1109 The Canadian Mineralogist Vol. 48, pp. 1109-1118 (2010) DOI : 10.3749/canmin.48.5.1109 THE CRYSTAL STRUCTURE OF WATKINSONITE, Cu2PbBi4Se8, FROM THE ZÁLESÍ URANIUM DEPOSIT, CZECH REPUBLIC Dan TOPa§ Department of Materials Research and Physics, Paris–Lodron University of Salzburg, Hellbrunnerstr. 34, A–5020 Salzburg, Austria Emil maKOViCKY Department of Geography and Geology, University of Copenhagen, Østervoldgade 10, DK–1350, Copenhagen, Denmark Jiří SEJKORa Department of Mineralogy and Petrology, National Museum, Václavské nám. 68, CZ–115 79, Praha 1, Czech Republic HERbERT DiTTRiCH Department of Materials Research and Physics, Paris–Lodron University of Salzburg, Hellbrunnerstr. 34, A–5020 Salzburg, Austria abSTRaCT The crystal structure of sulfur-free watkinsonite, ideally Cu2PbBi4Se8, monoclinic, a 12.952(4), b 4.152(1), c 15.155(5) Å, b 108.93(1)°, space group P21/m, Z = 2, part of the selenide mineralization of the uranium deposit of Zálesí, Czech Republic, has been solved by direct methods and reined to R1 = 4.61% on the basis of 1287 unique relections [Fo > 4s(Fo)] collected on a Bruker P3 diffractometer with a CCD detector and MoKa radiation. The crystal structure contains one Pb site, four Bi sites, two Cu sites and eight Se sites. Partial replacement of one Cu (in a linearly coordinated site) by Ag is observed. The crystal structure consists of (100)PbS slabs four atomic planes thick of a galena-like structure alternating with single layers of hexagonally packed selenium. The Pb atoms form asymmetric bicapped coordination prisms embedded in the surfaces of the galena-like pseudotetragonal slabs; atoms Bi1–3 occur as irregular coordination octahedra in the interior and at the surfaces of this slab. In addition, Bi4 forms a coordination prism similar to that of lead. Tetrahedrally coordinated copper Cu2 caps triangular voids of the hexagonal layer, whereas linearly coordinated Cu1 (with minor Ag), with the opposing Cu–Se bonds equal to 2.36 and 2.40 Å, respectively, forms foreshortened coordination octahedra, with four additional Cu–Se distances in the range 3.05–3.17 Å. The crystal structure of watkinsonite is closely related to that of berryite, Cu3Ag2Pb3Bi7S16, in which the foreshortened coordination octahedra are occupied by Ag, and Cu resides in the hexagonal layer. Keywords: sulfosalt, watkinsonite, crystal structure, berryite, Zálesí deposit, Czech Republic. SOmmaiRE Nous avons résolu par méthodes directes la structure cristalline de la watkinsonite dépourvue de soufre, dont la formule, de façon idéale, est Cu2PbBi4Se8, monoclinique, a 12.952(4), b 4.152(1), c 15.155(5) Å, b 108.93(1)°, groupe spatial P21/m, Z = 2, formant partie de la minéralisation en séléniures du gisement uranifère de Zálesí, en République Tchèque, et nous l’avons afinée jusqu’à un résidu R1 de 4.61% en utilisant 1287 rélexions uniques observées [Fo > 4s(Fo)], prélevées avec un diffractomètre Bruker P3 muni d’un détecteur CCD et avec rayonnement MoKa. La structure contient un site Pb, quatre sites Bi, deux sites Cu et huit sites Se. Nous observons le remplacement partiel d’un atome de Cu (dans un site à coordinence linéaire) par Ag. La structure cristalline est faite de dalles de (100)PbS d’une épaisseur de quatre plans atomiques, dont la structure ressemble à celle de la galène, qui alternent avec des couches distinctes de sélénium en agencement compact hexagonal. Les atomes de Pb logent dans des prismes de coordinence assymétriques et biterminés, encastrés dans la surface des dalles pseudotétragonales ressemblant à la galène; les atomes Bi1–3 se trouvent en coordinence octaédrique irrégulière à l’intérieur et à la surface de ces dalles. De § E-mail address: dan.topa@sbg.ac.at 1110 THE CanaDian minERalOgiST plus, l’atome Bi4 forme un prisme de coordinance semblable à celui qui renferme le Pb. Le Cu2, à coordinence tétraédrique, recouvre des lacunes triangulaires de la couche hexagonale, tandis que le Cu1 (avec un faible quantité de Ag), en coordinence linéaire, les liaisons Cu–Se étant égales à 2.36 et 2.40 Å, respectivement, forment des octaèdres de coordinence trappus ayant quatre liaisons Cu–Se additionnelles dans l’intervalle 3.05–3.17 Å. La structure cristalline de la watkinsonite montre une relation étroite avec celle de la berryite, Cu3Ag2Pb3Bi7S16, dans laquelle Ag occupe les octaèdres de coordinence trappus, et le Cu réside dans la couche hexagonale. (Traduit par la Rédaction) Mots-clés: sulfosel, watkinsonite, structure cristalline, berryite, gisement de Zálesí, République Tchèque. inTRODuCTiOn Watkinsonite is a selenosulide of lead, bismuth and copper, first described by Johan et al. (1987) from the selenide–telluride association of the Otish Mountains uranium deposit, Quebec, Canada. The empirical formula, Cu2.36Pb1.26Bi3.70Se6.21S1.74Te0.05, was approximated as Cu2PbBi4(Se,S)8 (Förster et al. 2005). The original crystallographic data indicate that watkinsonite is monoclinic, space group P2/m, P2, or Pm, with a 12.921, b 3.997, c 14.989 Å, and b 109.2°. A new ind of virtually sulfur-free watkinsonite from the Zálesí uranium deposit, Czech Republic, yielded material suitable for a crystal-structure analysis. In the paper on the crystal structure of berryite, Cu3Ag2Pb3Bi7S16, Topa et al. (2006) suggested that watkinsonite ought to be structurally related to the monoclinic polytype of berryite they analyzed, because its d 001 value (equal to 14.16 Å) is a half of the corresponding spacing (28.24 Å) in berryite, and the published unit-cell of watkinsonite is metrically identical to one of the two “component subcells” recognizable for the two variants of the pseudotetragonal layer (plus adjacent interlayer) in the structure of berryite– P21/m. This prediction has now been conirmed to a surprising extent. maTERial inVESTigaTED The scarce samples containing watkinsonite were collected from the waste pile of the mine dump material that probably originated from Adit No. 1 of the uranium mine, or from unnamed exploratory shafts in the area of the small uranium deposit of Zálesí. This deposit is situated at the southern margin of the settlement of Zálesí, about 6.5 km southwest of the Rychlebské Hory Mountains, northern Moravia, Czech Republic. A relatively abundant selenide mineralization is associated with the older, uraninite stage of mineralization that is close to the so-called “ive-elements” formation (U–Ni–Co–As–Ag) of hydrothermal ore veins (Fojt et al. 2005, Sejkora et al. 2006). Watkinsonite occurs there as irregular aggregates from 0.01 to about 4 mm, usually smaller than 0.5 mm. On a fresh fracture, it is yellowish to brownish grey (in comparison with clausthalite); it exhibits a metallic luster and a characteristic conchoidal fracture. Watkinsonite aggregates in a quartz gangue are usually intergrown with practically sulfur-free clausthalite (Pb0.97Ag0.01Cu0.01Bi0.01)S1.00Se0.99 (Fig. 1); uraninite, chalcopyrite, bornite, covellite, löllingite and S-rich umangite were also observed in close association. ExPERimEnTal Chemical analyses Quantitative chemical data for watkinsonite were obtained on two different electron microprobes. Initially, at the Laboratory of Electron Microscopy and Microanalysis of Masaryk University and Czech Geological Survey, Brno, watkinsonite (15 aggregates) and associated minerals were analyzed with a Cameca SX–100 electron microprobe in the wavelengthdispersion mode (WDS) with an accelerating voltage of 25 kV, a specimen current of 20 nA, and a beam diameter of about 1 m. The following standards and X-ray lines were used: Cu (Cu Ka), Ag (Ag La), PbSe (Se Lb), CuFeS2 (SKa), Bi2Te3 (TeLb), PbS (PbMa) and Bi (BiMb). Peak-counting times (CT) were 20 s for both main and minor elements; CT for each background was one half of peak time. Raw intesities were converted to the concentrations automatically using the PAP (Pouchou & Pichoir 1985) matrix-correction software package. At the Department of Geography and Geology, University of Salzburg, a JEOL Superprobe JXA–8600 apparatus was operated in WDS mode, at 25 kV and 35 nA, with a defocused beam (~3 m). The measurement time was 15 s for peak and 5 s for background counts. The following standards and X-ray lines were used: natural CuFeS2 (chalcopyrite; CuKa), natural PbS (galena PbLa), synthetic Bi2Se3 (BiLa, SeLa), synthetic Bi2S3 (SKa), synthetic CdTe (TeLa) and Ag metal (AgLa). The raw data were corrected with the on-line ZAF–4 procedure. Chemical data on the grain presented in Figures 1a and 1b, together with selected results of point analyses made in Brno, are shown in Table 1 and Figure 2. In the (Cu + Ag) – Bi – Pb diagram, the chemical composition of watkinsonite from Zálesí is positioned close to the structurally determined formula Cu2PbBi4Se8. THE CRYSTal STRuCTuRE Of waTKinSOniTE 1111 fig. 1. Back-scattered electron image (a) and optical image (b) of the grain of watkinsonite studied from Zálesí, Czech Republic. Symbols: wat: watkinsonite, cls: clausthalite, urn: uraninite. fig. 2. Chemical composition of watkinsonite from Zálesí (circles) and berryite from Grube Clara (Topa et al. 2006) (rhombs) in the central portions of the (Cu + Ag) – Bi – Pb diagram. 1112 THE CanaDian minERalOgiST Single-crystal X-ray diffraction For our single-crystal investigation, several grains of watkinsonite were extracted from the aggregates in a polished section. They were investigated with a Bruker AXS diffractometer equipped with a CCD area detector using graphite-monochromated MoKa radiation. An irregular fragment measuring approximately 0.03 3 0.05 3 0.09 mm was found to be suitable for structural investigation. Experimental data are listed in Table 2. The SMART (Bruker AXS, 1998) system of programs was used for unit-cell determination and data collection, SAINT+ (Bruker AXS, 1998) for the calculation of integrated intensities, and XPREP (Bruker AXS, 1998) for empirical absorption-correction based on pseudo-C scans. The space group P21/m, proposed by the XPREP program, differs from the original proposal derived from powder data by Johan et al. (1987). The structure of watkinsonite was solved by direct methods (program SHELXS of Sheldrick 1997a) and differenceFourier syntheses (program SHELXL of Sheldrick 1997b). Reinement data are given in Table 2; fractional coordinates and anisotropic displacement-parameters of atoms are listed in Table 3. The principal interatomic distances are presented in Table 4, and selected geometrical parameters for individual coordination polyhedra, calculated with the IVTON program (Balić-Žunić & Vicković 1996), are given in Table 5. A table of structure factors for watkinsonite may be obtained from the Depository of Unpublished Data on the Mineralogical Association of Canada website [document Watkinsonite CM48_1109]. Atom labeling is shown in Figure 3, and the crystal structure is illustrated in Figure 4. DESCRiPTiOn Of THE STRuCTuRE General features The structure of watkinsonite can be described in analogy to that of berryite (Topa et al. 2006) as an alternation of two types of layer, i.e., pseudotetragonal slabs that are four atomic planes thick, and pseudohexagonal layers that in these structures are reduced to single THE CRYSTal STRuCTuRE Of waTKinSOniTE sheets of hexagonally arranged anions with interstitial cations present in some positions of the sheet. The structure contains one lead site, four independent Bi sites that do not seem to exhibit signs of Pb-forBi substitution, and two copper sites. One of the latter sites, the linearly coordinated Cu1 site, accommodates the minor Ag present. There are eight independent Se sites. All atoms are at the (m) positions with y = 0.25 and 0.75. Whether there is any partial order of Se and S in distinct sites of the two types of layer in the case of watkinsonite from Otish Mountains (Johan et al. 1987), similar to that observed for the layered structure of proudite (Mumme et al. 2009), is not known at present. Coordination polyhedra The Cu1 site is an octahedrally coordinated (2 + 4) site in the pseudotetragonal layer. The two bonds oriented to the foreshortened apices of the coordination octahedron form a linear coordination. The opposing 1113 Cu–Se distances involved are equal to 2.358 and 2.401 Å, respectively; the shorter of them is oriented toward the exterior of the pseudotetragonal layer. The Cu1 site houses the bulk of analytically determined Ag (Tables 3, 4). The contents of silver in the structurally analyzed grain extracted from a margin of the watkinsonite aggregate appear slightly higher than the average obtained for the bulk (Table 1). These paired octahedral Cu1 sites are analogous to the paired octahedral Ag1 sites in berryite–P21/m (Topa et al. 2006), which have the short Ag–S bonds equal to 2.443–2.475 Å. In watkinsonite, the four long Cu1–Se distances in the octahedron are equal to 3.046–3.168 Å; those of Ag–S in berryite are 3.104–3.123 Å long. For an exact comparison with berryite, the difference between the radii of S2– and Se2– should be subtracted from the Cu–Se data. The Cu2 site is tetrahedrally coordinated (Table 4) and corresponds to the Cu2 site of berryite. Atom Cu2 forms [010] strings of edge-sharing tetrahedra, however, 1114 THE CanaDian minERalOgiST fig. 3. Atom labels in the structure of watkinsonite. All sites are fully occupied. with Cu–Cu distances equal to 2.58 Å, indicating Cu–Cu interactions. The marginal Bi octahedra of the pseudotetragonal layer, Bi1 and Bi2, show a considerable difference between the Bi–Se distance in the apex of the pyramid and that below the base of the tetragonal pyramid: 2.746 versus 3.270 Å and 2.735 versus 3.228 Å, respectively. The Bi4 site, which forms paired columns of square coordination pyramids of Bi, is the most eccentric of all Bi sites, with the Bi–S bond toward the apex of the pyramid equal to 2.718 Å, opposed by the long distances inside the capped trigonal coordination prism (Fig. 4), which are equal to 3.551 and 3.748 Å, respectively. The Bi3 site, occluded completely in the pseudotetragonal layer, is close to octahedral, with the apical distances equal to 2.888 and 2.993 Å, and a fairly symmetrical distribution of the remaining distances, from 2.880 to 3.037 Å. This results in an eccentricity value that is much lower than for the other Bi sites (Table 5). The polyhedron volume, however, is smallest of all the Bi sites (Table 5). This inding and the bond-valence results suggest that no recognizable Pb-for-Bi substitution takes place at the Bi3 site. THE CRYSTal STRuCTuRE Of waTKinSOniTE The single Pb site is an asymmetric bicapped trigonal prism site, with out-of-layer Pb–Se distances being substantially longer than the intralayer ones (Table 4). Modular description The pseudotetragonal (Q) slabs four atomic layers thick are outlined schematically by shading in Figure 4. The pseudohexagonal (H) layers are single layers of Se atoms halving the interspace between the Q slabs. The Bi4 positions are protruding from the surfaces of the pseudotetragonal layers, with coordination polyhedra including three atoms from the hexagonal layers. The long Bi4–Se7 distance “ties” the Bi4 atom to the Q layer. This situation corresponds in full to that in berryite (Topa et al. 2006), in which the bismuth site involved is Bi1 (Fig. 5). In watkinsonite, two Bi4 sites are paired, but in berryite, Bi1 columns pair with Pb3 columns. As in berryite, watkinsonite is a lock-in type of incommensurate two-layer structure, in which one common period comprises two pseudohexagonal 1115 subcells matching three pseudotetragonal subcells along the layer in the a direction, and this ratio is ixed by interlayer bonds. In berryite (Fig. 5), the pseudohexagonal layer is populated by an alternation of Cu1, Cu2 and Cu3 coordination triangles; only the space occupied by the Bi1–Pb3 coordination polyhedra was left unilled. In watkinsonite, both the space deined by the pair of Bi4–Bi4 polyhedra and that facing Pb are left empty, and only single [010] columns of this layer are occupied by Cu, in a compact manner. The presence of two Cu sites instead of three as in berryite is balanced out by the substitution of Pb3 in berryite by Bi4 in watkinsonite. According to the cations protruding from the surface of the Q layer, the sequence of Q layers in berryite–P21/m was described by Topa et al. (2006) as an alternation of “pseudotetragonal Pb–Pb layers” (with Pb3 protruding) with “pseudotetragonal Bi–Bi layers” (Bi1 protruding). They also argued that in the orthorhombic–Pcm21 polytype of berryite, the layers ought to be of the “Pb–Bi type”. Based on the original space-group assignment by Johan et al. (1987), Topa et fig. 4. The crystal structure of watkinsonite. In the order of decreasing size, spheres represent Se, Pb (blue), Bi, and Cu (red). The pseudotetragonal layers (Q) and the pavonite-related layers “a”, respectively, are shaded; the intervening pseudohexagonal (H) layers and the intermediate “b” intervals are left unshaded. In order to identify the single-sheet H layers, follow the pattern and spacing of Se atoms in the unshaded interspaces between two adjacent Q layers. Inspection of the Figure 5 will help as well. 1116 THE CanaDian minERalOgiST fig. 5. The crystal structure of the monoclinic polytype of berryite (Topa et al. 2006). For the majority of explanations, see the caption to Figure 4. The silver atoms are indicated in green. The component subcell centered on the Q1 layers (Pb–Pb type, see the text), metrically identical to the unit cell of watkinsonite, is drawn in red. al. (2006) assumed that the layers in watkinsonite also are of the “Pb–Bi type”, and the original space-group Pn was considered as the probable choice. The present data, however, indicate that the symmetry is P21/m, and the layers are of the “Bi–Bi type”, although the unitcell metric corresponds to that of the “Pb–Pb” subunit of berryite, illustrated in the central upper portions of Figure 3 in Topa et al. (2006). Lone-electron-pair micelles in watkinsonite and berryite are of the same type and position: the micelle deined by two Bi1 and two Bi3 atoms in the interior of the Q slab of watkinsonite (Fig. 4) is the same as the 2 Bi3 – 2 Bi7 micelle in berryite (Fig. 5), and the spaces that accommodate pairs of lone electrons under Bi4 coordination pyramids in watkinsonite correspond to such spaces under Bi1 polyhedra in berryite. All pairs of Cu1 coordination octahedra have the same orientation in the structure of watkinsonite, but the corresponding pairs of Ag sites alternate in orientation in monoclinic berryite (Figs. 4, 5). The distribution of copper The presence of linearly coordinated Cu (with only minor Ag) in the form of paired lattened Cu2Se6 octahedra in watkinsonite was an unexpected feature of its structure, and it resolves speculations about the accommodation of copper in this mineral. As a result of this inding, the coniguration of the Q layer in watkinsonite is exceedingly close to that of the “Pb–Pb” layer in monoclinic berryite. The only difference is that the protruding atoms on layer surfaces are Bi4 in watkinsonite as opposed to Pb in berryite. The other Pb site on the Q surface is common to both structures and also to both types of layer in berryite. The presence of smaller coordination-octahedra of Cu in the Q layer of watkinsonite (in comparison to the rest of the structure), instead of the Ag octahedra found in berryite, has a pronounced inluence on the distribution of Cu in the adjacent hexagonal layer. The coordination triangles that correspond to “Cu1” and “Cu3” positions of berryite lie in the extension of the space occupied by the octahedrally coordinated Cu1 in 1117 THE CRYSTal STRuCTuRE Of waTKinSOniTE fig. 6. Collage of slices of pavonite-like structure (one of which is shaded) cut out of the structure of watkinsonite. When joined by applying crystallographic shear, they generate a pavonite homologue N = 5. Selected foreshortened octahedra are indicated in pink. watkinsonite. Thus, the relevant coordination triangles, which were occupied in berryite, become too small to host a copper site. Copper is then forced to populate the Cu2 site, which corresponds to the “Cu2” position of berryite. RElaTiOnS TO OTHER SulfOSalTS In addition to the obvious modular afinity to the sulfosalts related to cannizzarite, such as berryite, watkinsonite is also related to the pavonite homologous series (Makovicky et al. 1977). If we divide the structure of watkinsonite into slices compatible with the pavonite structural principle (slices denoted as a and shaded in Fig. 6) and remove the intermediate parts (slices b, unshaded) by means of crystallographic shear, a structure of the pavonite homologue N = 5 will result. It has paired Bi pyramids and empty coordination octahedra in the thinner layers of the 5P structure, and it has ive octahedra strung along diagonals of the thicker structural layers of this structure. The same exercise gave a more complicated homologue of pavonite, with N = 5, 6, 5 ,6… for berryite–P21/m (Topa et al. 2006). COnCluSiOnS With the exception of one empty copper site (the two Cu sites Cu1 and Cu3 from berryite become equivalent in watkinsonite) and replacement of a marginal Pb atom by Bi, the structure of watkinsonite corresponds closely to that of the pseudotetragonal “Pb–Pb” layer of monoclinic berryite–P21/m, with the relevant parts of interlayer space and of the adjacent pseudohexagonal layer added, as predicted by Topa et al. (2006) (Figs. 4, 1118 THE CanaDian minERalOgiST 5). Thus it is one more success story of modular crystallography of sulfosalts. The most remarkable feature of the structure is the linear coordination of copper in the paired, foreshortened Cu1–Se6 coordination octahedra in the pseudotetragonal portions of the structure. This unusual coordination is closer to that of Ag+ and Hg2+ than to the usual tetrahedral or trigonal-planar Cu coordination observed in numerous sulfosalts. This structure type spans a range of compositions from mixed sulide– selenide to pure selenide, and can contain different Cu:Ag proportions in the foreshortened octahedral sites. aCKnOwlEDgmEnTS DT gratefully acknowledges the support of the Christian Doppler Research Society (Austria). EM was supported by the grant no. 272–08–0227 of the Research Council for Nature and Universe (Denmark). JS acknowledges the support by Ministry of Culture of the Czech Republic (DE07P04OMG003), and thanks Drs. Pavel Škácha (Charles University, Prague) and Radek Škoda (Masaryk Unversity, Brno) for the kind assistance. 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