Dunite

Dunite is an ultramafic intrusive igneous rock composed of more than 90% olivine, with minor pyroxene, chromite, and other accessory minerals. It is one of the densest and most magnesium-rich common rocks and is closely associated with peridotite.

Fresh dunite is yellowish-green to olive green, weathering to brown on exposed surfaces. It is named after Dun Mountain in New Zealand, where the dun (brownish) weathering color is striking.

Because it is so olivine-rich and mantle-derived, dunite gives geologists a rare direct window into the composition of Earth's upper mantle.

Dunite forms in the upper mantle and at the base of crustal magma chambers. It originates either as residual mantle rock left behind after partial melting extracts basaltic magma, or as a cumulate where dense olivine crystals settle and accumulate at the bottom of a cooling magma body.

It is brought to the surface in ophiolite complexes (slices of oceanic lithosphere thrust onto continents) and in layered intrusions. Notable localities include Dun Mountain in New Zealand, the Bushveld Complex in South Africa, and ophiolites in Oman and the Twin Sisters in Washington State.

Look for a dense, granular, yellowish to olive-green rock made almost entirely of sugary olivine grains, often with a characteristic brown weathering rind. It feels heavy for its size.

Olivine's hardness (6.5-7), glassy luster, and green color are key. Small black grains of chromite may be scattered through it. Streak is white to pale.

Look-alikes include other peridotites (contain more pyroxene) and serpentinite (softer, greasy feel, formed by alteration of dunite). When dunite alters with water it converts to serpentine.

Dunite is mined as a source of magnesium and as a refractory raw material; olivine from dunite is used in foundry sands, blast-furnace fluxes, and heat-resistant (refractory) bricks. Chromite-bearing dunites are sources of chromium ore.

Dunite is also studied intensively for carbon capture, because its olivine reacts with carbon dioxide to form stable carbonate minerals (mineral carbonation). Gem-quality olivine from such rocks yields peridot. Scientifically, dunite is invaluable as a sample of the Earth's mantle.