Peridotite is a dense, coarse grained ultrabasic rock, consisting mainly of the minerals olivine and pyroxene. Peridotite is also a group of mantle derived igneous rocks. They all are ultramafic or ultrabasic meaning they contain less than 45% silica and are high in iron and magnesium.
Members of the peridotite family include:
Dunite – predominately composed of olivine, with minor enstatite pyroxene and chromite.
Harzburgite – composed of olivine, enstatite, and minor chromite.
Lherzolite – composed of olivine, enstatite, diopside, and minor chromite and/or pyrope garnet.
Pyroxenite – composed largely of orthopyroxene and/or clinopyroxene, with lesser amounts of olivine, garnet, and spinel.
These peridotite rocks are usually subducted back into the mantle in subduction zones. However, they can be emplaced into or overthrust on continental crust during continental collisions (orogenies) or island arc collisions by the process of obduction. The occurances of these peridotites along with other mafic gabbros and basalts within continental crust are referred to as ophiolites.
A related rock type, eclogite, may be mostly metamorphosed peridotite from deep within the subduction environment.
The rocks of the peridotite family are uncommon at the surface and are highly unstable. Many, if not most, surface outcrops have been highly altered by retrograde metamorphism to serpentinites in which the pyroxenes and olivines have been converted to green serpentine along with amphibole minerals. This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures.