Peridotite is a dark-colored igneous rock consisting mostly of olivine and pyroxene. Peridotite is a hugely important rock type because it is believed that the Earth’s mantle is predominantly composed of this rock type.

At the surface, however, peridotite is relatively rare and often altered by hydrothermal processes or weathering because its constituent minerals are not stable outside of the mantle — their natural environment.

Olivine, the principal component of peridotite, makes up more than 40% of the pair olivine + pyroxene (or amphibole). A rock type with less olivine is named pyroxenite (or hornblendite if there is more amphiboles than pyroxenes). Peridotite is a plutonic rock, it is mostly composed of visible mineral grains. Other notable minerals that are often present are chromite, garnet, and plagioclase. Peridotite is an ultramafic rock (mafic minerals make up more than 90% of the rocks composition).

Peridotite is often massive (homogeneous) just like other plutonic rocks but it can be also layered. Layered peridotite is a cumulate rock formed by crystal settling from magma. Layered peridotites typically occur at the base of gabbroic intrusions. The Troodos ophiolite in Cyprus is a well-known locality where such rocks can be seen at the surface. Massive peridotite samples are often brought to the surface as xenoliths inside basalt.

Peridotite is the source rock of basalt. Basaltic magma forms when peridotite is partially melted. Basalt and peridotite differ in composition because rocks are mixtures of minerals but each mineral has its own melting temperature.

Some minerals start melting earlier and form basaltic magma which migrates upward because of lower density. Rest of the rock that did not melt have also different composition from the source rock, it is enriched in minerals with a higher melting temperature. This is the mechanism how different types of peridotite came to be. Lherzolite and wehrlite are sometimes referred to as a fertile mantle. These types of peridotite yield basaltic magma when they melt partially. What is left after the basaltic component is removed is called a depleted mantle. As a rock type it is harzburgite or in extreme cases dunite.

The main components of peridotite, olivine and pyroxene, are unstable minerals in the weathering environment. By the time these rocks reach the surface they are often heavily altered by a hydrothermal metamorphism or weathering. The rocks that are called peridotites are therefore often heavily metamorphosed and should be called serpentinite instead of peridotite which it once was.

Peridotite itself is not a notable mineral resource but valuable stuff may be associated with it. Chromite is a principal ore of chromium, talc is found in some metamorphosed peridotites, Serpentinite is used as a decorative stone because of its interesting texture, nickel and platinum are usually associated with ultramafic host rocks.

References

1. Le Maitre, R. W. (2005). Igneous Rocks: A Classification and Glossary of Terms: Recommendations of the International Union of Geological Sciences Subcommission on the Systematics of Igneous Rocks, 2nd Edition. Cambridge University Press.