Thermodynamic properties of strontianite-witherite solid solution (Sr,Ba)CO3 |
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| Authors: | I A Kiseleva A R Kotelnikov K V Martynov L P Ogorodova Ju K Kabalov |
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| Institution: | 1. Department of Geology, Moscow State University, 119899, Moscow, Russia
2. Institute of Experimental Mineralogy, Russian Academy of Sciences, 142432, Chernogolovka, Moscow district, Russia
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| 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  H
f,298.15
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: H
mix=X
Ba X
Sr(4.40±3.91)X
Ba+(28.13±3.91)X
Sr] kJmol–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 kJmol–1 (973 K) and W
Sr=W
Ba=12.09±0.95 kJmol– (1073 K) for trigonal carbonate solid solutions W
Sr=W
Ba=13.55±0.40 kJmol– (1073 K). |
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