dunite

Dunite is an ultramafic igneous rock composed almost entirely of the mineral olivine, with little or no feldspar and only minor pyroxene. Typically, olivine accounts for more than 90 percent of the rock, giving it a dense, coarse-grained texture and a greenish to olive color. Accessory minerals may include chromian spinel, magnetite, and minor chromite.

Dunite forms as a cumulate rock in primitive magmas or as mantle lithotype in ophiolites. It can represent the olivine-rich residue remaining after high-degree partial melting of peridotite or be part of the cumulate sequence crystallizing olivine from basaltic magma. It is commonly found in ultramafic intrusions and as slices within ophiolite complexes that represent slices of the upper mantle.

Name/etymology: The term dunite derives from the Dun Mountain in Nelson Province, New Zealand, where the rock type was first described.

Distinguishing features: Dunite is distinguished from harzburgite by its much higher olivine content and relatively scarce pyroxene. Peridotite is a broader category that includes dunite, harzburgite, and lherzolite, differentiated by their pyroxene to olivine ratios.

Occurrence and significance: Dunite occurs worldwide in ophiolites, ultramafic intrusions, and mantle xenoliths. It provides information about mantle composition and melting processes.

Uses: Olivine-rich rocks are sources of industrial olivine for refractories, foundry sands, and abrasives, and are studied for mantle geology.