View source for Pyroxenite

{{short description|Igneous rock}}
[[Image:Orthopyroxenite (ALH84001).gif|right|thumb|A sample of the orthopyroxenite [[meteorite]] [[ALH84001]]]]
'''Pyroxenite''' is an [[ultramafic]] [[igneous]] [[Rock (geology)|rock]] consisting essentially of [[mineral]]s of the [[pyroxene]] group, such as [[augite]], [[diopside]], [[hypersthene]], [[bronzite]] or [[enstatite]].  Pyroxenes have the general formula {{chem2|XY(Si,Al)2O6}},<ref name="Harmon_etal_2006">{{cite journal | title=Laser-induced breakdown spectroscopy – An emerging chemical sensor technology for real-time field-portable, geochemical, mineralogical, and environmental applications | first1=R.S. | last1=Harmon | first2=F.C. | last2=DeLucia | first3=C.E. | last3=McManus | first4=N.J. | last4=McMillan | first5=T.F. |  last5=Jenkins | first6=M.E. | last6=Walsh |  first7=A. | last7=Miziolek | journal=Applied Geochemistry | year=2006 | volume=21 | issue=5 | pages=730–747 | doi=10.1016/j.apgeochem.2006.02.003}}</ref> where X represents ions of [[calcium]] (Ca), [[sodium]] (Na), [[iron]] (Fe(II)) or [[magnesium]] (Mg) and more rarely [[zinc]], [[manganese]] or [[lithium]], and Y represents ions of smaller size, such as [[chromium]] (Cr), [[aluminium]] (Al), [[magnesium]] (Mg), [[cobalt]] (Co), [[manganese]] (Mn), [[scandium]] (Sc), [[titanium]] (Ti), [[vanadium]] (V) or even iron (Fe(II) or Fe(III)). Pyroxenes share a common structure consisting of single chains of silica [[tetrahedra]]; those that crystallize in the [[monoclinic]] system are known as '''clinopyroxenes''' and those that crystallize in the [[orthorhombic]] system are known as '''orthopyroxenes'''.  Accordingly, pyroxenites are classified into [[clinopyroxenite]]s and [[orthopyroxenite]]s. The [[websterite]]s contain both types of pyroxenes (see diagram below). Closely allied to this group are the [[hornblendite]]s, consisting essentially of [[hornblende]] and other [[amphibole]]s.

They are essentially of [[Igneous rock|igneous]] origin, though some pyroxenites are included in the [[Metamorphism|metamorphic]] [[Lewisian complex]] of [[Scotland]] where the pyroxene-rich rocks result from the type of [[contact metamorphism]] known as [[Metamorphic facies#Pyroxene-hornfels facies|pyroxene-hornfels facies]], have siliceous sediment or basaltic [[protoliths]], and are respectively metapelites and metabasites.

==Intrusive and mantle pyroxenites==
[[File:Metapyroxenite from the Ophiolite on Unst in Shetland Islands in Scotland.jpg|thumb|Metamorphosed clinopyroxenite, made of green diopside, from the Shetland [[ophiolite]], [[Unst]], Scotland]]
Igneous pyroxenites are closely allied to [[gabbro]]s and [[norite]]s, from which they differ by the absence of [[feldspar]], and to [[peridotite]]s, 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 [[crystal]]s which may be several inches in length. The principal accessory minerals, in addition to olivine and feldspar, are [[chromite]] and other [[spinel]]s, [[garnet]], [[magnetite]], [[rutile]], and [[scapolite]].

Pyroxenites can be formed as cumulates in [[Layered intrusion|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 (Earth)|upper mantle]]. The layers typically are a few centimeters to a meter or so in thickness. Pyroxenites that occur as [[xenolith]]s in [[basalt]] and in [[kimberlite]] have been interpreted as fragments of such layers. Although some mantle pyroxenites contain garnet, they are not [[eclogite]]s, 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).

==Pyroxenite lavas==
Purely pyroxene-bearing volcanic rocks are rare, restricted to [[Triodia (plant)|spinifex]]-textured [[Sill (geology)|sill]]s, [[lava tube]]s and thick flows in the [[Archean|Archaean]] [[greenstone belt]]s. Here, the pyroxenite lavas are created by in-situ crystallisation and accumulation of pyroxene at the base of a lava flow, creating the distinctive spinifex texture, but also occasionally mesocumulate and orthocumulate segregations. This is in essence similar to the formation of olivine spinifex textures in [[komatiite]] lava flows, the chemistry of the magma differing only to favor crystallisation of  pyroxene.

A type locality is the [[Gullewa Greenstone Belt]], in the [[Murchison (Western Australia)|Murchison]] region of [[Western Australia]], and the Duketon Belt near [[Laverton, Western Australia|Laverton]], where pyroxene spinifex lavas are closely associated with gold deposits.

==Distribution==
They frequently occur in the form of [[Dike (geology)|dike]]s or segregations in gabbro and peridotite: in [[Shetland]], Cortland on the [[Hudson River]], [[North Carolina]] (websterite), [[Baltimore]], [[New Zealand]], and in [[Saxony]]. They are also found in the [[Bushveld Igneous Complex]] in South Africa and The [[Great Dyke]] in Zimbabwe.<ref>Stratigraphy of the BIC http://jgs.lyellcollection.org/content/161/6/903/F2.large.jpg</ref>

[[Image:Peridotite Olivine-Orthopyroxene-Clinopyroxene - Common Peridotites highlighted.png|thumb|right|Classification diagram for peridotite and pyroxenite, based on proportions of olivine and pyroxene. The pale green area encompasses the most common compositions of peridotite in the upper part of the Earth's mantle]]
The pyroxenites are often subject [[serpentinite|serpentinization]] under low temperature retrograde metamorphism and [[weathering]]. The rocks are often completely replaced by [[Serpentine group|serpentine]]s, which sometimes preserve the original structures of the primary minerals, such as the lamination of hypersthene and the rectangular cleavage of augite. Under pressure-metamorphism hornblende is developed and various types of [[amphibolite]] and hornblende-[[schist]] are produced. Occasionally rocks rich in pyroxene are found as basic facies of [[nepheline syenite]]; a good example is provided by the [[melanite]] pyroxenites associated with the ''borolanite'' variety found in the Loch Borralan igneous complex of [[Scotland]].

==References==
{{Reflist}}

*[https://eos.org/research-spotlights/a-better-model-for-how-the-mantle-melts Lambart, S. L., and others, 2016, ''The role of pyroxenite in basalt genesis: Melt-PX, a melting parameterization for mantle pyroxenites between 0.9 and 5 GPa'', Journal of Geophysical Research – Solid Earth 121, p. 5708–5735]
*[http://www.sciencemag.org/cgi/content/abstract/316/5823/412 Sobolev, A. V., and others, 2007, ''The amount of recycled crust in sources of mantle-derived melts'', Science 316, p. 412-417] (abstract) Retrieved on 6 October 2007

==External links==
*{{Commons category inline|Pyroxenite}}
*{{cite EB1911|author=Flett, John Smith|authorlink=John Flett (geologist)|wstitle=Pyroxenite|volume=22|page=697}}

{{igneous rocks}}
{{Rock type}}
[[Category:Ultramafic rocks]]
[[Category:Plutonic rocks]]
[[Category:Volcanic rocks]]