The crystal structure of parnauite: a copper arsenate-sulphate with translational disorder of structural rods

New data for parnauite from the type locality, Majuba Hill, Nevada, USA (MH; type specimen), and also from Cap Garonne, Var, France (CG), and the Clara Mine, Baden-Wurttemberg, Germany, are presented. The average chemical composition of MH material is (Cu8.82Al0.16Fe0.02)(Sigma 9.00)(As1.78Al0.07Si0.08S0.07)(Sigma 2.00)O-8(SO4)(OH)(10) center dot 7H(2)O and that of CG parnauite, (Cu8.42Al0.21Zn0.10)(Sigma 8.73)(AsO4)(2)[(S0.97As0.10)(Sigma 1.07)O-4](OH9.23Cl0.77)(Sigma 10.00) center dot 7H(2)O. Both of these formulae confirm the 9:2:1 Cu:As:S ratio obtained from earlier descriptions of parnauite. Raman spectra for parnauite from both localities are very similar. Bands are assigned, but show no evidence of the presence of CO3, in contrast to previous studies, and no distinct Cu- Cl stretching mode. It appears that neither the minor CO3 and PO4 previously reported nor Cl are essential constituents of parnauite. Single-crystal XRD analysis indicates a primitive orthorhombic unit cell with dimensions 6 x 14 x 15 angstrom, similar to previous studies, but h = odd reflections were heavily streaked and diffuse, preventing full refinement. A 3 angstrom substructure was refined, with space group Pmn2(1), to R-1(F) = 0.0750 (MH). For a MH crystal, the subcell had a = 3.0113(4), b = 14.259(3), c = 14.932(2) angstrom, V = 641.13(16) angstrom(3) and Z = 1. The structure is of a new type, and contains Cu in 6 distinct sites, forming two three-polyhedron wide ribbons of edge-sharing Cu-O polyhedra extended parallel to the a-axis. The two ribbons lie back-to-back and are bridged by two AsO4 tetrahedra. The collection of 6Cu + 2As cations plus ligands forms a rod-like moiety extended parallel to a. These rods link through polyhedral corners to form complex, corrugated (010) layers. The interlayer space is occupied by H2O molecules. Thus, the disorder observed by XRD is of an unusual type, in which the shape of the unit mesh within layers is variable, rather than the stacking of the layers. Disorder arises because each AsO4 tetrahedron shares a face with a Cu(O,OH,H2O)(5- 6) polyhedron in the substructure, necessitating partial occupancy of both As and Cu sites. The S atoms were not located in the refinement, but four electron-density maxima in the interlayer region were interpreted as H2O molecules. Hence, the simplified structural formula derived from the substructure is (Cu-10 square(2))(As-2 square(2))O-8(OH)(14) center dot 8H(2)O, deviating from that obtained in chemical analyses. The discrepancy presumably arises due to strong delocalisation of the sulphur and the apical oxygen of the SO4 tetrahedron in the substructure. Short-range order of Cu-As and Cu-S parallel to a can occur independently in the relevant structural rods, which accounts for the observed long-range disorder. Cell parameters and substructures obtained from CG and Clara material are similar to those from the MH crystal. Site splitting of OH positions in the CG refinement indicates that Cl is distributed over several sites in the 3 angstrom substructure, making the mineral a Clrich variety of parnauite rather than a distinct mineral species.