Divalent metal halide double salts in equilibrium with their aqueous solutions I. Factors determining their composition

Using a few simple assumptions based on the hard and soft acid and base model and the bond-valence model, we explain why the five most common double salt compositions xMeX2 · yMe′X2 · zH2O (Me, Me′ = Mg, Ca, Mn, Fe, Co, Ni, Cu, Zn, Cd; X = Cl, Br) found in equilibrium with the corresponding saturated solutions at room temperature will have x:y:z equal to 2:1:12, 1:1:8, 1:1:6, 1:2:6, and 1:4:10 and predict that these will belong to nine structural types. About 90% of the double salts observed under these conditions have one of these compositions and the remainder have closely related structures. We also make predictions about the chemical species expected in saturated solutions.