RI 8873 Thermodynamic Properties of Hedenbergite, a Complex Silicate of Ca, Fe, Mn, and Mg

The thermodynamic properties of hedenbergite, of composition [(Ca0,9516) (Mg0,0583Fe2+0.7134 Fe3+0.0934 Mn0,1366) (Si206)], were experimentally determined at the Bureau of Mines. The standard enthalpy of formation determined by hydrofluoric acid solution calorimetry is AHf298 = -680.96±0.855 kcal/mol. The standard enthalpy of formation from the oxides is AR-96 = -24.55±0.485 kcal/mol. Low-temperature heat capacities were determined by adiabatic calorimetry from 12.3 to 300.1 K. The derived standard entropy is S-98 - 41.218±0.148 cal/mol-K. High-temperature heat capacities were measured with a differential scanning calorim¬eter from 397 to 1,147 K. These experimentally determined data were combined with data from the literature to calculate the Gibbs energies of formation and equilibrium constants of formation over the temperature range of measurements. Standard enthalpies of formation are given as a function of temperature. The standard Gibbs energy of formation at 298.15 K is AGf298 = -641.90±0.86 kcal/mol.