Journal of the Czech Geological Society 51/34(2006) 271 Crystal structure of vanadinite: Refinement of anisotropic displacement parameters Krystalová struktura vanadinitu: zpøesnìní anizotropních teplotních parametrù (2 figs, 5 tabs) FRANTIEK LAUFEK 1  ROMAN SKÁLA 2  JAKUB HALODA 1  IVANA CÍSAØOVÁ 3 1 Czech Geological Survey, Geologická 6, Praha 5, CZ-152 00 Czech Republic 2 Institute of Geology, Academy of Sciences of the Czech Republic, Rozvojová 135, Praha 6, CZ-165 02 Czech Republic 3 Faculty of Science, Charles University, Hlavova 8, Praha 2, CZ-128 43 Czech Republic The structure of vanadinite, Pb 5 (VO 4 ) 3 Cl, from Mibladén, Morocco, was refined from single-crystal X-ray data. The full anisotropic struc- tural refinement was carried out in the hexagonal space group P6 3 /m, unit cell parameters a = 10.2990(2), c = 7.3080(1) Å, V = 671.30(2) Å 3 , Z = 2, with an R factor of 0.0197. The full anisotropic crystal structure refinement results in smaller departures of bond valence sums for cations from the ideal value than the isotropic one. Key words: vanadinite; crystal structure; anisotropic displacement parameter; single-crystal X-ray refinement Introduction Vanadinite, Pb 5 (VO 4 ) 3 Cl, is an end member in the terna- ry system pyromorphite-vanadinite-mimetite. It belongs to the apatite-group (Mandarino  Back 2004). By anal- ogy with the other apatite-group minerals that crystallize in space group P6 3 /m, vanadinite was presumed to be hexagonal (Hendricks et al. 1932). Thirty years after the first description of the vanadinite structure, Trotter  Barnes (1958) studied the three-dimensional atomic ar- rangement in the phase. They refined the vanadinite struc- ture to R = 12 % using visual estimates of intensities from precession photographs. More recently, single crystal X-ray refinement of vanadinite was reported (Dai  Hughes 1989). In their refinement, the displacement pa- rameters for cations were refined as anisotropic whereas oxygen atoms were refined as isotropic only. Despite the abundance of museum-quality specimens, full anisotro- pic structural refinement of vanadinite is still lacking. Partly to blame are some serious experimental difficul- ties: the X-ray scattering is dominated by highly-absorb- ing lead atoms, and vanadium interaction with neutrons is negligible. The aim of this work was to perform full anisotropic structure refinement and evaluate the effects of anisotropic displacement parameters refinement on the crystal structure of vanadinite. Locality and properties Since 1979, Morocco has been an important source of su- perb large vanadinite crystals or aggregates thereof (White 1984). One of the major vanadinite localities in Morocco is carbonate-hosted stratabound lead deposit in Mibladén, upper-Moulouya district (32°46N, 4°38W; Fig. 1). This locality also provided the sample studied. The sample that contains vanadinite was purchased at a mineral fair in 2004 from a Czech mineral dealer. The sample was labelled va- nadinite from Mibladén, Morocco. Mineralization at Mibladén is comparable to other car- bonate-hosted lead-zinc deposits, sharing numerous char- acteristics with the Mississippi Valley type deposits (Jébrak et al. 1998). The lead ore at Mibladén is restricted to the Mesozoic units near the present outcrop limit of the formation, where the thickness of the sediments is minimal. Most of the lead ore is hosted by Domerian (=Midle Lias) dolomitic to carbonaceous formations (Fe- lenc  Lenoble 1965). At Mibladén, vanadinite occurs as an alteration prod- uct of galena in close association with barite, cerussite and anglesite. The crystals are characterized by their re- markable size (commonly up to 1 cm in length), a deep red colour and adamantine lustre. The crystals are short hexagonal prisms terminated by a basal pinacoid. This makes them different from the barrel-shaped crystals common to the apatite group minerals. Crystallographic data and chemical composition Data for crystal structure refinement were acquired using 4-circle single-crystal diffractometer Nonius KappaCCD at the Centre of Molecular Structures at the Faculty of Science, Charles University, Prague. The pro- gram COLLECT (Nonius 19972000), was used for data collection. Unit-cell refinement and data reduction was carried with program HKL-SCALEPACK (Otwinovski  Minor 1997). After correction the data for absorption (Spek 2001), the structure was solved applying Sir92 (Altomare et al. 1994) and subsequent refinement was carried out with SHELX97 program (Sheldrick 1997). The programs of the SHELX97 suite were operated through WinGX graphical interface (Farrugia 1999). Two refinements of the structure were carried out. In the first, we refined all the atoms anisotropically. In the second one, only heavy atoms were refined anisotropically, whereas oxygen displacement parameters isotropically. Other refined parameters were the same in both refine-