Thermodynamic data of lawsonite and zoisite in the system CaO-Al₂O₃-SiO₂H₂O based on experimental phase equilibria and calorimetric work

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 Δ_rH^Oxides = -168.7 ± 3.4 kJ mol^-1, or Δ_fH^o₂₉₈ = -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, Δ_fH₂₉₈^O (zoisite) = -6,888.99 kJ mol^-1, S₂₉₈^O (zoisite) = 297.71 J K^-1 mol^-1. Additionally, a recalculation of the bulk modulus of lawsonite yielded K = 120.7 GPa, which is in good agreement with recent experimental work.