The heat capacity of synthetic andradite garnet (Ca3Fe2Si3O12) was measured between 9.6 and 365.5 K by cryogenic adiabatic calorimetry and from 340 to 990 K by differential scanning calorimetry. At 298.15 K Cop,m and Som are 351.9 ?? 0.7 and 316.4 ?? 2.0 J/(mol??K), respectively. Andradite has a ??-peak in Cop,m with a maximum at 11.7 ?? 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe3+ ions. The Gibbs free energy of formation, ??fGom (298.15 K) of andradite is -5414.8 ?? 5.5 kJ/mol and was obtained by combining our entropy and heat capacity data with the known breakdown of andradite to pseudowollastonite and hematite at ??? 1410 to 1438 K. From a reexamination of the calcite + quartz = wollastonite equilibrium data we obtained ??fHom (298.15 K) = - 1634.5 ?? 1.8 kJ/mol for wollastonite. Between 300 and 1000 K the molar heat capacity of andradite can be represented by the equation Cop,m = 809.24 - 7.025 ?? 10-2T- 7.403 ?? 103T-0.5 - 6.789 ?? 105T-2. We have also used our thermochemical data for andradite to estimate the Gibbs free energy of formation of hedenbergite (CaFeSi2O6) for which we obtained ??fGom (298.15 K) = -2674.3 ?? 5.8 kJ/mol. ?? 1987.
Additional Publication Details
Heat capacity and thermodynamic properties of andradite garnet, Ca3Fe2Si3O12, between 10 and 1000 K and revised values for ??fGom (298.15 K) of hedenbergite and wollastonite