Pyroxene

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.

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 XYT₂O₆. The names of the common calcium–iron–magnesium pyroxenes are defined in the 'pyroxene quadrilateral'. The enstatite-ferrosilite series ([Mg,Fe]SiO₃) includes the common rock-forming mineral hypersthene, contains 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 no 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, pyroxene with more than 50 mol.% calcium is not possible. A related mineral, wollastonite (CaSiO₃), has the formula of the hypothetical calcium end member (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 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.

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

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

• Clinopyroxenes (monoclinic)
  • 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)
  • Enstatite, Mg₂Si₂O₆
  • Bronzite, intermediate between enstatite and hypersthene
  • Hypersthene, (Mg,Fe)SiO₃
  • Eulite, intermediate between hypersthene and ferrosilite
  • Ferrosilite, Fe₂Si₂O₆
  • Donpeacorite, (MgMn)MgSi₂O₆
  • Nchwaningite, Mn²⁺₂SiO₃(OH)₂·(H₂O)