Pyroxene

Diopside crystals from Afghanistan

Figure 1: A sample of pyroxenite (meteorite ALH84001 from Mars), a rock consisting mostly of pyroxene minerals

The pyroxenes (commonly abbreviated to Px) are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula is XY(Si,Al)₂O₆ (where X represents calcium, sodium, iron⁺² or magnesium and more rarely zinc, manganese or lithium and Y represents ions of smaller size, such as chromium, aluminium, iron⁺³, magnesium, cobalt, manganese, scandium, titanium, vanadium or even iron⁺²). Although aluminium substitutes extensively for silicon in silicates such as feldspars and amphiboles, 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 cystallize in the orthorhombic system are known as orthopyroxenes.

A microscopic image of pyroxene

The name pyroxene is derived from the Ancient Greek words for fire (πυρ) and stranger (ξένος). Pyroxenes were so named 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.

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 upper mantle of Earth is composed mainly of olivine and pyroxene. Pyroxene and feldspar are the major minerals in basalt and gabbro.

Chemistry and nomenclature of the pyroxenes

Figure 2: The nomenclature of the calcium, magnesium, iron pyroxenes.

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. 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 XYT₂O₆. The names of the common calcium–iron–magnesium pyroxenes are defined in the 'pyroxene quadrilateral' shown in Figure 2. The enstatite-ferrosilite series ([Mg,Fe]SiO₃) 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]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 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-hedenbergite (CaMgSi₂O₆ – CaFeSi₂O₆) 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.

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 2.

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

Fe3+

Y

Al

Fe3+

Ti4+

Cr

V

Ti3+

Zr

Sc

Zn

Mg

Fe2+

Mn

X

Mg

Fe2+

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:

1. Coupled substitutions of 1+ and 3+ ions on the X and Y sites respectively. For example, Na and Al give the jadeite (NaAlSi₂O₆) composition.
2. 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. NaFe²⁺_0.5Ti⁴⁺_0.5Si₂O₆.
3. The Tschermak substitution where a 3+ ion occupies the Y site and a T site leading to e.g. CaAlAlSiO₆.

In nature, more than one substitution may be found in the same mineral.

Pyroxene minerals

First X-ray diffraction view of Martian soil - CheMin analysis reveals feldspar, pyroxenes, olivine and more (Curiosity rover at "Rocknest", October 17, 2012).

• Clinopyroxenes (monoclinic; abbreviated CPx)
  • Aegirine, NaFe³⁺Si₂O₆
  • Augite, (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)₂O₆
  • Clinoenstatite, MgSiO₃
  • Diopside, CaMgSi₂O₆
  • Esseneite, CaFe³⁺[AlSiO₆]
  • Hedenbergite, CaFe²⁺Si₂O₆
  • Jadeite, Na(Al,Fe³⁺)Si₂O₆
  • Jervisite, (Na,Ca,Fe²⁺)(Sc,Mg,Fe²⁺)Si₂O₆
  • Johannsenite, CaMn²⁺Si₂O₆
  • Kanoite, Mn²⁺(Mg,Mn²⁺)Si₂O₆
  • Kosmochlor, NaCrSi₂O₆
  • Namansilite, NaMn³⁺Si₂O₆
  • Natalyite, NaV³⁺Si₂O₆
  • Omphacite, (Ca,Na)(Mg,Fe²⁺,Al)Si₂O₆
  • Petedunnite, Ca(Zn,Mn²⁺,Mg,Fe²⁺)Si₂O₆
  • Pigeonite, (Ca,Mg,Fe)(Mg,Fe)Si₂O₆
  • Spodumene, LiAl(SiO₃)₂
• Orthopyroxenes (orthorhombic; abbreviated OPx)
  • Hypersthene, (Mg,Fe)SiO₃
  • Donpeacorite, (MgMn)MgSi₂O₆
  • Enstatite, Mg₂Si₂O₆
  • Ferrosilite, Fe₂Si₂O₆
  • Nchwaningite, Mn²⁺₂SiO₃(OH)₂•(H₂O)

See also

• Clinopyroxene thermobarometry

References

• C.Michael Hogan. 2010. Calcium. eds. A.Jorgensen, C.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment.
• N. Morimoto, J. Fabries, A. K. Ferguson, I. V. Ginzburg, M. Ross, F. A. Seifeit and J.Zussman. 1989. "Nomenclature of pyroxenes" Canadian Mineralogist, Vol.27, pp. 143–156 http://www.mineralogicalassociation.ca/doc/abstracts/ima98/ima98(12).pdf

External links

• Mineral Galleries
• Video Section: Lunar Explorers (link to youtube :: The Lunar Crust)