Peridotites are dense, coarse-grained, dark-colored ultramafic plutonic rocks with at least 40% olivine by volume. Members are dunite, kimberlite, harzburgite, wehrlite and lherzolite. Others are hornblende and pyroxene-hornblende peridotites.
These silica-poor (< 45 wt.% SiO2) ultrabasic rocks have at least 90 vol. % mafic minerals. Their essential minerals are magnesium-rich olivine and pyroxenes. However, they also have minor amounts of mica, amphibole, and other minerals.
The name peridotite comes from peridot, a mostly yellow-green or olive-green olivine gemstone. However, peridotites are rocks, not minerals like peridots.
Where do they occur? Peridotite are a significant constituent of the upper mantle up to about 249 miles (≈ 400 km), with only small amounts in the crust. Those exposed on the Earth’s surface or submarine are usually altered or serpentinized.
Besides Earth, there have been blocks and clasts of peridotitic rocks in breccia from the moon. Scientists believe these blocks and clasts are from the lunar highland’s gabbro-anorthosite cumulates. Also, some meteorites have a peridotite-like composition.
Let us explore peridotite rocks. We will discuss what these rocks look like and their properties, including density, color, texture, and hardness.
Also, we will cover chemical and mineral composition, the different types, where they form and are found, and their significance or uses.
Quick overview and properties
- Name: Peridotite
- Rock type: Igneous
- Origin: Intrusive or plutonic
- Chemical composition: Ultramafic
- Color: Mostly light to dark green but may be greenish gray, pale yellow-greenish, red, dark grayish, brownish-black to nearly black, and rarely bluish.
- Texture: Coarse-grained or phaneritic, with some having a poikilitic or porphyritic texture.
- Cooling rate/history: Slow, deep inside the Earth
- Density: 3.3g/cm3
- Mohs hardness scale: 6.5-7
- Melting point: 1450 and 1750 °C at depths of between 30 and 150 km (see source)
- Extrusive equivalent: Komatiites ( most erupted in the Archean age; younger are rare)
- Tectonic environment: They occur mainly in the upper mantle, ophiolites (associated with obduction of oceanic crust in convergent boundaries), or at stable ancient cratons. Some layered intrusions are associated with divergent boundaries or rift zones.
What do peridotites look like?
In the field, their high density and appearance, i.e., color and texture, will help you identify peridotites. These rocks may somewhat resemble peridots in color but will appear dull, resinous, and not vitreous like peridots.
Usually, peridotites are dense, massive, or layered, greenish, greenish gray, pale yellow-green, reddish brown, bluish, brownish-black, dark gray to nearly black rocks with coarse-grained or cumulative texture. But they can have other textures, too.
However, color, texture, or appearance may vary from one specimen to another and from one variety to another.
Also, serpentinized and weathered specimens may have slightly different colors. Also, this process will alter other physical and chemical properties of these rocks.
Let us discuss more about colors, texture, form, and alteration.
1. Colors
Most peridotites are light, medium to dark greenish due to the high olivine content, sometimes with dark or black speckles. But they can be greenish-gray, pale-yellow-green, red, brown, blue, brownish-black, dark gray, or nearly black.
However, weathered outcrops will have a brownish crust, while submarines will be deep orange. Also, olivine alteration to iddingsite may create a dull, bright yellow to brown, especially on the crust.
2. Texture
Peridotites have mostly coarse-grained or aphanitic textures. However, these rocks may have cumulative, protogranular, poikilitic, pegmatitic, cataclastic, or granoblastic textures.
A cumulative (intercumulus) texture occurs in layered peridotites formed from cumulates. This texture has well-formed or euhedral interlocking crystals larger than 0.5cm in a bit finer-grained crystal matrix from liquids trapped in interstitial gaps. Such may have preferred crystal orientation, too.
Highly deformed and fractured mantle peridotites (especially kimberlites or basalt) will record deformation in their texture. Such rocks will have sheared or cataclastic texture characterized by larger crystals with deformation twinning structures interspaced with smaller, less strained crystals. This texture results from tectonic emplacement mode.
On the other hand, the less or undeformed rocks will have coarse grains with a protogranular texture characterized by smooth, curved boundaries.
Also, the undeformed peridotite will have nearly equigranular anhedral grains. These grains will have a granoblastic texture with polygonal boundaries/morphology meeting at three junctions, making interfacial angles of about 120 degrees on thin sections.
Granoblastic texture likely forms from slow cooling that resulted from recrystallization to reduce surface energy.
Besides the above, poikilitic peridotite textures are common. Such will have pyroxene oikocrysts enclosing many smaller, well-formed (idiomorphic) olivine grains.
Lastly, some peridotites have a pegmatitic texture. This texture is characterized by unusually large crystals, usually olivine, in a coarse-grained matrix. However, olivine may have altered to serpentine.
3. Form: Massive and layered
Peridotite rocks can be massive or layered and part of an igneous intrusive complex.
The layering, foliation, or concordant compositional banding may occur between various varieties like dunite, lherzolite, wehrlite, or harzburgite. Alternatively, they may be interlayered with related ultramafic, magnesium, and iron-rich rocks like pyroxenite or websterites.
Why does it happen? Layering happens due to the preference for fractional crystallization and settling of cumulates that are denser and crystallize at higher temperatures.
4. Altered and serpentinized peridotite
Peridotite rocks have high-temperature olivine and pyroxenes. Such minerals are unstable when on the Earth’s surface. Therefore, they will undergo alteration or serpentinization.
This alteration occurs via the hydration and oxidation of pyroxenes and olivine to form serpentine minerals, talc, brucite, and magnetite. Some of the serpentine minerals are antigorite, chrysolite, or lizardite.
Usually, the resulting alteration of product form depends on temperature. For instance, if it happens at moderately high temperatures, antigorite replaces chrysotile (usually fills cracks from hydration). However, at over 500°C, antigorite will decompose to talc and forsterite.
Lastly, serpentinization will cause a change in color, appearance, and other properties. For instance, the resultant green serpentinites are mechanically weak. As a result, most will undergo weathering, forming soil in which some plants will prefer growing. Thus, certain plants may indicate the presence of serpentinized and weathered peridotites.
Peridotite composition
Let us now look at the chemical and mineral composition of peridotites.
1. Chemical composition
Chemically, peridotites are ultramafic or ultrabasic plutonic rocks. These rocks are silica-poor (< 45 wt.%), high in magnesium and iron oxides, and low in aluminum, sodium, and potassium.
Usually, magnesium (Mg) accounts for 89 mol% of iron and magnesium content in these rocks. Therefore, most minerals are Mg-rich, as we will see next under mineralogy.
The typical percentage weight chemical composition of peridotite from Lizard Area, Cornwall, according to Green (1964), is SiO2: 44.8%, TiO2: 0.19%, Al2O3: 4.16%, Fe2O3: 1.36%, FeO: 6.85%, MnO: 0.11%, MgO: 39.2%, CaO: 2.42%, Na2O: 0.22%, K2O: 0.05%
Additionally, these rocks may have minor amounts of elements like cobalt, nickel, platinum, chromium, etc.
2. Peridotite mineral composition
Peridotites have mostly olivine with considerable amounts of pyroxenes and lesser amounts of amphibole and micas, with no quartz and no more than 5% feldspar. Accessory minerals are spinel, garnets, chromite, magnetite, ilmenite, perovskite, humite, and sulfides.
Not to overemphasize, peridotites are ultramafic rocks. Such rocks have mostly mafic (magnesium and iron) minerals, accounting for over 90% of this rock’s total mineral composition by volume. However, this definition alone isn’t sufficient.
Peridotites must have at least 40% olivine, considerable but lesser pyroxenes, and the other mentioned minerals above.
Going to specific minerals, this rock is dominated by magnesium-rich, iron-containing olive-green olivine. This olivine has a variable chemical formula (Mg, Fe)2 SiO4 and may include forsterite. Sometimes, it is enclosed in plagioclase feldspar or embedded into clinopyroxene, forming a poikilitic texture.
What about pyroxenes? Peridotites have considerable pyroxenes amounts, both orthopyroxene with orthorhombic crystal structure and clinopyroxenes. Enstatite is the most common orthopyroxene and may have iron substituting magnesium in some instances. However, bronzite or hypersthene do occur, too.
On the other hand, clinopyroxenes with a monoclinic crystal structure have a lower melting point than orthopyroxene or olivine. In this rock, diopside is the most common clinopyroxenes. However, sometimes, hedenbergite, in which iron substitutes magnesium, may occur.
Besides these two dominant minerals, peridotites may have micas, amphibole (hornblende or richterite or pargasite), plagioclase, and aluminous phase minerals.
Usually, micas present are biotite or phlogopite. They, together with plagioclase, often occur in interstices. However, hydrous minerals, including richterite, phlogopite, or pargasite, are less than 1%, and hornblende arises from hydrous fluid alterations.
What about the aluminous phase minerals? Usually, pressure will influence the aluminous phase, i.e., plagioclase, spinel, or garnet, thus can help know formation depth. Low pressure at shallower depths will form plagioclase, moderate spinel (aluminum-rich), and sometimes chrome-bearing picotite). Magnesium spinel may occur in basanite and alkali basalts.
On the other hand, mantle peridotites at high pressure due to overlaying rock will have garnets like pyrope or chrome pyrope. Thus, kimberlites will often have pyrope garnets.
Lastly, mineralogy will depend on the variety, e.g., kimberlite has omphacite.
Peridotite classification, types, and varieties
As already mentioned, ultramafic rocks have less than 45 wt.% SiO2 and over 90 vol% mafic minerals. These rocks can be further classified according to the composition of olivine, clinopyroxenes, orthopyroxenes, and hornblendes.
The types of peridotites are dunite, kimberlite and pyroxene peridotite which has harzburgite, wehrlite and lherzolite. Other are hornblende and pyroxene-hornblende.
Let us now discuss each of these types briefly.
1. Dunite
Dunite is a coarse-grained, pale olive to dark green peridotite rock with more than 90% olivine. However, it may appear tan to reddish brown when oxidized and occurs as cumulates in layered intrusions or the lower parts of ophiolite sequences.
On the olivine-clinopyroxene-orthopyroxene ternary diagram, dunite occupies the peak where olivine is.
2. Kimberlite
Kimberlites are rare, brecciated peridotite rocks rich in angular xenocrysts and xenoliths and may host diamonds in extremely rare cases.
Usually, non-altered kimberlites are greenish slate blue, dark blue-green, and dark gray to black. However, weathered and altered ones will appear decomposed and yellowish.
Lastly, kimberlitic rocks occur mostly in volcanic pipes, sills, and dikes in stable continental intraplate tectonic settings or cratons not associated with subduction, rifts, hotspots, or orogenesis.
3. Pyroxene peridotite
These are peridotite with 40% to 90% olivine by volume and are further classified as follows depending on the amounts of clinopyroxene and orthopyroxene:
a). Harzburgite
Harzburgite has 40% to 90% olivine, more than 5% orthopyroxenes (mainly enstatite and bronzite) and < 5% clinopyroxene. It dominates ophiolite’s peridotite layer and cumulates above dunite in layered intrusions.
How do they form? Harzburgites are residual/depleted upper mantle rocks formed after partial melting and extraction of basaltic magmas and dominate the upper mantle beneath continental cratons.
Harzburgite rock from an intrusion near Ambositra, Madagascar. Photo credit: Diorit, CC0, via Wikimedia Commons
b). Wehrlite
Wehrlite has 40%-90% olivine, more than 5% clinopyroxene, and less than 5% orthopyroxenes. It represents the fertile mantles and occurs above harzburgite in layered intrusions and between gabbro and other peridotite in ophiolites.
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c). Lherzolite
Lherzolite has 40%-90% olivine and intermediate orthopyroxene and clinopyroxene, i.e., more than 5% of either with minor chromium, garnets, and aluminum spinel.
In this rock, orthopyroxene is the dominant pyroxene, with chromium-rich clinopyroxene occurring in lesser amounts.
Lastly, garnet lherzolite is a type that probably makes a larger part of the Earth’s crust’s mantle.
4. Hornblende peridotite
Hornblende peridotites have 40-90% olivine and up to 50% hornblende (amphibole). This rock likely forms from the alteration of mantle rock by fluid, especially from the oceanic crust during subduction.
5. Pyroxene hornblende peridotite
It has 40-90% olivine and an intermediate amount of pyroxene and hornblende, i.e., either hornblende or pyroxene accounts for at least 5% to 95% of the combination of these two.
Naming peridotites
When naming peridotite varieties, you can use the name of significant mineral(s), including accessories. However, olivine must be >40% and the dominant mineral.
For those with up to 5% minerals like hornblende, spinel, garnet, etc., add the mineral name and word ‘bearing’ as a prefix. With this respect, you can have hornblende-, spinel-, or garnet-bearing peridotite like lherzolite or whichever variety.
For those with over 5% of a given mineral(s) but no more than 50%, use the mineral(s) as a prefix. For instance, you can have garnet, pyroxene, or spinel peridotite/ kimberlite/wehrlite, etc. When a given mineral exceeds 50%, use peridotitic + dominant minerals. For example, you can have peridotitic chromitite or pyroxenite.
Note: However, if a given mineral like pyroxene exceeds 60%, the rock becomes pyroxenite, which is darker. Those dominated with hornblende are hornblendite. Also, if feldspar exceeds 5%, you grade into a gabbro, norite, or troctolite.
Origin or formation
Most peridotites originate from the upper mantle, i.e., in the asthenosphere and upper, transitional, or plume mantles. In the upper mantle, they probably formed during early or young Earth accretion and differentiation and occur as blocks or fragments formed from mantle magma. The cooling rate was, of course, slow.
Although the mantle and asthenosphere have high temperatures, peridotites in the mantle don’t melt due to the high pressure of overlaying rocks.
Also, peridotites may form in layered mafic to ultramafic igneous intrusion complexes as cumulates of olivine, sometimes pyroxenes, and other minerals. These cumulates crystallize from basaltic magma mushes that cool slowly, forming a coarse-grained texture.
Usually, the resultant peridotitic rock composition varies depending on the amounts of olivine, pyroxenes, plagioclase, amphiboles, chromite, and other accessory minerals.
Lastly, basaltic magma mushes form from partially melting peridotites in the upper mantle. This partial melting may occur due to upwelling and compression of the asthenosphere, mantle plumes, and less often from flux melting (in subduction zones). Peridotite inclusions or nodules in these magmas are evidence of their origin.
Where is peridotite found?
Peridotite is a predominant rock in the Earth’s upper mantle at depths up to 400km, with only a minor yet important part of the Earth’s crust or surface. Depths beyond 400 km will convert olivine to wadsleyite.
This mantle-derived rock occurs as cumulates on continental crust, with submarines having abyssal peridotites.
Let us now look at the mantle, cumulate, and abyssal peridotites and where this rock is found.
1. Mantle peridotites
Mantle peridotite occurs as xenoliths or nodules in basalts, kimberlite, nephelinites, or part of the ophiolite complex (orogenic Andean type or alpine ophiolite peridotites). They rarely have plagioclase or spinel, unlike crustal. Also, they have higher magnesium and iron relative to silica compared to crustal due to high pressures.
Ophiolites represent uplifted and exposed sections of ancient oceanic crust and upper mantle emplaced on the continent’s crust or lithosphere. They will have pillow lavas, mafic flows, breccia, and chert followed by a gabbro section, down to ultramafic cumulates, and upper mantle peridotites, including deformed ones.
These ophiolites include Coast Range in California, USA), Semail in Oman and UAE, Troodos in Cyprus, and Dun Mountain-Maitai Terrane in New Zealand.
2. Cumulate peridotites
This type occurs on the lower part of ultramafic or mafic igneous intrusion complexes as cumulates. Some types may also occur in sills, dikes, volcanic pipes, or komatiite lava flows.
How do they form? Cumulate peridotites form when high-temperature minerals crystallize preferentially and settle at the bottom of intruding semi-solid magma mushes.
These layered intrusions may occur in stable continental cratons associated with plume magnetism. However, others are associated with rift magnetism, with some occurring in root zones of volcanoes.
Usually, peridotitic cumulates are layered and with different thicknesses. The lower parts of these layered intrusions will have chromitites, peridotite rocks like dunites, harzburgite, lherzolite, and other ultramafic rocks like bronzite pyroxenites.
On the other hand, you will find mafic rocks like anorthosite, norite, or gabbro toward the upper part.
Examples of igneous intrusions where cumulate peridotitic rocks occur include the Bushveld Igneous Complex in South Africa, the largest layered intrusion in the world. Other places are Great Dyke in Zimbabwe, Stillwater Complex in Montana, USA, Muskox Intrusion in Canada, and the Alaskan-type ultramafic complexes in Alaska, USA.
3. Abyssal peridotite
Abyssal peridotitic rocks occur beneath the oceanic crust (below gabbro, basalts, and sediments) and are often exposed near ocean floors at mid-ocean ridges.
These rocks likely form from partial melting residue and are depleted with light rare earth elements. Therefore, they are not in equilibrium with mid-oceanic ridge basalts.
Examples include several locations on the Mid-Atlantic Ridge Complex. Also, these rocks are in the Indian Ocean at Carlsberg and Southwest Indian ridges.
4. Specific locations
Peridotites occur in many countries on all continents. In the US, they are widespread in California, Montana, and Oregon. Also, they are in states like Colorado, Hawaii, North Carolina, Virginia, Idaho, Arizona, Georgia, Arkansas, etc. See geologic units with this rock in the US.
Elsewhere, they are widespread in Russia, Spain, Norway, New Zealand, Australia, Ireland, China, Greece, India, Canada, South Africa, Japan, France, Czech Republic and Brazil. Also, they occur in Mongolia, Zimbabwe, Ukraine, the UK, Turkey, Tanzania, Syria, Sweden, Portugal, Poland, Namibia, Germany, and many others.
Significance
Some of the peridotite nodules in kimberlites and basalt depths of about 200 km preserve stable isotope ratios of osmium and other elements. Such can help determine the age of Earth (when it formed), early mantle composition, evolution, processes involved, and much more.
What is peridotite used for?
Peridotites may help in carbon sequestration, contain diamonds, and have valuable minerals like nickel, chromium, copper, platinum, etc. Also, you can use them for construction or as furnace foundry bricks, among other uses.
Here is more on these uses.
- Peridotites present an affordable, safe, and permanent way of carbon sequestration – capturing and storing atmospheric carbon dioxide (CO2). Components of these rocks react with CO2 to form carbonates resembling limestone or marble. It is possible to enhance sequestration by Hydraulic fracturing and drilling. And as Kelemen & Matter (2008) estimate, the situ carbonation of peridotite from Oman can sequestrate over a billion tons of CO2.
- Some xenolithic mantle peridotites may contain valuable minerals like peridot, and some kimberlite host diamonds.
- Layered peridotitic rocks are associated with minerals rich in ores like nickel, chromium, platinum, copper, palladium, and some compounds of sulfides. For instance, podiform and cumulative layered chromite is associated with layered peridotites in Bushveld Complex and Great Dyke
- Some of these rocks, like dunite, have uses in building and construction as aggregate and architectural stones.
- They can make foundry or furnace bricks, and their high calcium oxide makes them suitable flux in metallurgical blast furnaces.
- These rocks are serpentine and talc protolith.
- Since they retain heat, peridotites make good sauna heat rocks, costing you about US$ 35-100 per 20- and 50-pound bag, respectively.
Frequently Asked Questions (FAQs)
Peridotites are denser than gabbros, and their color tends toward pale to dark green in most cases. In contrast, gabbros are less dense, mostly black to gray, and often have dark and off-white, pale brown, or pale gray speckles.
Compositionally, peridotites are ultramafic rocks (with over 90% mafic minerals) with less silica, Al2O3, and CaO than gabbro and no quartz. Also, these rocks have more magnesium-rich olivine than pyroxene. Conversely, gabbro is a mafic with small amounts of quartz and more pyroxene than olivine.
Peridotites are associated with komatiites. Komatiites are picritic ultramafic rocks of mostly the Archaean eon with some Proterozoic to Phanerozoic eons and have at least 18 wt. % magnesium oxide.
Also, according to Medaris et al. (2018), mantle peridotite massifs may have eclogite lenses, boudins, or layers. These eclogites originate from high-pressure omphacite and garnet crystallization in transient melts within the mantle.
Lastly, eclogite can occur from prograde high-pressure recrystallization of plagioclase in basic dikes within peridotites.
References
- Park, A. F. (1989). Peridotite. In Bowes, D. R. (ed.). The encyclopedia of igneous and metamorphic petrology. New York: Van Nostrand Reinhold.
- Bonewitz, R. (2012). Rocks and minerals (1st ed.). DK Pub.
- Deer, W. A., Howie, R. A., & Zussman, J. (2013). An introduction to the rock-forming minerals (3rd ed.). The Mineralogical Society.
- Best, M. G. (2013). Igneous and metamorphic petrology (2nd ed.). Blackwell Publishers.
- Gill, R. (2010). Igneous rocks and processes: A practical guide (1st ed.). Wiley-Blackwell.
- Haldar, S. K., & Tisľjar, J. (2014a). Introduction to Minerology and petrology (1st ed.). Elsevier.
- Green, D.H. (1964). ‘The petrogenesis of the high-temperature peridotite intrusion in the Lizard Area, Cornwall.’ Journal of Petrology, 5(1), pp. 134–188. doi:10.1093/petrology/5.1.134.
- Medaris, L. G., Brueckner, H. K., Cai, Y., Griffin, W. L., & Janák, M. (2018). Eclogites in peridotite massifs in the Western Gneiss region, Scandinavian Caledonides: Petrogenesis and comparison with those in the Variscan Moldanubian Zone. Lithos, 322, 325–346. https://doi.org/10.1016/j.lithos.2018.10.013
- Kelemen, P. B., & Matter, J. (2008). In situ carbonation of Peridotite for CO2 storage. Proceedings of the National Academy of Sciences, 105(45), 17295–17300. https://doi.org/10.1073/pnas.0805794105