The crystal structure of arsendescloizite, PbZn(OH)AsO4, from Tsumeb (Namibia)
Keller, Paul; Lissner, Falk; Schleid, Thomas
The crystal structure of arsendescloizite, PbZn(OH)[AsO4], orthorhombic P212121, a = 7.646(2), b = 9.363(2) and c = 6.077(1)Å, Z = 4, was deter mined by using the type crystal. The refinement of 607 unique reflections converged to R1 = 2.22 %. The bond distances are = 1.699 Å, = 2.136Å and = 2.660Å. No stereoactivity by the lone-pair electrons of Pb2+ was observed. The position of H was isotropically refined with a constrained donor-hydrogen distance dO-H ≈ 1.0 Å yielding positional parameters similar to that of austinite, CaZn(OH)[AsO4]. Arsendescloizite is the As-analogue of descloizite, PbZn(OH)[VO4], but belongs to the adelite-group of minerals. [Zn(OH)2O4] octahedra share edges to form linear chains that are further linked by vertices of the [AsO4] tetrahedra to an open framework structure. Its channels parallel to the a axis host edge- and vertex-sharing [Pb(OH)O7] polyhedra as well as the hydrogen bonds.
lead zinc arsenate •
crystal structure •