On the crystal chemistry of salt hydrates. III. The determination of the crystal structure of FeSO₄.7H₂O (melanterite)

FeSO 4. 71-I20 crystallizes in space group _P21/c with a = 14.072, b =6.503, c = 11.041/~, fl = 105 ° 34', Z =4.The crystal structure had been determined from Patterson and Fourier syntheses usingFho~ and Fhk0 data.The refinement was done by the method of least squares employing 738 observed structure factors from the zones hIcO, hO1, Olcl, h3l and h/el.The final R value, including the contribution of the hydrogen atoms is 4.5%.The iron ions occupy special positions and are surrounded octahedrally by six water molecules each.The six water oxygens form hydrogen bonds to the tetrahedral sulfate group and to the seventh water molecule which is not coordinated to Fe 2+.The mean bond length of Fe-O(w) is 2.124/~, of S-O 1"474/~ and of O-H • ' • O 2-82 A. One of the ironcoordinated water oxygen atoms receives an 'outward' hydrogen bond from the seventh water oxygen atom.The bond from iron to this water oxygen is 2.19/~ and thus significantly longer than the mean Fe-O(w) bond length.