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kimberlite A brecciated (see BRECCIA), potassic, ultrabasic igneous rock, consisting of megacrysts of olivine, enstatite, Cr-rich diopside, phlogopite, pyropealmandine garnet, and Mg-rich ilmenite, in a fine-grained groundmass of serpentine, phlogopite, carbonates, perovskite, and chlorite, in varying proportions. Kimberlites contain abundant xenoliths derived from the mantle and, as such, they are extremely useful for investigating mantle mineralogy and chemistry. Many of the mantle-derived xenoliths have been brought up from such great depth that they contain high-pressure minerals, the best known being diamond, the high-pressure form of carbon. Many of the megacrysts of enstatite, Cr-diopside and garnet are likewise thought to be derived from the upper mantle through which the kimberlite ascended, and as such should be termed ‘xenocrysts’. Near the surface, kimberlite is found in clusters of pipe-like bodies called diatremes. Mine workings show that they coalesce at depth and link up with dykes of non-fragmental kimberlite.