Apr 21 2014
Peridotite is a coarse-grained, dark-colored intrusive igneous rock, which consists of minerals such as pyroxene and olivine. It usually contains olivine as the primary mineral in addition to other mafic minerals, such as amphiboles and pyroxenes. It is very low in silica and high in magnesium.
Peridotite is common in the Earth’s mantle and basal crust. It is relatively rare and altered by weathering or hydrothermal processes, as its constituent minerals are unstable in the environment outside of the mantle.
Types of Peridotite
The peridotite family consists of different types of intrusive igneous rocks, which include the following:
- Lherzolite – It is type of peridotite which is primarily composed of olivine, in addition to significant quantities of clinopyroxene and orthopyroxene
- Harzburgite – This type includes olivine and orthopyroxene, with small quantities of garnet and spinel
- Dunite – This type is mainly composed of olivine and significant quantities of spinel, pyroxene, and chromite
- Kimberlite – This group of rocks consists of significant quantities of minerals, such as garnet, monticellite, diopside, serpentine, carbonates, pyroxenes, and phlogopite, in addition to 35% of olivine. In some cases, kimberlite also contains diamonds
- Wehrlite – This group of perioditites includes clinopyroxene and orthopyroxene, in addition to hornblende and olivine
Morphology of Peridotite
Peridotites are available in massive or layered form. Layered peridotites may contain gabbroic complexes in their base layers. Peridotites may include three main textures: well-formed olivine crystals surrounded by other minerals, equal-sized crystals having linear boundaries that converge at 120°, and long crystals with ragged curvilinear boundaries formed by internal deformation. Equal-sized crystals are formed as a result of recrystallization, due to slow cooling of rocks.
Some peridotites composed of amphibole have a concentric layered structure and constitute a part of plutons.
Distribution of Peridotite
Peridotites mainly occur in four main types of geologic environments, which are as follows:
- Dikes and irregular masses with rocks that are rich in soda and potash
- Volcanic pipes
- Alpine-type mountain belts as irregular, olivine-rich rocks, with or without gabbro
- Interlayered with magnesia-lime and iron-rich rocks in the lower areas of tabular-layered igneous masses or complexes
Peridotites also occur as fragments carried away by magmas from the Earth’s mantle. The peridotite below the ocean crust can be observed on the walls of rifts in the deep sea floor
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