Peridotite

Peridotite Classification and Composition

• Peridotite is a coarse-grained ultramafic rock with olivine making up 40% or more of the total volume.
• Coarse-grained igneous rocks with over 90% mafic minerals are classified as ultramafic rocks.
• Ultramafic rocks contain less than 45% silica.
• Dunite contains more than 90% olivine and is found in ophiolites and layered intrusions.
• Kimberlite is formed in volcanic pipes and contains at least 35% olivine.
• Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89.
• Olivine is the essential mineral found in all peridotites, typically olive-green in color.
• Pyroxenes are chain silicates, divided into orthopyroxenes and clinopyroxenes.
• Hornblende is an amphibole mineral present in peridotite due to alteration by hydrous fluids.
• Other accessory minerals in peridotites include chromite, spinel, garnet, biotite, and magnetite.

Peridotite Distribution and Location

• Peridotite is the dominant rock of the upper part of Earth's mantle.
• Peridotite nodules found in certain basalts provide samples of Earth's mantle brought up from depths ranging from 30km to 200km or more.
• Peridotite outcrops range in color from earthy bright yellow to dark green.
• Peridotite is found in ophiolites, which are interpreted as slices of oceanic lithosphere thrust onto continents.
• Peridotite is also found in volcanic pipes and is known for being the host rock to diamonds.

Peridotite Types

• Dunite contains more than 90% olivine and is found in ophiolites and layered intrusions.
• Harzburgite contains less than 5% clinopyroxene and makes up the bulk of the peridotite layer of ophiolites.
• Wehrlite contains less than 5% orthopyroxene and is found in the transition zone between the peridotite layer and overlying gabbro layer of ophiolites.
• Lherzolite has an intermediate content of clinopyroxene and orthopyroxene, making it a likely source rock for basaltic magma.
• Hornblende peridotite contains from 40% to 90% olivine and less than 5% pyroxene, found as rare xenoliths in andesites above subduction zones.

Peridotite Colors, Morphology, and Texture

• Classic peridotite is bright green with some specks of black.
• Peridotitic outcrops range in color from earthy bright yellow to dark green.
• Peridotitic rocks may exhibit a wide range of colors including blue, brown, and red.
• Peridotite hand samples tend to be darker green.
• Peridotite contains accessory minerals like chromite, which can affect the color of the rock.
• Peridotite weathers to form a distinctive brown crust in subaerial exposures.
• Peridotites can take on a massive form or may be in layers on a variety of size scales.
• Layered peridotites may form the base layers of layered intrusions.

Peridotite Origin and Associated Rocks

• Peridotites have two primary modes of origin: as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas.
• Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites.
• Alpine peridotites may be either of the ophiolite association or masses of subcontinental mantle emplaced along thrust faults in mountain belts.
• Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals.
• Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface.
• Eclogite is a rock similar to basalt in composition and is associated with peridotite in some xenolith occurrences.
• Peridotite may potentially be used in a low-cost method of capturing and storing atmospheric CO.
• Peridotite is named for the gemstone peridot.
• Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores.

Peridotite Data Sources