Peridotite (Dunite)

Peridotite or Dunite is an intrusive igneous rock, peridotite is coarsegrained and dense. It is light to dark green in color. Peridotite contains at least 40 percent olivine and some pyroxene. Unlike the olivine grains, the pyroxene grains in peridotite have a visible cleavage when viewed under a hand lens. Peridotite forms much of Earth’s mantle and can occur as nodules that are brought up from the mantle by kimberlite or basalt magmas. The rock is usually found interlayered with iron- and magnesium-rich rocks in the lower parts of layered igneous rock bodies, where its denser crystals first form through selective crystallization and then settle to the bottom of still-fluid or semi-solid crystallizing mushes. A peridotite specimen that has been altered by weathering becomes serpentinite. Peridotite and pyroxenite form in similar environments, but pyroxenite contains a higher percentage of pyroxene. Peridotites are important sources of chromium and nickel.

Group – plutonic.
Colour – generally dark greenish-grey.
Texture – phaneritic (coarse grained).
Mineral content – generally olivine with lesser pyroxene ( augite) (dunite is dominantly olivine), always contains some metallic minerals, e.g. chromite, magnetite.
Silica (SiO ₂) content – < 45%.

Peridotite Composition

Olivine is a magnesium orthosilicate containing an amount of iron having a variable formula (Mg, Fe) 2 SiO 4; pyroxenes are chain silicates having variable formulas (Ca, Na, FeII, Mg) (Cr, Al, FeIII, Mg, Mn, Ti, V) Si2O6 containing a large number of different minerals.

Magnesium rich olivine forms a large part of the peridotite and therefore has a high magnesium content. The layered magmatic complexes have much more diverse compositions depending on the fractions of pyroxenes, chromite, plagioclase and amphibole. Small minerals and mineral groups in peridotite include plagioclase, spinel (usually mineral chromite), garnet (especially mineral pyrope), amphibole and flogopite. Plagioclase in peridotite is stable in garnet at relatively low pressures (shell depths), alumina spinel at high pressures (at 60 km or more depths) and higher pressures.

Pyroxenites are associated with ultramafic rocks, mainly composed of orthopyroxene and / or clinopyroxy; Minerals that can be found in smaller amounts include olivine, garnet, plagioclase, amphibole and spinel.

Types of Peridotite

Lherzolite: This rock composed primarily of olivine with significant amounts of orthopyroxene and clinopyroxene. Some researchers believe that much of Earth’s mantle is composed of lherzolite.

Harzburgite: This rock composed primarily of olivine and orthopyroxene with small amounts of spinel and garnet.

Dunite: that is composed mainly of olivine and may contain significant amounts of chromite, pyroxene, and spinel.

Wehrlite: that is composed mainly of orthopyroxene and clinopyroxene, with olivine and hornblende.

Kimberlite: that is composed of at least 35% olivine with significant amounts of other minerals that might include phlogopite, pyroxenes, carbonates, serpentine, diopside, monticellite, and garnet. Kimberlite sometimes contains diamonds.

Peridotite Uses

As a source of valuable ores and minerals, including chromite, platinum, nickel and precious garnet; diamonds are obtained from mica-rich peridotite (kimberlite) in South Africa.

References

• Flexiblelearning.auckland.ac.nz. (2019). Geology – rocks and minerals. [online] Available at: https://flexiblelearning.auckland.ac.nz/rocks_minerals/rocks/peridotite.html [Accessed 4 Mar. 2019].
• King, H. (2019). Peridotite: Igneous Rock – Pictures, Definition & More. [online] Geology.com. Available at: https://geology.com/rocks/peridotite.shtml [Accessed 4 Mar. 2019].
• Ronald Louis Bonewitz, (2012) NATURE GUIDE AND MINERALS, Smithsonian NATURE GUIDE, LONDON, NEW YORK, MELBOURNE, MUNICH, AND DELHI