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 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 XBa (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).