New crystal-chemical and structural data of dietrichite, ideally ZnAl2(SO4)4•22H2O, a member of the halotrichite group

New crystal-chemical and structural data of a sample of dietrichite, ideally ZnAl2(SO4)4·22H2O, from the pyrite mine of Boccheggiano, Grosseto, Italy, are reported. This sample, unlike holotype dietrichite, is very close to the ideal chemical composition, in fact combined ICP and thermogravimetry indicate a formula (Zn0.98Fe0.07)Al1.91(SO4)4.03·21.88H2O based on 38O. The crystal structure has been refined by the Rietveld method on transmission X-ray powder diffraction data (Rp = 4.13%, Rwp = 5.44%, RB = 4.66%). Dietrichite is monoclinic P21/c, Z = 4, a = 6.1757(2), b = 24.262(1), c = 21.206(1) Å, = 100.436(3)°. The structure of dietrichite consists of one ZnO(H2O)5 octahedron, two independent Al(H2O)6 octahedra, and four independent SO4 tetrahedra per asymmetric unit. The only direct connection between polyhedra is by sharing of an oxygen atom, O(16), between S(4) and Zn. The structure contains 22 water molecules, 17 of which are octahedrally co-ordinated with Zn and Al cations whereas the remaining five molecules are only linked via hydrogen bonds to O or other H2O molecules. Hexagonal channels, running along [100], originate from a regular alternation of one ZnO(H2O)5 octahedron, two Al(H2O)6 octahedra, and three SO4 tetrahedra. Within the structure two types of channels may be identified, the first one containing three and the second two H2O molecules. Band positions of the IR spectrum of dietrichite are consistent with those of reference data.