year of publication
The thermodynamic properties of hedenbergite, of composition
[(Ca0.9516) (Mg0.0583 Fe2+0.7134 Fe3+0.0934 Mn0.1366)(Si2O6)],
were experimentally determined at the Bureau of Mines. The standard enthalpy of for
mation determined by hydrofluoric acid solution calorimetry is
ΔHf°298 = -680.96±0.855 kcal/mol.
The standard enthalpy of formation from the oxides is
ΔH°298 = -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°298 = 41.218±0.148 cal/mol·K.
High-temperature heat capacities were measured with a differential scanning calorimeter 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
ΔGf°298 = -641.90±0.86 kcal/mol.