THERMODYNAMICS OF THE AMPHIBOLES - FE-MG CUMMINGTONITE SOLID-SOLUTIONS

A thermodynamic solution model for the ferromagnesian amphiboles is de veloped. The model accounts explicitly for intersite nonconvergent cat ion ordering of Fe2+ and Mg between octahedral M1, M2, M3, and M4 site s and intrasite interaction energies arising from size-mismatch of unl ike cations. The model is formulated with 15 parameters: two standard- state contributions, three ordering energies involving the exchange of Fe2+ and Mg between the four crystallographically distinct sites, six reciprocal terms that describe the noncoplanarity of the Gibbs energy of mechanical mixing in composition-ordering space, and four regular- solution type parameters involving Fe2+-Mg interaction on each site. T he model may be readily collapsed to approximations that distinguish c ation ordering over only three sites (M13, M2, and M4) or two sites (M 123 and M4), or that assume the absence of ordering (i.e., a macroscop ic model), in which case the number of parameters decreases to 10, 6, and 3, respectively. The proposed model is calibrated for the ferromag nesian monoclinic amphiboles, under the assumptions of energetic equiv alency of the M1 and M3 sites and the absence of excess volume or exce ss vibrational entropy, using the X-ray site occupancy data of Hirschm ann et al. (1994) and the phase equilibrium data of Fonarev and Korolk ov (1980). Reference-state thermodynamic quantities for magnesio-cummi ngtonite [Mg7Si8O22(OH)(2)] and grunerite [Fe7Si8O22(OH)(2)] are deriv ed from previously published results. The calibrated model is internal ly consistent with the database of Berman (1988) and the work of Sack and Ghiorso (1989) on ferromagnesian orthopyroxene. Gibbs energy of mi xing, enthalpy of mixing, and activity-composition relation plots are constructed from the calibration.