The formation of pigeonite on the join hedenbergite–ferrosilite at 11.5 and 15 kbar: experiments and a solution model Available to Purchase

The three-pyroxene assemblage orthoferrosilite (OFs)-pigeonite (Pig)-hedenbergite (Hd_ss) 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 Hd_ss + Fayalite + Quartz while at and above 20 kbar the Hd_ss + 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 X_CFs and X_Hd 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