dunite

n.
A dense igneous rock that consists mainly of olivine and is a source of magnesium.

[After Mount Dun in northern South Island, New Zealand.]

dunitic du·nit'ic (dū-nĭt'īk, də-) adj.

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An ultramafic igneous rock composed of at least 90% olivine. Important accessory minerals (abundances usually less than 1%) found in different occurrences of dunite include chromian spinel, low- or high-calcium pyroxenes (enstatite and diopside), and plagioclase. If these minerals constitute greater than 10% of the rock, it is called chromitite, peridotite, or troctolite, respectively. Low-temperature alteration (less than 750°F or 400°C) causes hydration of olivine to the mineral serpentine and, where extensive, may transform dunite to the metamorphic rock serpentinite. The dun color is a characteristic feature of the weathered rock. See also Chromite; Olivine; Peridotite; Pyroxene; Serpentinite.

Dunite is ultrabasic in composition, meaning that it is low in silica compared to most crustal rocks (<45% silica) and is generally very high in magnesium (up to 54%). In some occurrences, notably in continental layered intrusions, however, the olivine may be very iron rich (variety hortonolite), and the rock can contain as much as 40% iron. Dunite is notably poor in alumina, soda, and lime; and while it may be relatively nickel rich, many other critical trace elements are nearly missing. Thus, weathering of dunite forms soils to which most terrestrial plants are poorly adapted, and such soils often host unusual plant species such as the carnivorous cobra lily and the miniature rhododendron Kalmiopsis leachiana found in the coast ranges of Oregon and California.

The most important occurrence of dunite is in its association with rocks believed to have come from the Earth's mantle (principally peridotite) where it marks the location of former conduits through which magmas have been transported out of the Earth to the crust. In the oceans, it is infrequently dredged from the great oceanic transform faults cutting the ocean ridges, exposed in tectonic windows in disrupted and uplifted ocean crust in association with serpentinized mantle peridotite. Important dunite bodies crosscutting tectonically exposed mantle sections on the walls and floors of rift valleys have been found in the central Atlantic and eastern Pacific oceans far from transform faults. The suggestion is that the flow of magma out of the mantle upwelling beneath ocean ridges is not uniform, and is therefore important in controlling the formation and structure of the two-thirds of the Earth's crust that is formed in the oceans. See also Earth; Mid-Oceanic Ridge; Rift valley; Transform fault.

Dunite has been extensively quarried as a building stone because of its unusual pale water-green color, but is little used now because of its high susceptibility to acid rain. Dunite is also used for refractory materials for furnaces. Olivine in dunite is occasionally of gem quality. Dunite is also the principal host of deposits of the mineral chromite. The hortonolite-dunite pipes found in the Bushveld Complex of South Africa have also been mined for platinum-group minerals. See also Gem; Igneous rocks; Petrology; Refractory.

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