Pyroxene: structure, chemistry, petrogenesis

Structure Pyroxene, [(Cavi, Mgvi,Fevi)SiivO3], is an important rock-forming inosilicate, occurring in both orthorhombic (orthopyroxene) and monclinic (clinopyroxene) forms. The pyroxenes consist of chains of silicate tetrahedra aligned along the c-axis of the unit cell.

The individual chains linked by cations in octahedral (and occasionally greater) coordination. The structure of orthopyroxene, with chain-linking cations, is shown below as projected along the c-axis of the orthorhombic unit-cell.
In orthopyroxene both M1 and M2 sites are octahedral and occupied by Mg2+ and Fe2+. In clinopyroxenes, the M2 sites are typically occupied by Ca2+, but may also be occupied in part by Na+, Mg2+ or Fe2+

Chemistry The quadrilateral pyroxenes have compositions that are bound by the diopside (CaMgSi2O6), hedenbergite (CaFeSi2O6), orthoferrosilite (Fe2Si2O6) and enstatite (Mg2Si2O6).

The pyroxene quadrilateral forms part of a larger ternary system (CaSiO3-MgSiO3-FeSiO3), that includes another single chain silicate wollastonite. The Ca-rich clinopyroxenes are separated from the orthopyroxenes, and from the Ca-poor clionpyroxene pigeonite by a solvus (shown in green in the above figure). The solvus with pigeonite closes with increasing Fe-content at any given temperature. The position of the solvus is temperature dependent, with the solvus gap expanding with decreasing temperature. Along the FeMg-1 exchanges the "quadrilateral" pyroxene solid-solutions have been divided into a host of mineral names. For the orthopyroxenes the related minerals are:
while, for the Ca-rich clinopyroxenes the related minerals are:
In addition to the MgCa-1 and FeMg-1 exchanges connecting diopside to the other "quadrilateral" pyroxenes, NaAlCa-1Si-1 forms another important exchange in the pyroxene group, that connects diopside with omphacite and jadeite. Thus, much of the compositional variation in the pyroxene group can be represented in the MgCa-1-FeMg-1-NaAlCa-1Si-1 space shown below.

Additional compositional variation within the pyroxenes is due to the NaFe3+Ca-1Si-1 and AlAlMg-1Si-1 (tschermak's) exchanges. The Fe3+-rich pyroxene is aegirine.

Petrogenesis The "quadrilateral" pyroxenes form an important mineral in mafic and ultramafic igneous rocks, and in high temperature "granulite facies" metamorphic rocks. Diposide also occurs in amphibolte facies calc-silicates and marbles.
Pigeonite is mainly restricted to volcanic rocks, but "inverted pigeonites" are found in layered ultramafic complexes and, very rarely, in high temperature meta-ironstones. The Na-pyroxenes are characteristic found in high pressure "eclogite-facies" metamorphism of mafic igneous rocks.