Heat capacity and thermodynamic properties of andradite garnet, Ca 3Fe 2Si 3O 12, between 10 and 1000 K and revised values for ΔfGom (298.15 K) of hedenbergite and wollastonite

Abstract

The heat capacity of synthetic andradite garnet (Ca 3Fe 2Si 3O 12) 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 C o p, m and S o m are 351.9 ± 0.7 and 316.4 ± 2.0 J/(mol·K), respectively. Andradite has a λ-peak in C o p, m with a maximum at 11.7 ± 0.2 K which is presumably associated with the antiferromagnetic ordering of the magnetic moments of the Fe 3+ ions. The Gibbs free energy of formation, Δ f G o m (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 Δ f H o m (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 C o p, m = 809.24 - 7.025 × 10−2 T− 7.403 × 10 3 T−0.5 − 6.789 × 10 5 T−2. We have also used our thermochemical data for andradite to estimate the Gibbs free energy of formation of hedenbergite (CaFeSi 2O 6) for which we obtained Δ f G o m (298.15 K) = −2674.3 ± 5.8 kJ/mol.