Partitioning of Co2+ and Mn2+ into meridianiite (MgSO4·11H2O): Ternary solubility diagrams at 270 K; cation site distribution determined by single-crystal time-of-flight neutron diffraction and density functional theory

We have grown single crystals of M2+SO4 hydrates at 270 K from aqueous solutions in the ternary systems CoSO4–MgSO4–H2O and MnSO4–MgSO4–H2O. These systems exhibit broad stability fields for a triclinic undecahydrate on the Mg-rich side (i.e., Co- or Mn-bearing meridianiite solid solutions) and stability fields for monoclinic heptahydrates on the Mg-poor side (i.e., Mg-bearing solid solutions of bieberite or mallardite). The solubility curves and distribution coefficients, describing the partitioning of M2+ ions between liquid and solid phases, have been determined by thermo-gravimetric and spectroscopic techniques. A subset of M2+SO4·11H2O specimens were selected for single-crystal time-of-flight neutron diffraction analysis in order to evaluate preferential occupancy of symmetry-inequivalent coordination polyhedra in the structure. Considering the nearly identical dimensions of the first coordination shells, there is a surprising difference in the distribution of Co and Mn over the two available sites.