Dunite granular, green igneous rock composed of coarse
grains of olivine, is the source of the world's supply of
chromium. It weathers to a dun brown color, as at Mount
Dun, in New Zealand, its namesake. Some dunites are rich
in chromite, magnetite, ilmenite, or pyrrhotite; they may
also contain a small amount of pyroxene. Dunite occurs in
layered, gabbroic igneous complexes (see gabbro). It
probably forms from the accumulation of dense, early
crystallizing grains of olivine that sink to the bottom of
low silica magma. Intrusions of dunite form sills or dikes.
Some dunite has been altered to form serpentine.