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Peridotite is a dense, coarse-grained ultramafic igneous rock consisting primarily of olivine and pyroxenes. It is the dominant rock type in the Earth's upper mantle. Its composition is characterized by a very low silica content (ultramafic) and high magnesium and iron content. The specific mineral assemblage varies, leading to different peridotite types such as dunite (mostly olivine), harzburgite (olivine and orthopyroxene), lherzolite (olivine, orthopyroxene, and clinopyroxene), and wehrlite (olivine and clinopyroxene). Peridotite often undergoes serpentinization, a hydrothermal alteration process where olivine and pyroxenes react with water to form serpentine minerals, often accompanied by magnetite and talc. This alteration can significantly change the rock's physical properties and appearance.
How to Identify
- Color
- Typically dark green to black, often with a yellowish-green hue from olivine. Can be mottled or speckled.
- Luster
- Dull to greasy on fresh surfaces, especially from olivine. Serpentinized varieties can have a waxy or silky luster.
- Texture
- Coarse-grained, phaneritic (crystals visible to the naked eye). Can be granular or poikilitic.
- Crystal Form
- Individual mineral grains (olivine, pyroxene) are typically anhedral to subhedral, interlocking. Olivine often forms equant grains.
- Cleavage
- Olivine has poor cleavage. Pyroxenes exhibit two distinct cleavages at approximately 90 degrees (orthopyroxene) or 87/93 degrees (clinopyroxene).
- Geological Environment
- Found in ophiolite complexes (obducted oceanic crust and upper mantle), as xenoliths in kimberlites and other volcanic rocks, and as cumulates in large layered mafic intrusions. Represents mantle material.
Key Facts
- Hardness: 6.5-7 (olivine), 5-6 (pyroxene), 2.5-4 (serpentine if altered)
- Specific Gravity: 3.2-3.4 g/cm³ (fresh peridotite), decreasing with serpentinization (2.5-2.8 g/cm³ for serpentinite)
- Crystal System: Orthorhombic (olivine, orthopyroxene), Monoclinic (clinopyroxene)
- Color: Dark green, yellowish-green, black, brownish-green.
- Luster: Vitreous to greasy (olivine), vitreous (pyroxene), waxy to silky (serpentine).
- Transparency: Opaque to translucent (olivine can be transparent in thin sections).
- Fracture: Conchoidal to uneven (olivine), uneven (pyroxene).
- Cleavage: Poor to indistinct (olivine), good in two directions at ~90° (orthopyroxene) or ~87/93° (clinopyroxene).
- Composition: Predominantly olivine ((Mg,Fe)2SiO4) and pyroxenes (Mg,Fe)SiO3 and Ca(Mg,Fe)Si2O6. Accessory minerals may include chromite, magnetite, garnet, spinel, amphibole, and phlogopite.
Quick Check
- Color: Dark green to black, often with yellowish-green olivine.
- Luster: Dull to greasy (fresh), waxy to silky (serpentinized).
- Streak: White to pale green (olivine), colorless (pyroxene), black (magnetite if present).
Physical Characteristics
- Crystal Habit: Granular, anhedral to subhedral interlocking crystals.
- Cleavage Type: Poor in olivine, distinct prismatic in pyroxenes.
- Fracture Type: Conchoidal to uneven.
- Tenacity: Brittle.
- Luster Type: Vitreous to greasy on fresh surfaces.
Formation
Peridotite is primarily formed in the Earth's mantle. It can be brought to the surface through various geological processes, including obduction in orogenic belts (ophiolites), as xenoliths in volcanic rocks, or as cumulates in large layered mafic intrusions. It forms at high temperatures and pressures, typically above 1000 °C and at depths greater than 30 km.
Usage
Peridotite itself has limited direct industrial uses. However, it is the primary source rock for chromium (from chromite), nickel (from pentlandite), and platinum group elements (PGEs) when these minerals are present as accessory phases. Serpentinized peridotite is used as a decorative stone and for some construction purposes. Olivine-rich peridotites (dunites) are sometimes used as refractory materials due to olivine's high melting point. Peridot, the gem variety of olivine, is found within peridotite.
Age Distribution
Ranges from Archean to Cenozoic, found in various tectonic settings.
Where to Find
Ophiolite Complexes Worldwide
Examples include the Troodos Ophiolite (Cyprus), Semail Ophiolite (Oman), Coast Range Ophiolite (California, USA), and various ophiolites in the Alps, Himalayas, and Appalachians. These represent fragments of oceanic lithosphere obducted onto continental margins.
Kimberlite Pipes
Diamond-bearing kimberlite pipes (e.g., in South Africa, Canada, Siberia) often contain peridotite xenoliths, providing direct samples of the Earth's mantle.
Layered Mafic Intrusions
Large igneous complexes such as the Bushveld Complex (South Africa), Stillwater Complex (Montana, USA), and Skaergaard Intrusion (Greenland) contain cumulate peridotites.
Oceanic Fracture Zones and Mid-Ocean Ridges
Peridotite can be exposed on the seafloor in areas of extensive faulting and slow-spreading ridges, where mantle material is brought closer to the surface.
Finding Tips
Look for Ophiolite Sequences
In areas known for ophiolites, peridotite will often be found alongside serpentinite, gabbro, and pillow basalts. It typically forms the basal unit of an ophiolite.
Identify Serpentinization
Many peridotites are partially or completely serpentinized. Look for greenish, often slickensided (polished by faulting) rocks with a waxy luster. Fresh peridotite is much harder and darker.
Check for Associated Minerals
Chromite (black, metallic, dense) and magnetite are common accessory minerals, especially in serpentinized peridotites. Garnet (pyrope) may be present in mantle xenoliths.
Consider Density
Peridotite is a dense rock. A hand sample will feel noticeably heavy for its size compared to more common crustal rocks.
Similar Rocks
Serpentinite
Serpentinite
Also known as: Serpentine rock
Pyroxenite
Pyroxenite
Also known as: Orthopyroxenite, Clinopyroxenite
Gabbro
Gabbro
Also known as: Black granite (commercial term)
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, dominated by (Mg,Fe)2SiO4 (olivine) and (Mg,Fe)SiO3 (pyroxene).
- Composition
- MgO (35-50 wt%), SiO2 (38-45 wt%), FeO (6-12 wt%), Al2O3 (1-5 wt%), CaO (1-5 wt%).
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