Thermochemistry of two calcium silicate carbonate minerals: scawtite, Ca7(Si6O18)(CO3)·2H2O, and spurrite, Ca5(SiO4)2(CO3)

Calcium silicate carbonate minerals are products of contact metamorphism and metasomatism processes. They are also possible products of low temperature hydrothermal reactions, including those in cementitious materials and possibly in the geologic CO2 sequestration environment. Two minerals in this class, scawtite, Ca7(Si6O18)(CO3)·2H2O, and spurrite, Ca5(SiO4)2(CO3), were synthesized and characterized in the present work. Their enthalpies of formation were determined by high temperature oxide melt solution calorimetry. The enthalpy of formation from the oxides is −689.5 ± 14.3 kJ/mol for scawtite and −455.1 ± 9.7 kJ/mol for spurrite, and the enthalpy of formation from the elements is −11564.5 ± 16.8 kJ/mol for scawtite and −5845.5 ± 10.9 kJ/mol for spurrite. By using an exchange reaction involving all solid phase reactants and products, the standard entropy of formation for scawtite was estimated. The energetics for several reactions in high temperature geochemical (metamorphic) processes have been determined. The calculated stability fields for these two minerals and calcium carbonate are presented for 25 °C and 80 °C, and the possibilities for these two minerals to precipitate under geologic CO2 sequestration conditions are discussed. Although calcium carbonate is the most likely phase during carbonation reactions in aqueous solution, scawtite and spurrite may precipitate near the surfaces of dissolving silicate minerals, clays, or cement phases, particularly in the caprocks.