Editing Pyroxene (section)

==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
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| image1 = Pyrox names.svg
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| 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 predominately 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 not 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{{snd}}CaFeSi2O6}}) solid solution. The divide is taken at >45&nbsp;mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50&nbsp;mol.% calcium are not possible. A related mineral [[wollastonite]] 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 are 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 that 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.