Stability and solubility of members of tin perovskites in the schoenfliesite subgroup, □2(BSn4+)(OH,O)6 (B = Ca, Fe3+, Mg, Mn2+, Zn, Cu)

•Enthalpies of formation of a suite of tin perovskites were measured by acid-solution calorimetry.•Tin perovskites are stable in alkaline solutions and may retain Sn in the environment.•High-temperature oxide-melt calorimetry and acid-solution calorimetry were tested and discussed on jeanbandyite. [Display omitted] In this work, we determined thermodynamic properties of a suite of tin perovskites □2(BSn4+)(OH,O)6 that correspond to the minerals burtite (B = Ca), jeanbandyite (Fe3+), schoenfliesite (Mg), wickmanite (Mn), vismirnovite (Zn), and mushistonite (Cu). Purity of the samples was verified by powder X-ray diffraction, chemical analysis (ICP-OES), solid state 119Sn magic-angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy, 57Fe Mössbauer spectroscopy, thermogravimetry (TG), and Karl-Fischer titration. Enthalpies of formation (ΔfHo) were determined by acid-solution calorimetry (in 5 N HCl at T = 298 K, with SnCl4 as the reference phase), that for jeanbandyite also in high-temperature oxide-melt calorimetry (in sodium molybdate at T = 973 K, with SnO2 as the reference phase). For the two methods, the difference for the ΔfHo values is 10.7 kJ·mol−1, a fair agreement. Because of the high H2O content of the studied phases, acid-solution calorimetry was given preference as it is less sensitive to errors in H2O determination. Entropy was estimated, including configurational or magnetic contribution. The resulting ΔfGo (kJ·mol−1) and logKsp values are: CaSn(OH)6−1885.8±7.6, 7.7; FeSn(OH)5O −1425.4±7.6, −5.8; MgSn(OH)6−1812.4±7.5, 3.3; ZnSn(OH)6−1519.6±7.6, 0.7; MnSn(OH)6−1598.7±7.7, 1.0; CuSn(OH)6−1311.2±7.9, 0.0. The logKsp value relate to reaction MSn(OH)6 + 6H+→ M2+ + Sn4+ + 6H2O or FeSn(OH)5O + 7H+→ Fe3+ + Sn4+ + 6H2O. The solid solution between vismirnovite and mushistonite was found to be thermodynamically non-ideal, with a mixing parameters W=−15.38 kJ·mol−1. Calculations of Gibbs free energies of selected reactions show that the tin perovskites are stabilized at basic pH. For example, burtite becomes stable with respect to cassiterite (SnO2) in systems buffered by calcite, gypsum or the C-S-H phase from cement at pH > 9–10.5. Similarly, jeanbandyite is stabilized in systems buffered by ferrihydrite [Fe(OH)3] but not in those buffered by goethite (FeOOH). Saturation indices calculated from polluted water in field settings show the same trend; the tin perovskites are stable under alkaline conditions and the solutions become supersaturated