Peridotite (Dunite)
Peridotite is a very dense, coarse-grained, olivine-rich, ultra- mafic intrusive rock. It is noted for its low silica content, and contains very little or no feldspar ( orthoclase, plagioclase). It is a common component of oceanic lithosphere, and is derived from the upper mantle. It is found on land as part of oceanic crust sequences called ophiolites which have been thrust in or on to a continental mass, or as localised intrusions.
Colour - generally dark greenish-grey.
Texture - phaneritic (coarse grained).
Mineral content - generally olivine with lesser pyroxene ( augite) (dunite is dominantly olivine), always contains some metallic minerals, e.g. chromite, magnetite.
Silica (SiO 2) content - < 45%.
Uses - as a source of valuable ores and minerals, including chromite, platinum, nickel and precious garnet; diamonds are obtained from mica-rich peridotite (kimberlite) in South Africa.
New Zealand occurrences - North Cape, Dun Mountain (southeast of Nelson), Fiordland.