Abstract:
The enthalpy of drop-solution in molten 2PbO·B2O3 of synthetic and natural lawsonite, CaAl2(Si2O7)(OH)2·H2O, was measured by high-temperature oxide melt calorimetry. The enthalpy of formation determined for the synthetic material is ΔfH Oxides=–168.7±3.4 kJ mol–1, or ΔfH 0298=–4,872.5±4.0 kJ mol–1. These values are in reasonable agreement with previously published data, although previous calorimetric work yielded slightly more exothermic data and optimisation methods resulted in slightly less exothermic values. The equilibrium conditions for the dehydration of lawsonite to zoisite, kyanite and quartz/coesite at pressures and temperatures up to 5 GPa and 850 °C were determined by piston cylinder experiments. These results, other recent phase equilibrium data, and new calorimetric and thermophysical data for lawsonite and zoisite, Ca2Al3(SiO4)(Si2O7)O(OH), were used to constrain a mathematical programming analysis of the thermodynamic data for these two minerals in the chemical system CaO–Al2O3–SiO2–H2O (CASH). The following data for lawsonite and zoisite were obtained: \(\) , \(\) , \(\) , \(\) . Additionally, a recalculation of the bulk modulus of lawsonite yielded K=120.7 GPa, which is in good agreement with recent experimental work.