Thermodynamic properties of strontianite-witherite solid solution (Sr,Ba)CO₃

Abstract

Structural parameters and thermodynamic properties of strontianite — witherite solid solutions have been studied by X-ray powder diffraction, heat flux Calvet calorimetry and cation-exchange equilibria technique. X-ray study of the synthetic samples have shown linear and quadratic (for c-parameter) composition dependencies of the lattice constants in the carbonate solid solution. The thermodynamic energy parameters demonstrate the non-ideal character of strontianite — witherite solid solutions. Enthalpies of solution of the samples have been measured in 2PbO*B₂O₃ at 973 K. The new data on the enthalpy of formation ΔH ⁰ _f,298.15 of SrCO₃ and BaCO₃ were obtained: -1231.4±3.2 and -1209.9±5.8 kJ*mol^-1 respectively. The enthalpy of mixing of the solid solution was found to be positive and asymmetric with maximum at X_Ba (carbonate)=0.35. The composition dependence of the enthalpy of mixing may be described by two — parametric Margules model equation: ΔH _mix=X _Ba✻X _Sr✻[(4.40±3.91)✻X _Ba+(28.13±3.91)✻X _Sr] kJ✻mol⁻¹ Cation-exchange reactions between carbonates and aqueous SrCl₂-BaCl₂ supercritical solutions (fluids) were carried out at 973 and 1073 K and 2 kbar. Calculated Margules model parameters of the excess free energy are: for orthorhombic carbonate solid solutions W _Sr=W _Ba=11.51±0.40 kJ✻mol⁻¹ (973 K) and W _Sr=W _Ba=12.09±0.95 kJ✻mol⁻ (1073 K) for trigonal carbonate solid solutions W _Sr=W _Ba=13.55±0.40 kJ✻mol⁻ (1073 K).