Peridotite

Description

Peridotite is a dense, coarse-grained ultramafic igneous rock consisting predominantly of olivine (typically >40% by volume), along with varying amounts of pyroxenes (orthopyroxene and clinopyroxene) and accessory minerals such as chromite, magnetite, ilmenite, and garnet. It is characterized by its high magnesium and iron content and very low silica content. The term 'peridotite' is a general classification, with specific types defined by the relative proportions of olivine, orthopyroxene, and clinopyroxene (e.g., dunite, harzburgite, lherzolite, wehrlite).

How to Identify

Color

Typically dark green to yellowish-green, black, or brownish-black. The green color is due to the high olivine content. Weathered surfaces can be reddish-brown due to oxidation of iron.

Luster

Vitreous to greasy on fresh olivine surfaces; pyroxenes can be vitreous. Serpentinized peridotite may have a waxy or silky luster.

Texture

Coarse-grained, phaneritic (visible crystals). Can be equigranular or porphyritic (larger olivine crystals in a finer matrix).

Crystal Form

Olivine crystals are typically anhedral to subhedral, often granular. Pyroxenes can be subhedral to euhedral.

Cleavage

Olivine has poor cleavage (or none), often exhibiting conchoidal fracture. Pyroxenes have distinct cleavage (two directions at nearly 90 degrees for orthopyroxene, and 87/93 degrees for clinopyroxene).

Geological Environment

Found in ophiolite complexes (obducted oceanic crust and mantle), as xenoliths in kimberlites and basalts, and as cumulates in layered mafic intrusions. Also occurs in the roots of ancient mountain belts and in mantle diapirs.

Key Facts

• Hardness: 6.5-7 (olivine), 5-6 (pyroxene)
• Specific Gravity: 3.2-3.4 g/cm³ (higher than most common rocks)
• Crystal System: Orthorhombic (olivine, orthopyroxene), Monoclinic (clinopyroxene)
• Color: Dark green, yellowish-green, black, brownish-black
• Luster: Vitreous to greasy
• Transparency: Opaque to translucent (olivine can be transparent in thin sections)
• Fracture: Conchoidal to uneven (olivine), uneven (pyroxene)
• Cleavage: Poor to absent (olivine), distinct (pyroxenes)
• Composition: Predominantly olivine (>40%), with varying amounts of orthopyroxene, clinopyroxene, and accessory minerals like chromite, garnet, and spinel.

Quick Check

• Color: Dark green to yellowish-green, black, brownish-black
• Luster: Vitreous to greasy (olivine), vitreous (pyroxene)
• Streak: White to colorless

Physical Characteristics

• Crystal Habit: Granular, anhedral to subhedral crystals of olivine and pyroxene.
• Cleavage Type: Olivine: Poor to absent. Pyroxenes: Two distinct cleavages at nearly 90 degrees (orthopyroxene) or 87/93 degrees (clinopyroxene).
• Fracture Type: Conchoidal to uneven (olivine), uneven (pyroxene).
• Tenacity: Brittle.
• Luster Type: Vitreous to greasy.

Formation

Peridotite forms primarily in the Earth's mantle, typically at depths greater than 30 km. It can be brought to the surface through several geological processes: 1. As xenoliths (fragments of mantle rock) entrained in rapidly ascending magmas (e.g., kimberlites, basalts). 2. As part of ophiolite complexes, which are slices of oceanic lithosphere (including mantle peridotite) obducted onto continental margins during tectonic collisions. 3. As cumulates in the lower parts of large layered mafic intrusions, where olivine and pyroxene crystallize and settle out of a basaltic or picritic magma. 4. As residual mantle after partial melting, where basaltic melt has been extracted, leaving behind a refractory peridotite residue.

Usage

Peridotite itself has limited direct industrial uses. However, it is the primary source rock for chromium (from chromite deposits), nickel (from sulfide mineralization), and platinum group elements (PGEs). Serpentinized peridotite (serpentinite) is used as a decorative stone and for carving. Gem-quality olivine (peridot) is a gemstone. Peridotite is also studied extensively for understanding mantle composition, dynamics, and the origin of magmas.

Age Distribution

Precambrian to Cenozoic, found in various tectonic settings, particularly in ancient cratons, ophiolite complexes, and as xenoliths in volcanic rocks.

Where to Find

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Scientific Classification

Mineral Class

Silicate (Nesosilicate for olivine, Inosilicate for pyroxenes)

Group

Ultramafic Igneous Rock

Crystal System

Orthorhombic (olivine, orthopyroxene), Monoclinic (clinopyroxene)

Chemical Formula

Variable, but dominated by (Mg,Fe)₂SiO₄ (olivine) and (Mg,Fe)SiO₃ (orthopyroxene) and Ca(Mg,Fe)Si₂O₆ (clinopyroxene).

Composition

High in MgO and FeO, low in SiO₂. Typically >40% olivine, with pyroxenes and accessory minerals.

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