Dunite

Dunite (/ˈduːnaɪt, ˈdʌnaɪt/), also known as olivinite (not to be confused with the mineral olivenite), is an intrusive igneous rock of ultramafic composition and with phaneritic (coarse-grained) texture.

The type of dunite found in the lowermost parts of ophiolites, alpine peridotite massifs, and xenoliths may represent the refractory residue left after the extraction of basaltic magmas in the upper mantle.

However, a more likely method of dunite formation in mantle sections is by interaction between lherzolite or harzburgite and percolating silicate melts, which dissolve orthopyroxene from the surrounding rock, leaving a progressively olivine-enriched residue.

Small layered intrusions may be of any geologic age, for example, the Triassic Palisades Sill in New York and the larger Eocene Skaergaard complex in Greenland.

A massive exposure of dunite in the United States can be found as Twin Sisters Mountain, near Mount Baker in the northern Cascade Range of Washington.