alkaline rock, any of various rocks in which the chemical content of the alkalies (potassium oxide and sodium oxide) is great enough for alkaline minerals to form. Such minerals may be unusually sodium rich, with a relatively high ratio of alkalies to silica (SiO2), as in the feldspathoids. Other alkaline minerals have a high ratio of alkalies to alumina (Al2O3), as in aegirine pyroxene and the sodic amphibole riebeckite.

English-speaking petrologists have followed Alfred Harker, who divided igneous rocks of Cenozoic age (that is, those laid down between about 65.5 million years ago and the present day) into calc-alkaline and alkaline suites. Alkaline rocks include many with unusual names, but the more common alkali-basalt, syenite, and phonolite are included in the group. The most common and widely distributed rocks of the world—e.g., granite, granodiorite, andesite, and basalt—do not contain the alkaline minerals. Alkaline rocks are generally considered to be abnormal types, and there have been many intensive studies of their origin, yielding a number of theories, each of which may be valid for a specific case.

This article was most recently revised and updated by John P. Rafferty.
Also called:
blue ground

kimberlite, a dark-coloured, heavy, often altered and brecciated (fragmented), intrusive igneous rock that contains diamonds in its rock matrix. 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, along with a similar rock called lamproite, is important for delivering diamonds to the crust through magmatic intrusions that solidify into pipelike structures.

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.

This article was most recently revised and updated by Amy Tikkanen.