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+
The quadrilateral pyroxenes have compositions that are bound by the diopside
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
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
(tschermak's) exchanges. The Fe3+-rich
pyroxene is aegirine.
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.