Dunite is an igneous, plutonic rock, of ultramafic composition, with coarse-grained or phaneritic texture. The mineral assemblage is greater than 90% olivine, with minor amounts of other minerals such as chromite. Dunite is the olivine-rich end-member of the peridotite group of mantle-derived rocks. Dunite is rarely found within continental rocks, but where it is found, it typically occurs at the base of ophiolite sequences where slabs of mantle rock from a subduction zone have been thrust onto continental crust by obduction during continental or island arc collisions (orogeny). Dunite typically undergoes retrograde metamorphism in near-surface environments and is altered to serpentinite.
Dunite may represent the refractory residue left after the extraction of basaltic magmas in the upper mantle. This is the type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, and xenoliths. Dunite may also form by the accumulation of olivine crystals on the floor of large basaltic or picritic magma chambers. These "cumulate" dunites typically occur in thick layers.
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).