Home » Rock Directory

Dunite

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

Yellowish green to green igneous rock composed almost entirely of olivine. Chromite and magnetite also occur in dunite, as do spinel, ilmenite, pyrrhotite, and platinum in some cases. Dunites may be a source of chromium. Places of occurrence include Dun Mtn., New Zealand (the source of its name), South Africa, and Sweden.

Dunite is a >90% olivine-bearing peridotite (an ultramafic intrusive igneous rock). How did dunite get its name? The type locality for dunite is Dun Mountain in New Zealand. It's been metamorphosed, but not serpentinized. Dunite tectonite has a grayish core of olivine plus some small black masses of chromite. The yellow-orangish weathering rind is referred to as "dun"-colored in New Zealand. The whole of Dun Mountain has surface-weathered dunite rocks of this color. Broken pieces show that the unweathered olivine is grayish. Rocks here are Early Permian in age (275-285 million years).

Dunite was named by the Austrian geologist, Ferdinand von Hochstetter in 1859 after Dun Mountain near Nelson, New Zealand. Dun Mountain was given its name because of the dun colour of the underlying ultramafic rocks. This color results from surface weathering that oxidizes the iron in olivine in temperate climates (weathering in tropical climates creates a deep red soil). Dun Mountain is separated from its sister massif, Red Mountain, at the southern end of South Island, New Zealand, by the Alpine Fault, an approximately 600 km long (?) right lateral strike slip fault similar to the San Andreas fault in California.

Terrestrially, dunite typically occurs in sills, or horizontal sheets intruded between other rocks, but may appear in other shapes, such as dikes and laccoliths, as well. The formation of the rock, which is a common component of the oceanic lithosphere, is generally believed to arise from the effect of gravity and other forces on accumulations of dense olivine grains. In addition to the massive bodies of the substance in New Zealand, dunite can be found in such diverse locales as Sweden, the Philippines, Alaska, and South Africa.

[ Close ]