We have grown single crystals of M(2+)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 center dot(H2O)-H-11 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. (C) 2017 The Author(s). Published by Elsevier B.V.