Peridotite Rock Type, Composition, Formation, Occurrence & Uses

Peridotite is a group of dense and coarse-grained ultramafic igneous rocks. The rock group contains kimberlite. The group can also contain chromite or diamonds. It is the dominant rock of the upper part of the earth’s mantle. The rock mostly represents the earth’s mantle than the crust.

Peridotite rocks are green, mainly due to their high olivine content. Peridotite is further divided into 5 different types based on mineral content.

Typical peridotite sample (dunite, left) and large olivine crystal (right)

Table of Contents

Types of Peridotite

Lherzolite: high percentage of orthopyroxene and clinopyroxene, believed to make up most of the earth’s mantle

Harzburgite: mainly olivine and orthopyroxene with trace amounts of spinel and garnet

Dunite: mainly olivine (more than 90%) with significant amounts of chromite, pyroxene, and spinel

Wehrlite: mainly orthopyroxene and clinopyroxene, with olivine and hornblende

Kimberlite: at least 35% olivine and large amounts of minerals like monticellite, garnet, phlogopite, pyroxenes, carbonates, serpentine, and diopside

Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle (partly adapted from Bodinier and Godard (2004)).

Rock Composition

Peridotites usually contain olivine as the primary mineral. Other minerals present in these rocks are pyroxenes and amphiboles. The silica content in peridotite is lower than in other igneous rocks. The quartz and feldspar content in these rocks is also low.

Peridotite Rock Formation

Peridotites are formed as a result of two activities. It was formed during the earth’s accretion and differentiation or by the crystallization of olivine and pyroxenes from ballistic and ultramafic magmas. Many peridotites are labeled as ophiolites in collisional mountain ranges. You may also see layered peridotites as igneous sediment rocks formed by the mechanical accumulation of dense olivine crystals. These rocks are formed from mantle-derived magma.

Where is Peridotite Found?

Peridotite is mined at a few locations in the world. The known reserves are in St. John’s Island in the Red Sea and San Carlos Apache Indian Reservation in Arizona.

Peridotite’s layered intrusions are also associated with sulfide or chromite ores. Sulfides form nickel ores and platinoid metals; platinum. Most platinum is mined from the Bushveld Igneous Complex in South Africa and the Great Dyke of Zimbabwe.

Alpine peridotite from the Ivrea zone in the Alps of Italy (dunite from Finero)

Peridotite Uses

Peridotite is a source of many valuable ores and minerals. It is mined to get high-value minerals like chromite, platinum, nickel, and garnet. South African peridot mines also produce diamonds.