The crystal structure of ransomite, CuFe2(SO4)4±6H2O, is monoclinic, P21/c, and has been refined to R = 0.12. Iron coordination octahedra with an average Fe—O distance of 1.98 Å are concentrated in bands one octahedron wide and two deep which extend in the a axial direction. Individual iron octatehedra of the bands are linked by sulfate tetrahedra and translational equivalent bands are connected through these tetrahedra by distorted copper coordination octahedra to form sheets parallel to (010). The sheets are joined by hydrogen bonds to account for the perfect (010) cleavage. Two groups of Cu—0 bonds of the distorted copper coordination octahedron average 1.99 Å and 2.44 Å. A short S—O bond of 1.41 ± 0.01 A is noted in one of the sulfur coordination tetrahedra.
The complexity of metal sulfate hydrate structures is suggested to be a function of the percentage of oxygen atoms of the sulfate tetrahedra that are involved in coordination of the metal cations of the structure.