Excess heat capacity and entropy of mixing in the high-structural state (K,Ca)-feldspar binary

Low- and high-temperature heat capacities were measured for a series of synthetic high-structural state (K,Ca)-feldspars (Or–An) using both a relaxation and a differential scanning calorimeter. The data were collected at temperatures between 5 and 800 K on polycrystalline samples that had been synthesised and characterised in a previous study. Below T = 300 K, Or₉₀An₁₀, and Or₈₀An₂₀ showed excess heat capacities of mixing with maximum values of ~3 J mol⁻¹ K⁻¹. The other members of this binary (An > 20 mol%) had lower excess heat capacity values of up to ~1 J mol⁻¹ K⁻¹. Above T = 300 K, some compositions exhibited negative excess heat capacities of mixing (with maximum values of −2 J mol⁻¹ K⁻¹). The vibrational entropy at 298.15 K for Or₉₀An₁₀ and Or₈₀An₂₀ deviated strongly from the behaviour of a mechanical mixture, with excess entropy values of ~3.5 J mol⁻¹ K⁻¹. More An-rich members had only small excess vibrational entropies at T = 298.15 K. The difference in behaviour between members with An > 20 mol% and those with An ≤ 20 mol% is probably a consequence of the structural state of the (K,Ca)-feldspars, i.e., (K,Ca)-feldspars with An ≤ 20 mol% have monoclinic symmetry, whereas those with An > 20 mol% are triclinic. At T = 800 K, the vibrational entropy values were found to scatter around the values expected for a mechanical mixture and, thus, correspond to a quasi-ideal behaviour. The solvus for the (K,Ca)-feldspar binary was calculated based on the entropy data from this study in combination with enthalpy and volume of mixing data from a previous study.