The three-pyroxene assemblage orthoferrosilite (OFs)-pigeonite (Pig)-hedenbergite (Hdss) is stable on the CaFeSi2O6–Fe2Si2O6 join at pressures above 11.5 kbar and below approximately 20 kbar. It is found at 825±10°C (11.5 kbar) and at 855±10°C (15 kbar). At lower pressures the low-Ca pyroxenes decompose to Hdss + Fayalite + Quartz while at and above 20 kbar the Hdss + Pig pair is replaced by a single clinopyroxene (Cpx). Under the assumption that (a) Pig has the C2/c space group at elevated temperatures and (b) Ca enters only the M2 site of Cpx, the Cpx solution can be described by an asymmetric Margules model:
where units are kJ and kbar, and XCFs and XHd are the mole fractions of Fe2Si2O6 and of CaFeSi2O6 in Cpx. The Gibbs free energy for the reaction OFs = CFs (C2/c) is given by
ΔG1=+1.8430.001676T(Kelvin)+0.03686P kJ.

The Opx solution is assumed to be symmetric, with WG = 15 kJ.

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