Thermal and Thermochemical Study of Thaumasite

— A sample of natural thaumasite Ca 3.0 Si(OH) 6 (CO 3 ) 0.9 (SO 4 ) 1.1 ·12.3H 2 O (N’Chwaning mine, Kalahari manganese ore field, South Africa) was studied by powder X-ray diffraction, infrared absorption and Raman spectroscopy, thermal analysis, and microcalorimetry. The process of thermal transformation of thaumasite was studied using the results of FTIR and Raman spectroscopy. The enthalpy of formation from elements Δ f H 0 (298.15 K) = −8816 ± 30 kJ/mol was determined by high-temperature melt solution calorimetry. The value of the absolute entropy was estimated, and the enthalpy and Gibbs energy of formation of thaumasite of theoretical composition were calculated: 945.4 ± 1.8 J/(mol K), −8699 ± 30 kJ/mol, −7577 ± 30 kJ/mol, respectively.