Pyroxene
111194
220399541
2008-06-19T17:55:30Z
66.31.245.69
/* Chemistry and nomenclature of the pyroxenes */
[[Image:Peridot in basalt.jpg|thumb|'''Figure 1:''' [[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)]]
The '''pyroxenes''' are a group of important rock-forming [[silicate minerals]] found in many [[igneous]] and [[metamorphic rock|metamorphic]] [[rock (geology)|rocks]]. They share a common structure comprised of single chains of silica tetrahedra and they crystallize in the [[monoclinic]] and [[orthorhombic]] systems. Pyroxenes have the general formula XY(Si,Al)<sub>2</sub>O<sub>6</sub> (where X represents [[calcium]], [[sodium]], [[iron]]<sup>+2</sup> and [[magnesium]] and more rarely [[zinc]], [[manganese]] and [[lithium]] and
Y represents ions of smaller size, such as [[chromium]], [[aluminium]], iron<sup>+3</sup>, magnesium, manganese, [[scandium]], [[titanium]], [[vanadium]] and even iron<sup>+2</sup>).
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.
The name pyroxene comes from the Greek words for ''fire'' and ''stranger''. Pyroxenes were named this way because of their presence in volcanic lavas, where they are sometimes seen as crystals embedded in volcanic [[glass]]; it was assumed they were impurities in the glass, hence the name "fire strangers".
However, they are simply early forming minerals that crystallized before the lava erupted.
The upper [[Mantle (geology)|mantle]] of Earth is composed mainly of [[olivine]] and pyroxene. A piece of the mantle is shown in Figure 1 (orthopyroxene is black, diopside (containing chromium) is bright green, and olivine is yellow-green) and is dominated by olivine, typical for common [[peridotite]]. Pyroxene and [[feldspar]] are the major minerals in [[basalt]] and [[gabbro]].
==Chemistry and nomenclature of the pyroxenes==
[[Image:pyrox names.png|left|350px|thumbnail|'''Figure 2:''' The nomenclature of the calcium, magnesium, iron pyroxenes.]]
IMPORTANT NOTE: The ternary diagram here of the pyroxene group is incorrect; it lists Hedenbergite as FeCaSi2O, when really it is FeCaSi2O6. I Don't know how to correct this without creating a whole new picture file, so if whoever made this could please correct that, it would be nice to have proper mineral formulas. Thank you.
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 tetrahederal 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. 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 (Morimoto ''et al.'', 1989).
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 XYT<sub>2</sub>O<sub>6</sub>. The names of the common calcium - iron - magnesium pyroxenes are defined in the 'pyroxene quadrilateral' shown in Figure 2. The [[enstatite|enstatite-ferrosilite]] series ([Mg,Fe]SiO<sub>3</sub>) contain up to 5 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]] ([Mg,Fe,Ca][Mg,Fe]Si<sub>2</sub>O<sub>6</sub>) 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 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]] (CaMgSi<sub>2</sub>O<sub>6</sub> - CaFeSi<sub>2</sub>O<sub>6</sub>) solid solution. The divide is taken at >45 mol.% Ca. As the calcium ion cannot occupy the Y site, pyroxenes with more than 50 mol.% calcium are not possible. A related mineral [[wollastonite]] has the formula of the hypothetical calcium end member but important structural differences mean that it is not grouped with the pyroxenes.
[[Image:Na pyrox trig.png|right|350px|thumbnail|'''Figure 3:''' The nomenclature of the sodium pyroxenes.]]
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 nomenclature shown in Figure 3. 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 mol.% calcium, magnesium or iron(II) components are known as [[omphacite]] and [[aegirine-augite]], with 80% or more of these components the pyroxene falls in the quadrilateral shown in figure 1.
Table 1 shows the wide range of other cations that can be accommodated in the pyroxene structure, and indicates the sites that they occupy.
{|
|+ '''Table 1:''' 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 then filling the site with 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 (NaAlSi<sub>2</sub>O<sub>6</sub>) 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.'' NaFe<sup>2+</sup><sub>0.5</sub>Ti<sup>4+</sup><sub>0.5</sub>Si<sub>2</sub>O<sub>6</sub>.
# The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to ''e.g.'' CaAlAlSiO<sub>6</sub>.
In nature, more than one substitution may be found in the same mineral.
==Pyroxene minerals==
*Clinopyroxenes ([[monoclinic]])
** [[Aegirine]] (Sodium Iron Silicate)
** [[Augite]] (Calcium Sodium Magnesium Iron Aluminium Silicate)
** [[Clinoenstatite]] (Magnesium Silicate)
** [[Diopside]] (Calcium Magnesium Silicate, CaMgSi<sub>2</sub>O<sub>6</sub>)
** [[Esseneite]] (Calcium Iron Aluminium Silicate)
** [[Hedenbergite]] (Calcium Iron Silicate)
** [[Hypersthene]] (Magnesium Iron Silicate)
** [[Jadeite]] (Sodium Aluminium Silicate)
** Jervisite (Sodium Calcium Iron Scandium Magnesium Silicate)
** Johannsenite (Calcium Manganese Silicate)
** Kanoite (Manganese Magnesium Silicate)
** Kosmochlor (Sodium Chromium Silicate)
** Namansilite (Sodium Manganese Silicate)
** Natalyite (Sodium Vanadium Chromium Silicate)
** [[Omphacite]] (Calcium Sodium Magnesium Iron Aluminium Silicate)
** Petedunnite (Calcium Zinc Manganese Iron Magnesium Silicate)
** [[Pigeonite]] (Calcium Magnesium Iron Silicate)
** [[Spodumene]] (Lithium Aluminium Silicate)
*Orthopyroxenes ([[orthorhombic]])
** [[Hypersthene]]
** Donpeacorite, (MgMn)MgSi<sub>2</sub>O<sub>6</sub>
** [[Enstatite]], Mg<sub>2</sub>Si<sub>2</sub>O<sub>6</sub>
** [[Ferrosilite]], Fe<sub>2</sub>Si<sub>2</sub>O<sub>6</sub>
** Nchwaningite (Hydrated Manganese Silicate)
*[[Schefferite]], Ca(Mg,Fe,Mn)Si<sub>2</sub>O<sub>6</sub>
*[[Zinc schefferite]], Ca(Mg,Mn,Zn)Si<sub>2</sub>O<sub>6</sub>
*[[Jeffersonite]], Ca(Mg,Fe,Mn,Zn)Si<sub>2</sub>O<sub>6</sub>
*[[Leucaugite]], Ca(Mg,Fe,Al)(Al,Si)<sub>2</sub>O<sub>6</sub>
*[[Calcium-Tschermak's molecule]], CaAlAlSiO<sub>6</sub>
==References==
*Morimoto, N., J. Fabries, A.K. Ferguson, I.V. Ginzburg, M. Ross, F.A. Seifeit and J. Zussman (1989) "Nomenclature of pyroxenes" Canadian Mineralogist Vol.27 pp143-156 http://www.mineralogicalassociation.ca/doc/abstracts/ima98/ima98(12).pdf
==External links==
*[http://mineral.galleries.com/minerals/silicate/pyroxene.htm Mineral Galleries]
[[Category:Inosilicates]]
[[cs:Pyroxen]]
[[de:Pyroxengruppe]]
[[et:Pürokseenid]]
[[es:Piroxeno]]
[[eo:Pirokseno]]
[[fr:Pyroxène]]
[[ko:휘석]]
[[hr:Pirokseni]]
[[it:Pirosseno]]
[[he:פירוקסן]]
[[lt:Piroksenas]]
[[hu:Piroxén]]
[[nl:Pyroxeen]]
[[ja:輝石]]
[[no:Pyroksen]]
[[pl:Pirokseny]]
[[pt:Piroxena]]
[[ro:Piroxeni]]
[[sk:Skupina pyroxénu]]
[[fi:Pyrokseeni]]
[[sv:Pyroxen]]
[[tr:Piroksen]]
[[zh:辉石]]