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-Württemberg, Germany, are presented. The average chemical composition of MH material is (Cu_8.82Al_0.16Fe _0.02)_Σ9.00(As_1.78Al_0.07Si _0.08S_0.07)_Σ2.00O₈(SO ₄)(OH)₁₀ · 7H₂O and that of CG parnauite, (Cu_8.42Al_0.21Zn_0.10)Σ8. 73(AsO₄)₂[(S_0.97As_0.10) _Σ1.07O₄](OH_9.23Cl_0.77) _Σ10.00 · 7H₂O. 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 × 14 × 15 Å, similar to previous studies, but h = odd reflections were heavily streaked and diffuse, preventing full refinement. A 3 Å substructure was refined, with space group Pmn2₁, to R₁(F) = 0.0750 (MH). For a MH crystal, the subcell had a = 3.0113(4), b = 14.259(3), c = 14.932(2)Å, V = 641.13(16) ų 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 || 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 H ₂O molecules. Hence, the simplified structural formula derived from the substructure is (Cu₁₀□₂) (As₂□₂)O₈(OH)₁₄·8H ₂O, 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 || 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 3Å substructure, making the mineral a Clrich variety of parnauite rather than a distinct mineral species.