THE CRYSTAL-STRUCTURE OF GEMINITE, CU2+(ASO3OH)(H2O), A HETEROPOLYHEDRAL SHEET STRUCTURE

The crystal structure of geminite, Cu2+(AsO3OH)(H2O), triclinic, a 9.841(2), b 10.818(2), c 15.733(3) Angstrom, alpha 95.71(2), beta 90.94(2), gamma 103.11(2)degrees, V 1621.9(6) Angstrom(3), C (1) over bar, Z=16, has been solved by direct methods and refined to an R index of 3.0% for 3263 observed reflections, measured with MoK alpha X-radiation. There are four unique Cu2+ sites, each coordinated by six anions in a [4 + 2]-distorted octahedral arrangement; this distortion of the coordination polyhedra is associated with the Jahn-Teller effect. There are four unique As5+ sites, each coordinated by three oxygen atoms and an OH group in a distorted tetrahedral arrangement; the three As-O distances are shorter than the As-OH distance in each tetrahedron, in accord with the prediction of bond-valence theory. The Cu phi(6) polyhedra share edges to form [Cu phi(4)] chains parallel to [010]; As phi(4) tetrahedra cross-link these chains into sheets parallel to (001), and there are two symmetrically distinct but topologically identical sheets in the structure. These sheets are linked into a three-dimensional structure by a network of H bonds, accounting for the perfect cleavage and platy habit of geminite.