First-principles study of anhydrite, polyhalite and carnallite

We report density functional calculations of the structures and properties of anhydrite (CaSO4), polyhalite (K2SO4·MgSO4·2CaSO4·2H2O) and carnallite (KCl·MgCl2·6H2O). Densities of states are systematically investigated and phonon analysis using density functional perturbation theory is performed at constant equilibrium volume for anhydrite and polyhalite in order to derive their isochoric thermal properties. Thermal properties at constant atmospheric pressure are also calculated using the quasi-harmonic approximation. The computed molar entropy and isobaric heat capacity for anhydrite reproduce experimental data up to 800 K to within 3% and 10%, respectively, while further experimental work is needed to assess our theoretical predictions for polyhalite.