The crystal structure of parnauite: a copper arsenate–sulphate with translational disorder of structural rods
- Author(s)
- Stuart J. Mills, Anthony R. Kampf, A M McDonald, Luca Bindi, Andrew, G. Christy, Uwe Kolitsch, Georges Favreau
- Abstract
New data for parnauite from the type locality, Majuba Hill, Nevada, USA (MH; type specimen), and also from CapGaronne, Var, France (CG), and the Clara Mine, Baden-Wu¨rttemberg, Germany, are presented. The average chemical composition ofMH material is (Cu8.82Al0.16Fe0.02)9.00(As1.78Al0.07Si0.08S0.07)2.00O8(SO4)(OH)10 7H2O and that of CG parnauite,(Cu8.42Al0.21Zn0.10)8.73(AsO4)2[(S0.97As0.10)1.07O4](OH9.23Cl0.77)10.00 7H2O. Both of these formulae confirm the 9:2:1Cu: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 stretchingmode. 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 A ° , similar to previousstudies, but h ¼ odd reflections were heavily streaked and diffuse, preventing full refinement. A 3 A ° substructure was refined, withspace group Pmn21, to R1(F) ¼ 0.0750 (MH). For a MH crystal, the subcell had a ¼ 3.0113(4), b ¼ 14.259(3), c ¼ 14.932(2)A ° , V ¼641.13(16) A ° 3 and Z ¼ 1. The structure is of a new type, and contains Cu in 6 distinct sites, forming two three-polyhedron wideribbons of edge-sharing Cu-O polyhedra extended parallel to the a-axis. The two ribbons lie back-to-back and are bridged by twoAsO4 tetrahedra. The collection of 6Cu þ 2As cations plus ligands forms a rod-like moiety extended || a. These rods link throughpolyhedral corners to form complex, corrugated (010) layers. The interlayer space is occupied by H2O molecules. Thus, the disorderobserved by XRD is of an unusual type, in which the shape of the unit mesh within layers is variable, rather than the stacking of thelayers.Disorder arises because each AsO4 tetrahedron shares a face with a Cu(O,OH,H2O)5-6 polyhedron in the substructure, necessitatingpartial occupancy of both As and Cu sites. The S atoms were not located in the refinement, but four electron-density maxima in theinterlayer region were interpreted as H2O molecules. Hence, the simplified structural formula derived from the substructure is(Cu10&2)(As2&2)O8(OH)148H2O, deviating from that obtained in chemical analyses. The discrepancy presumably arises due tostrong delocalisation of the sulphur and the apical oxygen of the SO4 tetrahedron in the substructure. Short-range order of Cu-As andCu-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 ofOH positions in the CG refinement indicates that Cl is distributed over several sites in the 3A ° substructure, making the mineral a Clrichvariety of parnauite rather than a distinct mineral species.
- Organisation(s)
- Department of Mineralogy and Crystallography
- External organisation(s)
- Australian National University, Naturhistorisches Museum Wien (NHM), Museums Victoria, Natural History Museum of Los Angeles County, Laurentian University, University of Florence, Independent researcher
- Journal
- European Journal of Mineralogy: an international journal of mineralogy, geochemistry and related sciences
- Volume
- 25
- Pages
- 693-704
- No. of pages
- 12
- ISSN
- 0935-1221
- DOI
- https://doi.org/10.1127/0935-1221/2013/0025-2329
- Publication date
- 2013
- Peer reviewed
- Yes
- Austrian Fields of Science 2012
- 105113 Crystallography, 105116 Mineralogy
- Portal url
- https://ucrisportal.univie.ac.at/en/publications/755c3c64-3fbb-4ddf-8bda-795cc3fb74af