Peridotite: Composition, Types & Uses

Peridotite is a dense, coarse-grained ultramafic igneous rock that forms deep within the Earth’s mantle. Dominated by the mineral olivine, with significant amounts of pyroxenes and accessory phases such as spinel, garnet, and chromite, it is the most important rock type of the upper mantle and a cornerstone of igneous petrology. Its low silica content—less than 45%—and high concentrations of magnesium and iron give it both its distinctive greenish hue and its remarkable density, typically 3.2 to 3.4 grams per cubic centimeter, making it heavier than most crustal rocks such as granite.

The name “peridotite” is derived from peridot, the gem variety of olivine, reflecting its mineralogical composition. Unlike volcanic rocks that crystallize quickly from cooling lava at the surface, peridotite forms slowly under immense pressure within the mantle, recording billions of years of Earth’s dynamic history in its chemistry and textures. Tectonic processes sometimes bring it to the surface, either as large bodies of intrusive rock or as xenoliths—fragments carried upward in basaltic or kimberlitic magmas.

Peridotite is a dense, coarse-grained ultramafic igneous rock.

Peridotite’s significance extends far beyond mineralogy. It is the source rock for most basaltic magmas that form oceanic crust and fuel volcanic eruptions. Studying peridotite provides direct insight into mantle processes, plate tectonics, and the mechanisms that drive earthquakes and continental drift. It is also central to resource exploration, hosting chromite, diamonds, and platinum-group elements, and has recently gained attention as a potential natural reservoir for carbon sequestration.

Peridotite Composition

Peridotite is an ultramafic igneous rock dominated by mafic minerals (>90%), making it unusually rich in magnesium and iron while poor in silica and alkalis. Its mineralogy records the processes of the Earth’s upper mantle, where it is the most abundant rock type.

Chemical Composition

Peridotite is chemically defined by:

High MgO (35–50%) and FeO (8–15%)
Low SiO₂ (<45%), Al₂O₃ (<5%), and CaO (<5%)
Mg# (100 × Mg/[Mg+Fe]) of 88–92, reflecting its mantle derivation

Varieties such as harzburgite are depleted by partial melting, while lherzolite represents more fertile, near-primitive mantle material. These differences make peridotite an archive of mantle melting and magma generation.

Mineralogy

Major Minerals

Olivine ((Mg,Fe)₂SiO₄)

The dominant mineral, typically comprising 40–90% of the rock. Olivine is usually magnesium-rich (forsterite component), imparting the characteristic green color and a high melting temperature (approaching 1,800 °C). It largely governs peridotite’s density, strength, and resistance to melting. Alteration of olivine produces serpentine, magnetite, and talc, weakening the rock significantly.

Peridotite types: lherzolite, wehrlite, harzburgite & dunite, classified by olivine, orthopyroxene & clinopyroxene mineral proportions.

Ultramafic peridotite types: lherzolite (olivine, orthopyroxene, clinopyroxene), wehrlite (olivine, clinopyroxene), harzburgite (olivine, orthopyroxene), and dunite (mostly olivine).

Pyroxenes (10–40%)

Pyroxenes are chain silicate minerals that significantly influence mantle rock properties, including fertility (ability to produce melts) and rheology (deformation behavior). They occur in mantle peridotites like harzburgite and lherzolite and include two main types:

Orthopyroxene (enstatite, (Mg,Fe)SiO₃): Forms prismatic crystals and is a major component of harzburgite and lherzolite. It enhances the rock’s rigidity and refractory (melt-resistant) nature, making it common in depleted mantle rocks that resist melting.
Clinopyroxene (diopside–augite, CaMgSi₂O₆): Rich in calcium, aluminum, and sodium, clinopyroxene is more abundant in fertile lherzolite. Its presence increases the rock’s potential to generate basaltic magmas, contributing key elements to melt production.

Amphibole (Hornblende group)

Present in metasomatized or hydrated peridotites. With a double-chain silicate structure incorporating OH⁻, amphiboles indicate interaction with fluids and are key markers of mantle hydration.

Accessory and Depth-Sensitive Minerals

The accessory minerals in peridotite (typically 5–10%) are critical indicators of its depth and pressure history. Their presence allows peridotites to be classified into distinct types:

Plagioclase Peridotite: Forms at shallow depths (<20 km). The presence of plagioclase feldspar is rare in true mantle samples.
Spinel Peridotite: Stable at mid-lithospheric depths (~20–60 km). Spinel (MgAl₂O₄ or chromian variety) is a common indicator of shallow mantle conditions and often forms octahedral crystals.
Garnet Peridotite: Forms at high pressures and great depths (>60 km). Garnet (e.g., pyrope, Mg₃Al₂Si₃O₁₂) is typical of the deep subcontinental lithospheric mantle and subduction zones.

Other accessory minerals like chromite, magnetite, sulfides, amphibole, and phlogopite typically occur in trace amounts. Their presence often records the rock's history of fluid infiltration, melting, or metasomatic alteration.

Garnet peridotite specimen, rich in green olivine and red pyrope garnets, from near Åheim, Norway.

This depth-dependent stability of aluminum-bearing phases (plagioclase → spinel → garnet) provides a powerful tool for estimating the pressure–temperature conditions under which a peridotite formed.

Physical and Geophysical Properties of Peridotite

Peridotite’s properties reflect its mineralogical and chemical makeup, explaining its central role in mantle geodynamics.

Color: Peridotite is typically dark green to gray due to its olivine and pyroxene content. When weathered, it develops a brown crust from iron oxidation. Unique varieties like "Verde Prato" get distinctive colors from minerals like chromium spinels.

Density: Peridotite has a high density (3.2–3.4 g/cm³), driven by its iron-rich minerals. This density, which is significantly greater than the average crustal rock (~2.7 g/cm³), is a key factor in driving subduction.

Hardness: Mohs 6.5–7, controlled by olivine. Despite this hardness, hydration (serpentinization) greatly weakens the rock mechanically.

Strength & Rheology: Dry peridotite is mechanically strong, but hydration lowers strength, promoting subduction zone processes and earthquake nucleation. Mantle convection occurs through ductile creep of olivine aggregates.

Melting Behavior: Solidus ranges from ~1,200–1,500 °C under mantle pressures. Volatiles such as water and CO₂ lower the solidus, triggering partial melting that produces basaltic magmas.

Magnetic Properties: Fresh peridotite is weakly magnetic, but serpentinized varieties may contain magnetite, enhancing magnetic susceptibility.

Seismic Properties: High P- and S-wave velocities relative to crustal rocks. Variations in velocity with depth reflect mineral phase transitions (e.g., olivine → wadsleyite → ringwoodite), which help map mantle discontinuities.

Olivine-rich peridotite, a coarse-grained, ultramafic intrusive igneous rock with a crystalline texture.

Texture and Fabric of Peridotite

Peridotite typically displays a phaneritic (coarse-grained) texture, reflecting its slow crystallization deep in the mantle. Grain sizes usually range from 1–5 mm, though deformation and recrystallization can significantly modify the original fabric. Olivine and pyroxene crystals are generally visible, and in some mantle-derived samples, deformation produces foliation or lineation, recording patterns of mantle flow.

Several characteristic textures and fabrics are recognized:

Equigranular texture – Crystals of similar size, common in undeformed mantle xenoliths, indicating relatively static mantle conditions.
Porphyroclastic texture – Large, strained olivine grains encircled by finer recrystallized rims, produced by mantle deformation and high-temperature creep. This is a key indicator of mantle flow.
Cumulus texture – In layered intrusions, early-formed olivine crystals settle and accumulate, creating dense olivine-rich layers (dunite).
Foliation and lineation – Preferred orientation of olivine and pyroxene crystals due to shearing, reflecting tectonic strain and mantle-scale flow.

These textures together preserve evidence of both primary crystallization and subsequent deformation, making peridotite a valuable record of mantle processes.

Peridotite thin section showing olivine, clinopyroxene, and spinel

Peridotite thin section showing olivine cumulus crystals with serpentine-filled fractures, intercumulus clinopyroxene, and spinel under cross-polarized light.

Peridotite in Thin Section

Peridotite is composed primarily of olivine (40–90%), pyroxenes (10–40%), and minor spinel or garnet. Viewed in a thin section (a 30-micron-thick rock slice) under a polarizing microscope, peridotite reveals its mineralogy, textures, and deformation history:

Olivine:

Plane-polarized light (PPL): Colorless to pale yellow or green, with high relief (distinct grain boundaries) and conchoidal fractures. Altered grains show serpentine in a mesh-like texture.
Cross-polarized light (XPL): High birefringence produces vibrant second- to third-order colors (pinks, greens, blues). Undulose extinction (wavy darkening when rotated) indicates deformation.

Orthopyroxene (enstatite, (Mg,Fe)SiO₃):

PPL: Pale green to brown, with two cleavages at ~90° and moderate relief. May show exsolution lamellae (fine layers from mineral unmixing during cooling).
XPL: Low birefringence (grey to yellow first-order colors) and straight extinction (grains darken uniformly). Common in harzburgite, enhancing rigidity.

Clinopyroxene (diopside–augite, CaMgSi₂O₆):

PPL: Colorless to pale green, with cleavages at ~120° and moderate to high relief.
XPL: Higher birefringence (second-order blue, orange, or green) with inclined extinction (darkens at angles up to 45°). Often twinned. Abundant in fertile lherzolite, aiding basaltic melt production.

Harzburgite thin section: olivine with high-relief fractures and bright interference colors, alongside orthopyroxene with lower relief and gray interference colors. Mark Brandriss Collection.

Accessory Minerals:

Spinel (e.g., chromian spinel): Isotropic (dark in XPL), reddish-brown to opaque in PPL.
Garnet (e.g., pyrope): Isotropic, pink to red in PPL, with high relief. Often surrounded by strain shadows (distorted zones from deformation).

Textures: Peridotite shows granular (uniform grains) or porphyroclastic (large grains in a finer matrix) textures. Harzburgite (olivine + orthopyroxene) is depleted, with less clinopyroxene, while lherzolite (olivine + orthopyroxene + clinopyroxene) is fertile, reflecting greater melt potential.

Alteration: Hydration forms serpentine, creating:

Mesh texture: Serpentine replacing olivine, preserving crystal outlines.
Bastite texture: Fibrous serpentine after pyroxene.
Magnetite-rich veins in serpentinized peridotites enhance magnetic properties.

Structural Features: Exsolution lamellae in pyroxenes record cooling history, while strain shadows and recrystallized grains indicate mantle deformation. Advanced techniques like electron backscatter diffraction (EBSD) reveal lattice-preferred orientation (LPO), influencing mantle seismic anisotropy (how seismic waves travel through the mantle).

Lherzolite thin section featuring abundant olivine, orthopyroxene, and clinopyroxene minerals, distinguished by unique cleavage patterns and interference colors under polarized light.

Classification: Types of Peridotite

Peridotite is classified by the proportional amounts of its main minerals: olivine, orthopyroxene, and clinopyroxene, along with accessory phases like spinel or garnet. These variations determine whether a rock is fertile—retaining calcium, aluminum, and other components capable of producing basaltic melts—or refractory, meaning depleted by previous melt extraction and resistant to further melting. The different types of peridotite, such as lherzolite, harzburgite, and dunite, illustrate mantle processes ranging from magma generation to residual depletion.

Peridotite classification ternary diagram by Streckeisen (1976) illustrating olivine, orthopyroxene, and clinopyroxene proportions.

Dunite

Dunite is the most olivine-rich peridotite, consisting of more than 90% magnesium-rich forsterite with only minor chromite, pyroxene, or spinel. Dense, greenish, and highly refractory, it represents residues of the mantle after extensive partial melting. Dunite commonly occurs in ophiolite complexes, alpine peridotite massifs, and layered mafic–ultramafic intrusions, where it may form as veins or olivine cumulates. Economically, dunite is significant because of its association with chromite deposits and its occasional inclusion of gem-quality peridot. As the petrological end-member of mantle depletion, it provides important evidence for high-degree melting in Earth’s interior.

Dunite, a type of peridotite, composed primarily of olivine.

Harzburgite

Harzburgite is a refractory peridotite composed primarily of olivine and abundant orthopyroxene, with only trace amounts of clinopyroxene, typically less than five percent. Its composition reflects significant melt extraction, during which basaltic components are removed, leaving behind a depleted mantle residue. Harzburgite is widely developed in ophiolite complexes and the subcontinental lithospheric mantle, where it frequently represents the residual mantle beneath mid-ocean ridges. Its rigidity contributes to the strength of the lithosphere, while its mineralogy provides vital clues to mantle melting, the formation of oceanic crust, and the chemical evolution of Earth’s interior.

Harzburgite sample: ultramafic rock with dominant olivine (green) and orthopyroxene (dark) minerals.

Lherzolite

Lherzolite is the most fertile and common variety of peridotite, consisting of olivine with substantial amounts of both orthopyroxene and clinopyroxene. This balanced mineralogy makes lherzolite a key reference for the composition of the primitive upper mantle. It is the principal source rock of basaltic magmas generated at mid-ocean ridges and hotspots. Lherzolite is often sampled as xenoliths brought to the surface by basaltic eruptions, such as those at Kilbourne Hole in New Mexico and on Oahu, Hawaii, and it also occurs in ophiolite complexes. Because of its fertility, abundance, and role in magma generation, lherzolite is central to models of mantle geochemistry and the processes that sustain Earth’s magmatic systems.

Lherzolite sample showcasing olivine (light green), orthopyroxene (black), and clinopyroxene (dark green) minerals, visible to the naked eye.

Wehrlite

Wehrlite is a clinopyroxene-rich peridotite composed mainly of olivine and clinopyroxene, with little or no orthopyroxene. Moderately fertile, it occupies a transitional position between lherzolite and pyroxenite in mantle classification. Its formation is often linked to melt–rock interaction, where ascending magmas infiltrate and enrich mantle peridotite in clinopyroxene. Wehrlite is typically found in mantle xenoliths, in the transitional zones of ophiolite complexes, and in Alaskan-type ultramafic intrusions associated with convergent margins. Although relatively rare compared to other peridotite types, it provides valuable insights into mantle metasomatism, chemical heterogeneity, and the processes that modify mantle composition over time.

Wehrlite peridotite sample: ultramafic rock featuring olivine (lighter green) and clinopyroxene (dark green) minerals.

Accessory Variants

Garnet peridotite: Stable at depths >60 km, with garnet as the aluminum phase. It is refractory, diagnostic of deep lithospheric mantle, and commonly linked to diamond-bearing cratonic roots.

Spinel peridotite: Stable at 20–60 km depths, with spinel as the aluminum phase. It ranges from fertile to refractory, is common in ophiolites, and typifies mantle beneath mid-ocean ridges.

Hornblende peridotite: Contains >5% hornblende with reduced pyroxenes. Formed by hydrous metasomatism in subduction zones, it is fertile but unstable in dry mantle and often sampled as xenoliths in arc volcanoes.

Serpentinization and Serpentinized Peridotite

Peridotite, the dominant rock of Earth’s upper mantle, is stable under the high pressures and temperatures of Earth’s mantle. However, once it is exposed to fluids at or near Earth’s surface, or within the oceanic lithosphere, it becomes chemically unstable. Under these conditions, it undergoes serpentinization—a hydration and metamorphic reaction that alters its primary minerals, olivine and pyroxene.

Process

Mineral alteration: Olivine and pyroxenes react with water to form serpentine group minerals (lizardite, chrysotile, antigorite), accompanied by magnetite, brucite, and sometimes talc or carbonates.

Reaction dynamics: The transformation is exothermic, releases heat, and increases rock volume by up to ~40%. This creates distinctive replacement textures, such as mesh patterns in former olivine or bastite after pyroxene.

Geological setting: Serpentinization typically occurs at temperatures below ~500 °C, particularly at mid-ocean ridges, transform faults, and subduction forearcs where mantle rocks encounter seawater or metamorphic fluids.

Serpentinite formed from metamorphosed Precambrian peridotite, Ishpeming Greenstone Belt, Michigan's Upper Peninsula, USA.

Consequences

Hydrogen and methane production: Oxidation of Fe²⁺ to Fe³⁺ generates H₂ and CH₄, fueling chemosynthetic ecosystems such as those at the Lost City hydrothermal field.
Mechanical weakening: Serpentinized peridotite becomes less dense (~2.5–2.9 g/cm³) and mechanically weaker than unaltered mantle rock, influencing fault slip, subduction dynamics, and earthquake generation.
Carbon sequestration: Reaction with CO₂ can produce magnesite or dolomite, making serpentinized peridotite both a natural carbon sink and a target for engineered carbon storage.
Geophysical signature: Increased porosity and reduced seismic velocities give serpentinized mantle distinctive properties that can be detected with seismic surveys, helping map hydrated lithosphere.

Serpentinized peridotite from Thetford Mines, Quebec, showcasing metamorphic alteration of olivine-rich peridotite to serpentinite.

Formation and Mantle Processes

Peridotite, the primary mantle rock, records dynamic processes—partial melting, metasomatism, and deformation—that determine its fertile or refractory nature and link to its classification (e.g., lherzolite, harzburgite) and petrographic features in thin sections. These processes shape its mineralogy and textures, reflecting mantle evolution in tectonic and magmatic systems.

Partial Melting

Partial melting in the upper mantle (20–100 km depth) occurs when reduced pressure causes clinopyroxene in fertile lherzolite to melt, forming basaltic magma that builds oceanic crust at mid-ocean ridges and hotspots. The remaining refractory residue, depleted of calcium and aluminum, often becomes harzburgite or dunite, as seen in ophiolites with reduced clinopyroxene in thin sections. This process drives magma generation and transforms fertile peridotites into depleted types, shaping the oceanic lithosphere.

Metasomatism

Metasomatism chemically alters peridotite via fluids or melts, often in subduction zones where water-rich fluids from subducting plates infiltrate the mantle wedge. This introduces elements like potassium, forming hydrous minerals such as amphibole (e.g., hornblende) or phlogopite, transforming refractory harzburgite into fertile rock. In thin sections, metasomatized peridotites show secondary minerals replacing pyroxenes, lowering the melting point. Hornblende-bearing peridotites, found as xenoliths in subduction-related volcanoes, highlight fluid-driven mantle changes.

Deformation

Deformation from mantle convection shears peridotite, aligning olivine and pyroxenes into foliated textures—layered patterns in ophiolite or xenolith samples. In lherzolite or harzburgite, porphyroclastic textures form, with large strained grains amid finer recrystallized ones, visible in thin sections. Occurring in mid-ocean ridges or subduction zones, deformation creates lattice-preferred orientation (LPO), influencing seismic anisotropy. Foliated peridotites in the Oman ophiolite, for example, record mantle flow, evidencing tectonic processes.

These processes govern peridotite’s role in mantle dynamics, from fertile magma sources to depleted residues, as seen in its diverse types and petrographic signatures.

Geochemistry

Peridotite geochemistry preserves clues to mantle heterogeneity and melting history:

Major elements – Rich in magnesium and iron, but low in aluminum and calcium.
Trace elements – Abundances distinguish fertile (enriched) from refractory (depleted) peridotites, reflecting degrees of melt extraction.
Isotopes – Sr, Nd, and Pb isotopes trace mantle sources and evolution, while Os isotopes date ancient melt depletion events older than 2 billion years.
Volatiles – Addition of water, CO₂, and alkalis during metasomatism strongly influences melting behavior and magma generation.

Geological Occurrence: Where Peridotite is Found

The most significant occurrences include:

Ophiolites: These are massive slabs of oceanic crust and the underlying upper mantle that have been thrust onto a continent during tectonic collisions—a process called obduction. Famous examples, like the Semail Ophiolite in Oman and the Troodos Ophiolite in Cyprus, offer extensive exposures of mantle peridotite.

Mantle Xenoliths: Peridotite often reaches the surface as xenoliths, which are foreign rock fragments carried upward by rising magma. Deep-seated volcanic eruptions, such as the kimberlite pipes of South Africa and Siberia, act like elevators, bringing peridotite (and sometimes diamonds) from the mantle to the surface. Basalts in places like Hawaii also frequently contain peridotite xenoliths.

Tectonic Uplift: In mountain-building zones, tectonic forces can uplift and expose slices of mantle rock. These are known as Alpine-type massifs and are found in ranges like the Western Alps and the Pyrenees.

Mid-Ocean Ridges: Along the seafloor, peridotite forms the uppermost mantle directly beneath the oceanic crust. It can be exposed on the seafloor by faulting and sampled by dredging.

Harzburgite peridotite outcrop from the Oman Ophiolite

Representative Examples of Peridotite

Semail Ophiolite, Oman – One of the world’s best-exposed mantle sections, revealing large peridotite sequences, melt-rock reactions, and serpentinization.
Josephine Ophiolite and Rattlesnake Hills, USA – Classic North American occurrences of mantle peridotite.
Lherz Massif, France – Type locality for lherzolite, a fertile peridotite.
Alpine peridotite massifs (European Alps) – Exhumed mantle rocks from continental collision zones.
Abyssal peridotites – Samples from mid-ocean ridges, recording partial melting beneath spreading centers.
Kimberlite localities (Kimberley, South Africa; Yakutia, Russia) – Sources of peridotite xenoliths and diamonds transported from depths >150 km.

Uses and Economic Importance

Although not widely employed as a building stone due to its susceptibility to alteration, peridotite and its derivatives are geologically and economically significant:

Gem-quality olivine (peridot): The gemstone peridot, the August birthstone, is derived from olivine-rich peridotites, particularly dunite bodies.

Diamonds: Mantle-derived kimberlites that contain peridotite xenoliths are the world’s main diamond hosts. Diamonds crystallize at depths >150 km and are transported rapidly to the surface in volcanic pipes.

Chromite and Platinum Group Elements (PGEs): Layered peridotite intrusions, such as the Bushveld Complex (South Africa), are among the world’s largest sources of chromite (for stainless steel) and PGEs (used in catalytic converters, electronics, and hydrogen fuel technologies).

Nickel and Cobalt: Lateritic soils developed on weathered peridotites are an important source of Ni and Co, both critical for batteries.

Magnesium source: Olivine-rich rocks provide magnesium for industrial uses, including refractory materials.

Asbestos (historical): Serpentinized peridotite yields chrysotile asbestos, once widely mined but now restricted due to health hazards.

Construction aggregate: Crushed peridotite and serpentinite are locally used as road metal and building aggregate.

Carbon capture potential: Serpentinized and fresh peridotite naturally react with CO₂ to form stable carbonates such as magnesite (MgCO₃) and dolomite. Large ophiolite bodies, notably in Oman, are studied for engineered CO₂ sequestration, as they could store billions of tons of carbon. Experiments explore accelerating this process through drilling, fracturing, and fluid injection.

Green gem-quality olivine (peridot) crystals from a peridotite.

Petrological and Geochemical Importance

Peridotite is the most important mantle rock for understanding Earth’s geodynamics:

Mantle source rock: Partial melting of fertile peridotite (especially lherzolite) generates basaltic magmas at mid-ocean ridges, volcanic arcs, and hotspots.
Indicator of mantle composition: Mantle xenoliths of peridotite provide direct samples of the lithospheric and asthenospheric mantle, recording mineralogy, chemistry, and deformation.
Geochemical reservoirs: Peridotite compositions constrain mantle heterogeneity, melt extraction histories, and recycling of oceanic crust through subduction.
Metasomatic record: Trace-element and isotopic signatures reveal episodes of melt infiltration and fluid addition in subduction and intraplate settings.
Analytical methods: Peridotites are studied using thin-section petrography, electron microprobe and SEM for mineral chemistry, LA-ICP-MS and SIMS for trace elements and isotopes, X-ray diffraction for mineral identification, high-pressure experimental petrology to determine melting relations, and geophysical techniques (e.g., seismic tomography) to infer mantle structure and distribution.

Why it matters: Studying peridotite allows geologists to reconstruct mantle melting, magma generation, lithospheric stability, metasomatism, and the deep processes that produce strategic resources such as diamonds, Ni-Co ores, chromite, and PGEs.

Planetary Significance

Peridotite is not unique to Earth but represents a fundamental mantle rock type across the solar system:

Mars and Venus: Both planets are believed to have peridotite-dominated mantles, with basaltic volcanism derived from partial melting of mantle peridotite.
Moon: Lunar basalts originate from melting of a peridotitic mantle beneath the lunar crust.
Asteroids: Pallasite meteorites—composed of olivine crystals embedded in Fe-Ni metal—are interpreted as fragments of differentiated asteroidal mantles, closely resembling peridotite.

Thus, peridotite forms a cosmochemical bridge linking Earth’s mantle to the deep interiors of other rocky planets and asteroids, making it central not only to geology but also to planetary science.