Yves Moëlo

Lead-antimony sulfosalts from Tuscany (Italy). IV. Crystal structure of pillaite, Pb9Sb10S23CIO0.5, an expanded monoclinic derivative of hexagonal Bi(Bi2S3)9I3, from the zinkenite group

The crystal structure of pillaite, a recently described Pb-Sb oxy-chloro-sulfosalt from the Buca della Vena mine, Tuscany (Italy), was solved and refined to a residual R of 0.045 in the monoclinic system, with space group C2/m, and a = 49.49(1), b = 4.1259(8), c = 21.828(4) A, β = 99.62(3) 0 , V = 4394(2) A3. The structural formula is Pb 9.16 Sb 9.84 S 23 ClO 0.5 , corresponding to the electron-microprobe one Cu 0.10 Pb 9.16 Sb 9.84 S 22.94 Cl 1.06 O 0.5 (ideally Pb 9 Sb 10 S 23 ClO 0.5 ), with Z = 4 and D cale. = 5.87. The crystal structure is organized around pseudo-threefold axes where chlorine atoms are in a trigonal prismatic coordination with Pb atoms. An oxygen position, half-filled, is bonded to two Sb atoms, like in scainiite. Pairs of pseudo-trigonal rods parallel to the elongation b-direction are connected along [102] to form rod-layers. The pillaite structure is an expanded homologue of hexagonal Bi(Bi 2 S 3 ) 9 I 3 . Among lead sulfosalts, it belongs to the zinkenite group of cyclically twinned structures. Pillaite is the first example of a Pb-Sb sulfosalt where small amounts of two anion species simultaneously play a specific crystal chemical role, which is essential for the stabilization of the crystal structure: chlorine, like in dadsonite, and oxygen, like in scainiite.

Lead-antimony sulfosalts from Tuscany (Italy): II - Crystal structure of scainiite, Pb14Sb30S54O5, an expanded monoclinic derivative of Ba12Bi24S48 hexagonal sub-type (zinkenite group)

The crystal structure of scainiite, Pb 14 Sb 30 S 54 O 5 , has been solved from single-crystal X-ray data and refined to an R index of 8.9 % based on 3703 unique reflections. The structure is monoclinic C 2/ m , with cell parameters a 51.996(8), b 8.148(1), c 24.311(4) A, β 104.09(1)°, V 9990(10) A 3 , and with Z = 4. The structure analysis was essential in determining the position of oxygen atoms in a special double-ribbon, Sb 8 (Pb,Sb) 2 S 10 O 5 , directly related to the structure of kermesite, Sb 2 S 2 O. The general organization of the structure can be described according to the general principles developed by Makovicky (1993) for complex sulfides related to the PbS and SnS archetypes. This structure falls into the group of “pure rod structures” (zinkenite group), and is an expanded homologue of cyclic hexagonal Ba 12 Bi 24 S 48 . The expansion of the parent structure is obtained by shearing along a zig-zag surface, that creates lozenge shaped spaces in place of the original hexagonal channels, and filling these spaces by the kermesite-type fragments.

Lead-antimony sulfosalts from Tuscany (Italy). III. Pillaite, Pb9Sb10S23ClO0.5, a new Pb-Sb oxy-chloro-sulfosalt, from Buca della Vena mine

The new mineral pillaite, Pb9Sb10S23ClO0.5, an oxy-chloro-sulfosalt, has been found at Buca della Vena mine in the Apuan Alps, northern Tuscany, Italy. It crystallized in small fractures of dolomitic lenses in massive Fe- Ba ore, together with many Pb-Sb sulfosalts, among which the oxy-sulfosalt scainiite. Pillaite occurs as black acicu- lar crystals with metallic lustre, elongated // (010), up to 1 cm long and less than 0.1 mm thick. The mineral is brittle, opaque; VHN50 = 175 kg/mm2, dcalc = 5.77 g/cm3. In reflected light it shows a weak anisotropy and bireflectance, and

Lead-antimony sulfosalts from Tuscany (Italy). VI. Pellouxite, ∼ (Cu,Ag)2Pb21Sb23S55ClO, a new oxy-chloro-sulfosalt from Buca della Vena mine, Apuan Alps

Pellouxite, the second natural oxy-chloro-sulfosalt after pillaite, has been discovered in cavities of calcite veins in the small Fe-Ba deposit of Buca della Vena mine, Apuan Alps (Tuscany, Italy). The new mineral name honours professor Alberto Pelloux (1868–1948), curator of the mineralogical Museum at the University of Genova. Pellouxite occurs as black acicular flattened crystals with metallic lustre, up to 1 mm long and less than 0.05 mm thick. Main reflectance values [λ (nm), R air / R oil %]: 470, 38.3/23.8; 546, 37.3/22.4; 589, 36.9/21.5; 650, 35.2/19.6. It has monoclinic symmetry, space group C 2/ m , with a = 55.824(11), b = 4.0892(8), c = 24.128(5) A, β = 113.14(3)° and V = 5065(2) A 3 . The eight strongest lines of the X-ray powder diffraction pattern are [d(A), I obs (hkl)]: 4.002, 38 (606, 406); 3.878, 24 (206, 112); 3.562, 31 (804, 1202); 3.423, 100 (807, 1604, 407); 3.009, 25 (808, 912, 713); 2.948, 27 (1313, 1111); 2.265, 19 (409); 2.048, 20 (020). Electron probe microanalysis gives (mean of 15 spots; wt.%, error σ): Pb 47.17(20), Sb 31.16(23), Cu 0.89(5), Ag 0.59(5), S 19.08(6), Cl 0.33(3), O 0.39(15), Total 99.60. The unit formula, on the basis of Pb + Sb = 22 at. according to the crystal structure study, is (Cu 0.64 Ag 0.25 )Pb 10.36 Sb 11.64 S 27.07 Cl 0.42 O 1.11 ( Z = 4). Taking into account a 2 b superstructure, the crystal chemical formula is (Cu,Ag) 2-x Pb 21-x Sb 23+x S 55 ClO (x = 0.12; d calc. = 5.97 g/cm 3 ), derived from the stoichiometric one (Cu,Ag) 2 Pb 21 Sb 23 S 55 ClO. Pellouxite, structurally related to scainiite, is an expanded monoclinic derivative of synthetic hexagonal Ba 12 Bi 24 S 48 . It belongs to the zinkenite group of lead sulfosalts with cyclically twinned structures and their derivatives. Like for pillaite, its formation is the result of a complex sulfide deposition from hydrothermal brines, at relatively high temperature, and fO 2 /fS 2 conditions controlled by the pyrite-hematite-magnetite buffer.

Le district à antimoine-or Dúrico-Beirão (Portugal): évolution paragénétique et géochimique; implications métallogéniques

The veins of this Hercynian district occur within the epimetamorphic series of the Dúrico-Beirão anticlinorium, along the edges of a straight Carboniferous syncline. A comparative paragenetic study, microprobe analyses of arsenopyrite and sphalerite, and a preliminary fluide inclusion study improve the knowledge to the metallogenic process.Pure or Ag-bearing gold is concentrated in the veins during all metallogenic process, in the different stages: with arsenopyrite-pyrite, or jamesonite, or berthieritestibnite I, or galena-fülöppite-pyrargyrite-stibnite II-carbonates (late hydrothermal remobilization stage), or Sboxides (weathering).The presence of discrete W-Sn minerals in the first stage of the Sb-Au veins supports the possibility of a genetic link with the W-Sn deposits related to nearby granites. The Pb-Zn-Ag veins located in the south-eastern margin of the district present epithermal characters, and are probably more recent. The Dúrico-Beirão district can thus be compared to other Hercynian metallogenic provinces (French Massif Central, Central Marocco), considered as examples of geochemical interference between Pb and Sb by metallogenic superimposition.As consequences for exploration, these results suggest a southward extension of the Sb-Au district and the presence of Ag-rich gold in the Pb-Zn stage.

Lead-antimony sulfosalts from Tuscany (Italy). VII. Crystal structure of pellouxite, ∼ (Cu,Ag) 2 Pb 21 Sb 23 S 55 ClO, an expanded monoclinic derivative of Ba 12 Bi 24 S 48 hexagonal sub-type (zinkenite group)

The crystal structure of pellouxite, a natural Pb-Sb-(Cu,Ag) oxy-chloro-sulfide from Tuscany (Italy), has been solved and refined using single-crystal X-ray-diffraction, with a final R = 0.044. The symmetry is monoclinic, space group C 2/ m , with a = 55.824(11), b = 4.0892(8), c = 24.128(5) A, β = 113.14(3)°, V = 5065(2) A 3 . The structural formula is close to (Cu 0.68 Ag 0.26 )Pb 10.44 Sb 11.56 S 27 (Cl 0.5 S 0.5 )O 0.5 , with Z = 4, or, ideally (according to ordering revealed by a weak 2b superstructure), (Cu,Ag) 2 Pb 21 Sb 23 S 55 ClO, with Z = 4. Among cations, there are 9 pure Pb positions, 10 pure Sb positions, and 3 Pb/Sb and one Cu/Ag pairs. Together with 27 S atoms, there are one Cl/S pair, as well as one isolated half-filled O position. All the minor elements (Cu with Ag, Cl and O) appear essential for the organization of the crystal structure. The Cu/Ag pair is located within a triangular bipyramid (four S and one Cl/S); vacancies on this site (fixed to 0.06 %) have been correlated to an equivalent Sb excess in the neighbouring Pb/Sb pair according to the substitution Pb 2+ + (Cu,Ag)+ → Sb 3+ + vacancy. The isolated oxygen is strongly bonded to two Sb atoms, like in scainiite and pillaite. The Cl/S pair is situated in a triangular prismatic coordination (4 Pb + 2 Sb). Following the principles of modular analysis, the whole structure is described in two ways: 1 - a combination of three types of 1 D and 2D building blocks, each of them organized around Sb 3+ lone-electron-pair micelles, or 2 - a single type of complex rod-layer parallel to (201), organized around columns with pseudo-trigonal symmetry. Pellouxite constitutes the second example of oxychloro-sulfosalt after pillaite. It is an expanded monoclinic derivative of synthetic hexagonal Ba 12 Bi 24 S 48 , structurally related to scainiite, Pb 14 Sb 30 S 54 O 5 (topological similarity for about 90 % of the atom positions). It belongs to the zinkenite group of cyclically twinned sulfosalt structures and derivatives.

Gramaccioliite-(Y), a new mineral of the crichtonite group from Stura Valley, Piedmont, Italy

Gramaccioliite-(Y) is a new mineral within the crichtonite group. It was found in the gneiss of the Hercynian Massif of Argentera, at Sambuco (Stura Valley, Piedmont, Italy). It occurs as black lamellar mm-sized crystals. Associated minerals are quartz, albite, muscovite, anatase, brookite, rutile, fluorapatite, xenotime, pyrite, a mineral of the synchysite series, dessauite-(Y) and senaite. Optically, gramaccioliite-(Y) is opaque and shows very weak anisotropy and low bireflectance; pleochroism is not observed. Chemical analysis carried out by means of EPM led to the following simplified formula: (Pb, Sr)(Y, Mn)(Ti, Fe)18Fe2O38. Gramaccioliite is trigonal, space group R3, α = 9.186(4) A, α = 68.82(4)°. The strongest five reflections in the X-ray powder diffraction pattern [ d in A ( I) (hkl )] are: (3.002) (100) (300), (1.606) (95) (1.3.10), (2.892) (70) (116), (2.258) (70) (134), (1.809) (60) (318) (indices are referred to the hexagonal setting). The crystal structure of gramaccioliite-(Y) has been refined ( R = 0.086) from single crystal X-ray diffraction data. The structure corresponds to that of the other members of the crichtonite group. During the study of gramaccioliite-(Y), a mineral described in literature as a Re-rich member of the crichtonite group from an unknown locality has been re-examined. New EPM data indicated the complete lack of rhenium in this mineral and allowed to correctly identify it as a second finding of gramaccioliite-(Y).

Chemistry of some Pb-(Cu, Fe)-(Sb, Bi sulfosalts from France and Portugal. Implications for the crystal chemistry of lead sulfosalts in the Cu-poor part of the Pb2S2-Cu2S-Sb2S3-Bi2-Bi2S3 system

SummaryElectron microprobe analysis of Pb-Cu(Fe)-Sb-Bi sulfosalts from Bazoges and Les Chalanches (France), and Pedra Luz (Portugal), give new data about (Bi, Sb) solid-solution and incorporation of the minor elements Cu, Fe or Ag in jaskolskiite, and in izoklakeite-giessenite and kobellite-tintinaite series. Jaskolskiite from Pedra Luz has high Sb contents (from 17.9 to 20.7 wt.%), leading to the extended general formula: CuxPb2+x(Sb1−yBiy)2−xS5, with 0.10 ⩽x ⩽ 0.22 and 0.19 ⩽y ⩽ 0.41. Fe-free, Bi-rich izoklakeite from Bazoges has high Ag contents (up to 2.2 wt. %), leading to the simplified formula Cu2Pb22Ag2(Bi, Sb)22S57; in Les Chalanches it contains less Ag content (1.2 wt.%), but has an “excess” of Cu that gives the formula: Cu2.00 (Cu0.49Ag1.18)Σ=1.67Pb22.70(Bi12.63Sb8.99)Σ=21.62S57.27.In tintinaite from Pedra Luz, the variation of the Fe/Cu ratio can be explained by the substitution: Cu + (Bi, Sb) → Fe + Pb; Fe-free kobellite from Les Chalanches has a Cu-excess, corresponding to the formula Cu2.81Ag0.54Pb9.88(Bi10.37Sb5.21)Σ=15.38S35.09. Eclarite from the type locality, structurally related to kobellite, shows a Cu excess too. In natural samples of the kobellite homologous series, Fe is positively correlated with Pb, and its contents never exceed that of Cu. Ag substitutes for Pb, together with (Bi, Sb). Taking into account the possibility of Cu excess, but excluding formal Cu2+ and Fe3+, general formulae can be written:n -for the izoklakeite-giessenite series:n

Structure cristalline d'une ménéghinite naturelle pauvre en cuivre, Cu0,58Pb12,72(Sb7,04Bi0,24)S24

begin{gathered} (Cu_{2 - x} Fe_x )[Pb_{26 - 2z + x} (Ag,Cu)_z (Sb_{1 - y} Bi_y )_{20 + z - x} ]_{sum = 46} S_{57,} hfill with:0 leqslant x leqslant 1; 0.44 leqslant y leqslant 0.85;0 leqslant z leqslant 2.2 hfill end{gathered}