page_title: Pyroxenite
section_title: Intrusive and mantle pyroxenites
hierarchical_section_title: Pyroxenite / Intrusive and mantle pyroxenites
caption_attribution_description: English: Metapyroxenite (8.0 cm across) composed of chromian diopside pyroxene ((Ca,Cr)MgSi2O6 - calcium chromium magnesium silicate). Because this pyroxenite (a clinopyroxenite, really) is all diopside, a better rock name is diopsidite. This is from an ophiolite complex (Shetland Ophiolite Complex; a.k.a. Unst Ophiolite Complex) exposed on the island of Unst in the northeastern Shetland Islands (in the North Sea, between Scotland &amp; Norway). The rocks in the area represent ~basal crustal rocks metamorphosed during the Caledonian Orogeny (~425-500 million years ago). Published research indicates that the original rocks were from a late Neoproterozoic-Cambrian intracontinental, ocean-floored rift basin. This metapyroxenite is from the upper pyroxenite layer of the Unst Ophiolite Complex. Locality: outcrop at Skeo Taing, shores of Balta Sound, eastern margin of island of Unst, northeastern Shetland Islands, Scotland.
context_page_description: Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both types of pyroxenes. Closely allied to this group are the hornblendites, consisting essentially of hornblende and other amphiboles.
  They are essentially of igneous origin, though some pyroxenites are included in the metamorphic Lewisian complex of Scotland. The pyroxene-rich rocks, which result from the type of contact metamorphism known as pyroxene-hornfels facies, have siliceous sediment or basaltic protoliths, and are respectively metapelites and metabasites.
context_section_description: Igneous pyroxenites are closely allied to gabbros and norites, from which they differ by the absence of feldspar, and to peridotites, which are distinguished from them by containing more than 40% olivine. This connection is indicated also by their mode of occurrence, for they usually accompany masses of gabbro and peridotite and seldom are found by themselves.
  They are often very coarse-grained, containing individual crystals which may be several inches in length. The principal accessory minerals, in addition to olivine and feldspar, are chromite and other spinels, garnet, magnetite, rutile, and scapolite.
  Pyroxenites can be formed as cumulates in ultramafic intrusions by accumulation of pyroxene crystals at the base of the magma chamber. Here they are generally associated with gabbro and anorthite cumulate layers and are typically high up in the intrusion. They may be accompanied by magnetite layers, ilmenite layers, but rarely chromite cumulates.
  Pyroxenites are also found as layers within masses of peridotite. These layers most commonly have been interpreted as products of reaction between ascending magmas and peridotite of the upper mantle. The layers typically are a few centimeters to a meter or so in thickness. Pyroxenites that occur as xenoliths in basalt and in kimberlite have been interpreted as fragments of such layers. Although some mantle pyroxenites contain garnet, they are not eclogites, as clinopyroxene in them is less sodic than omphacite and the pyroxenite compositions typically are unlike that of basalt. Pyroxenites might play an important role in basalt genesis (e.g., Lambart et al., 2016), either by contributing directly to the magma production, or indirectly as the result of reaction between peridotite and magma derived from partial melting of eclogite (e.g., Sobolev and others, 2007).
caption: Metamorphosed clinopyroxenite, made of green diopside, from the Shetland ophiolite, Unst, Scotland
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