A neutron-diffraction study of hemimorphite

Abstract

Abstract The hydrogen atoms in hemimorphite, Zn4Si2O7(OH)2 · H2O, have been located and its crystal structure refined using 415 three-dimensional singlecrystal neutron-diffraction data. The mineral is orthorhombic, space group Imm2, with a = 8.367(5), b = 10.730(6), c = 5.115(3) Å, and Z = 2. The structure consists of three-membered rings of corner-sharing Zn(OH)O3 ( × 2) and SiO4 tetrahedra arranged in compact sheets parallel to (010). Three oxygen atoms in each tetrahedron are bonded to two zinc atoms and one silicon atom, while a fourth oxygen atom forms a bridging bond to an equivalent cation in an adjacent sheet. The water molecules are oriented parallel to (010) inside large cavities between the tetrahedral sheets and are held in place by hydrogen bonds to and from the hydroxyl groups of the Zn – OH – Zn bridging linkages. Mulliken population analyses calculated using constant bond lengths and the observed angles within and between the tetrahedra allow a rationalization of the bond-length variations in the SiO4 group, but are less successful in the case of Zn(OH)O3. Detailed analysis of observed bond length and calculated overlappopulation variations in a variety of tetrahedral oxyanions suggests that the poorer agreement in the case of the Zn tetrahedron is more a function of the larger overall size of the group than of the relative ionic character of the bonds.