Thermodynamics of the amphiboles: Fe-Mg cummingtonite solid solutions

A thermodynamic solution model for the ferromagnesian amphibolites is developed. The model accounts explicitly for intersite nonconvergent cation ordering of Fe²⁺ and Mg between octahedral M1, M2, M3, and M4 sites and intrasite interaction energies arising from size-mismatch of unlike cations. The proposed model is calibrated for the ferromagnesian monoclinic amphiboles, under the assumptions of energetic equivalency of the M1 and M3 sites and the absence of excess volume or excess vibrational entropy, using the X-ray site occupancy data of Hirschmann et al. (1994) and the phase equilibrium data of Fonarev and Korolkov (1980). Reference-state thermodynamic quantities for magnesio-cummingtonite [Mg₇Si₈O₂₂(OH)₂] and grunerite [Fe₇Si₈O₂₂(OH)₂] are derived from previously published results. -from Authors