Thermodynamics of minerals of variable composition: Andradite-grossularite and pistacite-clinozoisite solid solutions

Exchange-mineral equilibria with Al and Fe³⁺ aqueous chloride solutions (aq.), Andr + AlCl _aq ³ = FeCl _aq ³ + Gros, (1) Psc + AlCl _aq ³ = FeCl _aq ³ + Czo, (2) were studied under the following experimental conditions: 500°;C and 580°;C and 1 and 2 kbar, respectively, with an overall concentration of metals in the aqueous solutions of about 0.5 M and pH 3. The mixing functions of the components in garnet and epidote were calculated from the experimental data. Thermodynamic treatment of experimental evidence for reaction (1) led to the conclusion that, within the accuracy of experiment, garnet in the andradite-grossularite series was an ideal solid solution. However, epidote solid solution markedly departed from the ideal, as was shown by concentration and pressure-temperature (PT) dependencies of Gibbs's molar excess energies and by mixing-volume concentration dependence.