View source for Pyroxene

{{Short description|Group of inosilicate minerals with single chains of silica tetrahedra}}
[[File:Diopside-172005.jpg|thumb|Pyroxene ([[diopside]]) crystals from [[Afghanistan]]]]

The '''pyroxenes''' (commonly abbreviated '''Px''') are a group of important rock-forming [[Silicate minerals#Inosilicates|inosilicate]] [[mineral]]s found in many [[Igneous rock|igneous]] and [[metamorphic rock|metamorphic]] [[rock (geology)|rocks]].  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)). Although aluminium substitutes extensively for silicon in silicates such as [[feldspar]]s and [[amphibole]]s, the substitution occurs only to a limited extent in most pyroxenes. They share a common structure consisting of single chains of silica [[tetrahedra]]. Pyroxenes that crystallize in the [[monoclinic]] system are known as '''clinopyroxenes''' and those that crystallize in the [[orthorhombic]] system are known as '''orthopyroxenes'''.

The name ''pyroxene'' is derived from the [[Ancient Greek]] words for 'fire' ({{Wikt-lang|grc|πυρ}}, {{grc-transl|πυρ}}) and 'stranger' ({{Wikt-lang|grc|ξένος}}, {{grc-transl|ξένος}}). Pyroxenes were so named due to their presence in volcanic [[lava]]s, where they are sometimes found as [[phenocryst|crystals embedded]] in [[volcanic glass]]; it was assumed they were impurities in the glass, hence the name meaning "fire stranger". However, they are simply early-forming minerals that crystallized before the lava erupted.

The [[upper mantle (Earth)|upper mantle]] of Earth is composed mainly of [[olivine]] and pyroxene minerals. Pyroxene and [[feldspar]] are the major minerals in [[basalt]], [[andesite]], and [[gabbro]] rocks.<ref>{{Cite journal|last1=Deegan|first1=Frances M.|last2=Whitehouse|first2=Martin J.|last3=Troll|first3=Valentin R.|last4=Budd|first4=David A.|last5=Harris|first5=Chris|last6=Geiger|first6=Harri|last7=Hålenius|first7=Ulf|date=2016-12-30|title=Pyroxene standards for SIMS oxygen isotope analysis and their application to Merapi volcano, Sunda arc, Indonesia|url=http://www.sciencedirect.com/science/article/pii/S0009254116305496|journal=Chemical Geology|language=en|volume=447|pages=1–10|doi=10.1016/j.chemgeo.2016.10.018|bibcode=2016ChGeo.447....1D|issn=0009-2541|url-access=subscription}}</ref><ref>{{Cite journal|last1=O’Driscoll|first1=Brian|last2=Stevenson|first2=Carl T. E.|last3=Troll|first3=Valentin R.|date=2008-05-15|title=Mineral Lamination Development in Layered Gabbros of the British Palaeogene Igneous Province: A Combined Anisotropy of Magnetic Susceptibility, Quantitative Textural and Mineral Chemistry Study|journal=Journal of Petrology|volume=49|issue=6|pages=1187–1221|doi=10.1093/petrology/egn022|issn=1460-2415|doi-access=free}}</ref>

==Structure==
Pyroxenes are the most common single-chain silicate minerals (the only other important group of single-chain silicates, the [[pyroxenoid]]s, are much less common.) Their structure consists of parallel chains of negatively-charged silica tetrahedra bonded together by metal cations. In other words, each silicon ion in a pyroxene crystal is surrounded by four oxygen ions forming a tetrahedron around the relatively small silicon ion. Each silicon ion shares two oxygen ions with neighboring silicon ions in the chain.<ref name=Nesse2000p261>{{cite book |last1=Nesse |first1=William D. |title=Introduction to mineralogy |date=2000 |publisher=Oxford University Press |location=New York |isbn=9780195106916 |page=261}}</ref>

The tetrahedra in the chain all face in the same direction, so that two oxygen ions are located on one face of the chain for every oxygen ion on the other face of the chain. The oxygen ions on the narrower face are described as apical oxygen ions. Pairs of chains are bound together on their apical sides by Y cations, with each Y cation surrounded by six oxygen ions. The resulting pairs of single chains have sometimes been likened to [[I-beam]]s. The I-beams interlock, with additional X cations bonding the outer faces of the I-beams to neighboring I-beams and providing the remaining charge balance. This binding is relatively weak and gives pyroxenes their characteristic [[cleavage (mineralogy)|cleavage]].<ref name=Nesse2000p261/>
<gallery>
File:Ino a.jpg|A single chain of silicon tetrahedra viewed in the [100] direction
File:Ino b.jpg|A single chain of silica tetrahedra viewed in the [010] direction
File:Diopside c scaled ibeam.jpg|Structure of pyroxene looking along the silica chains. "I-beams" are outlined in green. Silicon ions are oversized to emphasize the silicon chains. 
</gallery>

==Chemistry and nomenclature==
The chain silicate structure of the pyroxenes offers much flexibility in the incorporation of various [[cations]] and the names of the pyroxene minerals are primarily defined by their chemical composition. 
Pyroxene minerals are named according to the chemical species occupying the X (or M2) site, the Y (or M1) site, and the tetrahedral T site. Cations in Y (M1) site are closely bound to 6 oxygens in octahedral coordination. Cations in the X (M2) site can be coordinated with 6 to 8 oxygen atoms, depending on the cation size. {{As of|1989}}, twenty mineral names are recognised by the International Mineralogical Association's Commission on New Minerals and Mineral Names and 105 previously used names have been discarded.<ref>{{cite journal |first1=N. |last1=Morimoto |first2=J. |last2=Fabries |first3=A.K. |last3=Ferguson |first4=I.V. |last4=Ginzburg |first5=M. |last5=Ross |first6=F.A. |last6=Seifeit |first7=J. |last7=Zussman |year=1989 |title=Nomenclature of pyroxenes |journal=Canadian Mineralogist |volume=27 |pages=143–156 |archive-url=https://web.archive.org/web/20080309160117/http://www.mineralogicalassociation.ca/doc/abstracts/ima98/ima98(12).pdf |archive-date=9 March 2008 |url-status=dead |url=http://www.mineralogicalassociation.ca/doc/abstracts/ima98/ima98(12).pdf}}</ref>

{{Multiple image
| align = 
| direction = 
| total_width = 500
| image1 = Pyrox names.svg
| alt1 = 
| caption1 = Pyroxene quadrilateral nomenclature of the calcium, magnesium, iron pyroxenes
| image2 = Na pyrox trig.svg
| caption2 = Pyroxene triangle nomenclature of the sodium pyroxenes
| header = Pyroxene nomenclature
}}

A typical pyroxene has mostly silicon in the tetrahedral site and predominantly ions with a charge of +2 in both the X and Y sites, giving the approximate formula {{chem2|XYT2O6}}. The names of the common calcium{{ndash}}iron{{ndash}}magnesium pyroxenes are defined in the 'pyroxene quadrilateral'. The [[enstatite|enstatite-ferrosilite]] series ({{chem2|[Mg,Fe]SiO3}}) includes the common rock-forming mineral [[hypersthene]], contains up to 5&nbsp;mol.% calcium and exists in three polymorphs, [[orthorhombic]] orthoenstatite and protoenstatite and [[monoclinic]] clinoenstatite (and the ferrosilite equivalents). Increasing the calcium content prevents the formation of the orthorhombic phases and [[pigeonite]] ({{chem2|[Mg,Fe,Ca][Mg,Fe]Si2O6}}) only crystallises in the monoclinic system. There is no complete [[solid solution]] in calcium content and Mg-Fe-Ca pyroxenes with calcium contents between about 15 and 25&nbsp;mol.% are not stable with respect to a pair of exolved crystals. This leads to a [[miscibility gap]] between pigeonite and [[augite]] compositions. There is an arbitrary separation between augite and the [[diopside|diopside-hedenbergite]] ({{chem2|CaMgSi2O6\sCaFeSi2O6}}) solid solution. The divide is taken at >&nbsp;45&nbsp;mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxene with more than 50&nbsp;mol.% calcium is not possible. A related mineral, [[wollastonite]] ({{chem2|CaSiO3}}), has the formula of the hypothetical calcium end member ({{chem2|Ca2Si2O6}}), but important structural differences mean that it is instead classified as a pyroxenoid.

Magnesium, calcium and iron are by no means the only cations that can occupy the X and Y sites in the pyroxene structure. A second important series of pyroxene minerals is the sodium-rich pyroxenes, corresponding to the 'pyroxene triangle' nomenclature. The inclusion of sodium, which has a charge of 1+, into the pyroxene implies the need for a mechanism to make up the "missing" positive charge. In [[jadeite]] and [[aegirine]], this is added by the inclusion of a 3+ cation (aluminium and iron(III), respectively) on the Y site. Sodium pyroxenes with more than 20&nbsp;mol.% calcium, magnesium or iron(II) components are known as [[omphacite]] and [[aegirine-augite]]. With 80% or more of these components, the pyroxene is classified using the quadrilateral diagram.[[File:PIA16217-MarsCuriosityRover-1stXRayView-20121017.jpg|thumb|200px|right|First [[X-ray crystallography#Mineralogy and metallurgy|X-ray diffraction view]] of [[Martian soil]] – [[CheMin|CheMin analysis]] reveals [[feldspar]], pyroxenes, [[olivine]] and more ([[Curiosity rover]] at "[[Rocknest (Mars)|Rocknest]]")<ref name="NASA-20121030">{{cite web |last=Brown |first=Dwayne |title=NASA Rover's First Soil Studies Help Fingerprint Martian Minerals |url=http://www.nasa.gov/home/hqnews/2012/oct/HQ_12-383_Curiosity_CheMin.html |date=October 30, 2012 |publisher=[[NASA]] |access-date=October 31, 2012}}</ref>]]

A wide range of other cations can be accommodated in the different sites of pyroxene structures.

{|
|+'''Order of cation occupation in the pyroxenes'''
|-
|'''T'''
|
|Si
|Al
|Fe<sup>3+</sup>
|-
|'''Y'''
|
|
|Al
|Fe<sup>3+</sup>
|Ti<sup>4+</sup>
|Cr
|V
|Ti<sup>3+</sup>
|Zr
|Sc
|Zn
|Mg
|Fe<sup>2+</sup>
|Mn
|-
|'''X'''
|
|
|
|
|
|
|
|
|
|
|
|Mg
|Fe<sup>2+</sup>
|Mn
|Li
|Ca
|Na
|}In assigning ions to sites, the basic rule is to work from left to right in this table, first assigning all silicon to the T site and then filling the site with the remaining aluminium and finally iron(III); extra aluminium or iron can be accommodated in the Y site and bulkier ions on the X site.

Not all the resulting mechanisms to achieve charge neutrality follow the sodium example above, and there are several alternative schemes:
#[[Coupled substitution]]s of 1+ and 3+ ions on the X and Y sites respectively. For example, Na and Al give the jadeite {{chem2|(NaAlSi2O6}}) composition.
# Coupled substitution of a 1+ ion on the X site and a mixture of equal numbers of 2+ and 4+ ions on the Y site. This leads to ''e.g.,'' {{chem2|NaFe(2+)0.5Ti(4+)0.5Si2O6}}.
# The Tschermak substitution where a 3+ ion occupies the Y site and a T site, leading to ''e.g.,'' {{chem2|CaAlAlSiO6}}.
In nature, more than one substitution may be found in the same mineral.

==Pyroxene minerals==
[[File:Microscopic image Pyroxene.jpg|thumb|A [[thin section]] of green pyroxene]]
[[Image:Peridot in basalt.jpg|thumb|[[Mantle (geology)|Mantle]]-[[peridotite]] [[xenolith]] from San Carlos Indian Reservation, Gila Co., Arizona, USA. The xenolith is dominated by green peridot [[olivine]], together with black orthopyroxene and [[spinel]] crystals, and rare grass-green diopside grains. The fine-grained gray rock in this image is the host basalt.(unknown scale).]]

[[File:Orthopyroxenite (ALH84001).gif|thumb|A sample of [[pyroxenite]] (meteorite [[ALH84001]] from Mars), a rock consisting mostly of pyroxene minerals]]

*Clinopyroxenes ([[monoclinic]])
**[[Aegirine]], {{chem2|NaFe(3+)Si2O6}}
** [[Augite]], {{chem2|(Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6}}
** [[Clinoenstatite]], {{chem2|MgSiO3}}
** [[Diopside]], {{chem2|CaMgSi2O6}}
** [[Esseneite]], {{chem2|CaFe(3+)[AlSiO6]}}
** [[Hedenbergite]], {{chem2|CaFe(2+)Si2O6}}
** [[Jadeite]], {{chem2|Na(Al,Fe(3+))Si2O6}}
** Jervisite, {{chem2|(Na,Ca,Fe(2+))(Sc,Mg,Fe(2+))Si2O6}}
** Johannsenite, {{chem2|CaMn(2+)Si2O6}}
** [[Kanoite]], {{chem2|Mn(2+)(Mg,Mn(2+))Si2O6}}
** [[Kosmochlor]], {{chem2|NaCrSi2O6}}
** Namansilite, {{chem2|NaMn(3+)Si2O6}}
** Natalyite, {{chem2|NaV(3+)Si2O6}}
** [[Omphacite]], {{chem2|(Ca,Na)(Mg,Fe(2+),Al)Si2O6}}
** Petedunnite, {{chem2|Ca(Zn,Mn(2+),Mg,Fe(2+))Si2O6}}
** [[Pigeonite]], {{chem2|(Ca,Mg,Fe)(Mg,Fe)Si2O6}}
** [[Spodumene]], {{chem2|LiAl(SiO3)2}}
*Orthopyroxenes ([[orthorhombic]])
** [[Enstatite]], {{chem2|Mg2Si2O6}}
** [[Bronzite]], intermediate between enstatite and hypersthene
** [[Hypersthene]], {{chem2|(Mg,Fe)SiO3}}
** Eulite, intermediate between hypersthene and ferrosilite
** [[Ferrosilite]], {{chem2|Fe2Si2O6}}
** Donpeacorite, {{chem2|(MgMn)MgSi2O6}}
** Nchwaningite, {{chem2|Mn(2+)2SiO3(OH)2*(H2O)}}

==See also==
{{Portal|Minerals}}
* [[Clinopyroxene thermobarometry]]
* [[Emerald]] – sometimes substituted in jewelry by hiddenite, a green variety of spodumene.
* [[Rhodonite]]
* [[Wollastonite]]

==References==
{{Reflist}}
* C. Michael Hogan (2010). [http://www.eoearth.org/article/Calcium?topic=49557 ''Calcium''. eds. A. Jorgensen, C. Cleveland. Encyclopedia of Earth]. National Council for Science and the Environment.

==External links==
*[https://web.archive.org/web/20080212014138/http://mineral.galleries.com/minerals/silicate/pyroxene.htm Mineral Galleries]
*''Video Section'': Lunar Explorers [https://www.youtube.com/watch?v=FGYC0u566gs (link to youtube: The Lunar Crust)]
*{{cite EB1911 |wstitle=Pyroxene |volume=22 |page=696 |short=1}}

{{Minerals}}
{{Authority control}}

[[Category:Inosilicates]]
[[Category:Pyroxene group|*]]
[[Category:Metasilicates]]