Thermodynamic mixing properties and behavior of grossular-spessartine, (CaxMn1-x)3Al2Si3O12, solid solutions

The heat capacity, C-p, for nine synthetic solid-solution members of the grossular(Gr)-spessartine(Sp) binary was measured in the temperature range between 2 and 300 K using relaxation calorimetry and from 280 to 764 K using DSC methods. All solid-solution garnets exhibit normal heat capacity behavior from 764 K down to low temperatures. Below 7 K, a heat-capacity anomaly, originating from a paramagnetic to antiferromagnetic phase transition related to an ordering of the electron spins of the Mn cation, is observed. This lambda-anomaly is sharp and pronounced for Mn-rich solid-solution members and it becomes more flattened and rounded with decreasing Mn concentration in the garnet. The corresponding peak(Neel)-temperature decreases from 6.2 K in pure Sp to less than 2 K in Gr-rich garnets with X-Mn(grt) < 0.5. The calorimetric entropy, S-298, at 298 K for all intermediate composition garnets shows a slight positive deviation from ideality, resulting in an excess calorimetric entropy, Delta S-ex,S-cal, of approximately 2 J/mol K. The vibrational and magnetic parts of the total calorimetric entropy, S-vib and S-mag, respectively, were separated using the single-parameter phonon dispersion model of Komada and Westrum (1997). A symmetric entropy interaction parameter of W-S,CaMn(grt) = 3.8 +/- 2.0 J/cation mol K is calculated for Ca-Mn mixing in the Gr-Sp binary based on garnets with X-Mn(grt) >= 0.5. More Gr-rich garnets have impurity phases, complicating their exact C-p behavior and, thus, they were not used in this calculation. S-mag behavior was also determined for intermediate garnets where the lambda-anomaly is well pronounced and measurable. A thermodynamic analysis of the displaced phase equilibrium experiments of Koziol (1990) and the exchange experiments of Gavrieli et al. (1996) was also made using the calorimetrically derived W-S, CaMn(grt). The analysis yields a symmetric enthalpy interaction parameter of W-H, CaMn(grt) = 3.2 +/- 0.3 kJ/cation mol, giving a maximum excess enthalpy, Delta H-ex, of 0.8 kJ/cation mol for the Gr-Sp binary. Delta H-ex calculated from line broadening of IR powder spectra of the same studied Gr-Sp garnets (Rodehorst et al., 2004) agrees within error with that derived from the phase equilibrium and calorimetry results. Delta G(ex) for Gr-Sp garnets is slightly positive at 500 degrees C and becomes more negative with increasing temperatures. Its absolute values are small though and, thus, Ca-Mn mixing in garnet can be considered nearly ideal at most geologically relevant P-T conditions. (C) 2014 Elsevier Ltd. All rights reserved.