Ultramafic rocks

thumb|False-color scanning electron microscope image of kimberlite from South Africa. [[Olivine crystals (green) are in a fine-grained matrix made up of clay minerals and carbonates (presented in blue, purple and buff colors).]]

Peridotite ( ) is a dense, phaneritic (coarse-grained) igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high proportions of magnesium-rich olivine, with appreciable iron. Peridotite is derived from Earth's mantle, either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting the relative proportions of pyroxenes, chrom

thumb|right|upright=1.2|Small volcanic bomb of (black) [[basanite with (green) dunite]]

thumb|Komatiite lava at the type locality in the Komati Valley, Barberton Mountainland, South Africa, showing the distinctive "spinifex texture" formed by dendritic plates of olivine (scale shown by a hammer on the right edge of photo)

igneous rock type

thumb|Minette (a type of lamprophyre), from Jáchymov in the Czech Republic

thumb|Pyroxenite from Rustenburg, South Africa Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenes have the general formula , where X represents ions of calcium (Ca), sodium (Na), iron (Fe(II)) or magnesium (Mg) and more rarely zinc, manganese or lithium, and Y represents ions of smaller size, such as chromium (Cr), aluminium (Al), magnesium (Mg), cobalt (Co), manganese (Mn), scandium (Sc), titanium (Ti), vanadium (V) or even iron (Fe(II) or Fe(III)). Pyroxenes share a common

thumb|Lherzolite thumb|right|Lherzolite at Etang de Lers, Ariège, France thumb|Garnet lherzolite, a xenolith from a kimberlite pipe, [[Kimberley, South Africa. Field of view ~1.6 cm across. Purplish red = pyrope garnet. Bright green = chromian diopside. Dark greenish-black = orthopyroxene. Olive-green = olivine.]]

Harzburgite, an ultramafic, igneous rock, is a variety of peridotite consisting mostly of the two minerals olivine and low-calcium (Ca) pyroxene (enstatite); it is named for occurrences in the Harz Mountains of Germany. It commonly contains a few percent chromium-rich spinel as an accessory mineral. Garnet-bearing harzburgite is much less common, found most commonly as xenoliths in kimberlite.

variety of high-magnesium olivine basalt that is very rich in the mineral olivine

thumb|Nephelinite lava (grey) containing a xenolith of [[peridotite (yellow), Kaiserstuhl, Germany.]] Nephelinite is a fine-grained or aphanitic igneous rock made up almost entirely of nepheline and clinopyroxene (variety augite). If olivine is present, the rock may be classified as an olivine nephelinite. Nephelinite is dark in color and may resemble basalt in hand specimen. However, basalt consists mostly of clinopyroxene (augite) and calcic plagioclase.

thumb|Websterite highlighted in a ternary diagram thumb|Cumulate rock|Cumulate layers of websterite in the [[McMurdo Dry Valleys in Antarctica]]

thumbnail|right|Wehrlite is a mixture of olivine and clinopyroxene.|300px thumb|right|Micrograph|Photomicrograph of a [[thin section of wehrlite, in cross-polarised light]] Wehrlite is an ultramafic and ultrabasic rock that is a mixture of olivine and clinopyroxene. It is a subdivision of the peridotites.