Pyroxene

  The pyroxenes are a group of important rock-forming silicate minerals found in many igneous and metamorphic rocks. They share a common structure comprised of single chains of silica tetrahedra and they crystalise in the monoclinic and orthorhombic systems. Pyroxenes have the general formula XY(Si,Al)₂O₆ (where X represents calcium, sodium, iron⁺² and magnesium and more rarely zinc, manganese and lithium and Y represents ions of smaller size, such as chromium, aluminium, iron⁺³, magnesium, manganese, scandium, titanium, vanadium and 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.

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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 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

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 octahedral X (or M1) and Y (or M2) sites and the tetrahederal T site. 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 of the octahedral (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 calcum 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.

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 X 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.

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 X site and bulkier ions on the Y 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 Y and X sites respectively. For example Na and Al give the jadeite (NaAlSi₂O₆) composition.
2. Coupled substitution of a 1+ ion on the Y site and a mixture of equal numbers of 2+ and 4+ ions on the X site. This leads to e.g. NaFe²⁺_0.5Ti⁴⁺_0.5Si₂O₆.
3. The Tschermak substitution where a 3+ ion occupies the X 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

• Clinopyroxenes (monoclinic)
  • Aegirine (Sodium Iron Silicate)
  • Augite (Calcium Sodium Magnesium Iron Aluminium Silicate)
  • Clinoenstatite (Magnesium Silicate)
  • Diopside (Calcium Magnesium Silicate, CaMgSi₂O₆)
  • 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)MgSi₂O₆
  • Enstatite, Mg₂Si₂O₆
  • Ferrosilite, Fe₂Si₂O₆
  • Nchwaningite (Hydrated Manganese Silicate)

• Schefferite, Ca(Mg,Fe,Mn)Si₂O₆
• Zinc schefferite, Ca(Mg,Mn,Zn)Si₂O₆
• Jeffersonite, Ca(Mg,Fe,Mn,Zn)Si₂O₆
• Leucaugite, Ca(Mg,Fe,Al)(Al,Si)₂O₆
• Calcium-Tschermak's molecule, CaAlAlSiO₆

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