Similarities and peculiarities between the crystal structures of the hydrates of sodium sulfate and selenate

Salt hydrates of different water content and different crystal structures (Na₂SO₄·7H₂O, tetragonal, space group P4₁2₁2 and Na₂SeO₄·7.5H₂O, monoclinic, space group C2/c) can act mutually as a crystal nucleus, so that either of these salts can initiate crystallization of the other from their supersaturated solutions (a kind of heteroepitaxy). This phenomenon is attributed to similarities between particular elements of their structures.