals are present in greater or lesser amounts. It's also stuffed with xenoliths, samples of rocks gathered along the way.

Kimberlite pipes (which also are called kimberlites) are scattered by the hundreds in the most ancient continental areas, the cratons. Most are a few hundred meters across, so they can be hard to find. Once found, many of them become diamond mines. South Africa seems to have the most, and kimberlite gets its name from the Kimberley mining district in that country. This specimen, however, is from Kansas and contains no diamonds. It's not very precious, just very interesting.

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