The thermodynamic behaviour of three synthetic, Ca3(Al0.25Fe0.753+)2Si3O12, Ca3(Al0.50Fe0.503+)2Si3O12 and Ca3(Al0.75Fe0.253+)2Si3O12 and three grossular-rich natural solid-solution garnets were investigated. Synthetic garnets were crystallized at high P and T in a piston-cylinder device. They were characterized by optical microscope examination and X-ray diffraction. The larger natural crystals of gem-like quality were characterized by optical microscope examination, microprobe analysis and IR spectroscopy. The heat capacity, Cp, behaviour of both types of garnets was measured using relaxation calorimetry between 2 K and 300 K and by Differential Scanning Calorimetry (DSC) between 143 K and 920 K and between 282 K and 764 K in two different labs. The low-temperature Cp results show a magnetic phase transition for all garnets that decreases in magnitude and sharpness with increasing grossular content in the solid solution. The Néel temperature decreases from 11.3 ± 0.1 K for end-member andradite to 3.0 ± 0.1 for the most andradite-poor composition Ca3(Al0.75Fe0.253+)2Si3O12. The vibrational and magnetic, Cvib and Cmag, contributions to the measured Cp at T < 300 K, were separated by applying a single-parameter phonon dispersion model. The results were used to calculate the vibrational entropy, Svib, and the magnetic entropy, Smag, at 298.15 K. The sum of both gives the calorimetric entropy, Scal, at 298.15 K, whereby “calorimetric” means that any configurational entropy contribution is excluded. An analysis of the results shows linear shifts in Svib and Smag as a function of composition across the And-Gro binary join indicating ideal thermodynamic mixing behaviour for Ca3(AlxFe1-x3+)2Si3O12 garnets. There are no measurable calorimetric, vibrational and magnetic excess entropies of mixing and, thus, ΔSvib,ex ≈ 0 and ΔSmag,ex ≈ 0 and ΔScal,ex ≈ 0. A revised value for the standard enthalpy of formation, ΔH°f, of andradite from the elements was calculated using the recently published calorimetric value of 325.0 ± 2.0 J K−1 mol−1 for andradite and published phase-equilibrium results on three andradite-bearing reactions. The resulting value is ΔH°f = −5763.3 ± 1.5 kJ mol−1. The effect of using the recent thermodynamic data for andradite is examined and discussed by calculating several reactions relevant to the formation of skarn and by constructing a phase diagram for a calc-pelite containing the assemblage grossular-almandine-andradite garnet, epidote, clinopyroxene, calcite, plagioclase and quartz.

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