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PIZGRISCHITE, (Cu,Fe)Cu₁₄PbBi₁₇S₃₅, A NEW SULFOSALT FROM THE SWISS ALPS: DESCRIPTION, CRYSTAL STRUCTURE AND OCCURRENCE

Nicolas Meisser^1,, Kurt Schenk², Peter Berlepsch³, Joël Brugger⁴, Michel Bonin⁵, Alan J. Criddle^6,, Philippe Thélin⁷ and François Bussy⁷

¹ Musée Cantonal de Géologie and Institut de Minéralogie et Géochimie, Université-Dorigny, CH–1015 Lausanne, Switzerland
² Laboratoire de Cristallographie 1, EPFL, Dorigny, CH–1015 Lausanne, Switzerland
³ Heinrichsgasse 8, CH–4055 Basel, Switzerland
⁴ School of Earth and Environmental Sciences, The University of Adelaide, North Terrace, 5005 Adelaide, South Australia, and Division of Mineralogy, South Australian Museum, North Terrace, 5000 Adelaide, South Australia
⁵ Laboratoire de Cristallographie 1, EPFL, Dorigny, CH–1015 Lausanne, Switzerland
⁶ Department of Mineralogy, Natural History Museum, Cromwell Road, London SW7 580, U.K,
⁷ Institut de Minéralogie et Géochimie, Université-Dorigny, CH–1015 Lausanne, Switzerland

E-mail address: nicolas.meisser{at}unil.ch

Pizgrischite, (Cu,Fe)Cu₁₄PbBi₁₇S₃₅, is a new mineral species named after the type locality, Piz Grisch Mountain, Val Ferrera, Graubünden, Switzerland. This sulfosalt occurs as thin, striated, metallic lead-grey blades measuring up to 1 cm in length, embedded in quartz and associated with tetrahedrite, chalcopyrite, pyrite, sphalerite, emplectite and derivatives of the aikinite–bismuthinite series. In plane-polarized light, the new species is brownish grey with no perceptible pleochroism; under crossed nicols in oil immersion, it presents a weak anisotropy with dark brown tints. Minimum and maximum reflectance values (in %) in air are: 40.7–42.15 (470 nm), 41.2–43.1 (546 nm), 41.2–43.35 (589 nm) and 40.7–43.3 (650 nm). Cleavage is perfect along {001} and well developed on {010}. Abundant polysynthetic twinning is observed on (010). The mean micro-indentation hardness is 190 kg/mm² (Mohs hardness 3.3), and the calculated density is 6.58 g/cm³. Electron-microprobe analyses yield (wt%; mean result of seven analyses): Cu 16.48, Pb 2.10, Fe 0.77, Bi 60.70, Sb 0.35, S 19.16, Se 0.04, total 99.60. The resulting empirical chemical formula is (Cu_15.24Fe_0.80Pb_0.60)_16.64(Bi_17.07Sb_0.17)_17.24(S_35.09Se_0.03)_35.12, in accordance with the formula derived from the single-crystal refinement of the structure, (Cu,Fe)Cu₁₄PbBi₁₇S₃₅. Pizgrischite is monoclinic, space group C2/m, with the following unit-cell parameters: a 35.054(2), b 3.91123(1), c 43.192(2) Å, β 96.713(4)°, V 5881.24 ų, Z = 4. The strongest seven X-ray powder-diffraction lines [d in Å (I)(hkl)] are: 5.364(40)(04), 4.080(50)(05), 3.120(40)(118), 3.104(68)(18), 2.759(53) (11), 2.752(44)(910) and 1.956(100)(020). The crystal structure is an expanded monoclinic derivative of kupikite. Pizgrischite belongs to the cuprobismutite series of bismuth sulfosalts but, sensu stricto, it is not a homologue of cuprobismutite. At the type locality, pizgrischite is the result of the Alpine metamorphism under greenschist-facies conditions of pre-Tertiary hydrothermal Cu–Bi mineralization.

Keywords: pizgrischite, new mineral species, sulfosalt, crystal structure, kupikite, Alps, Switzerland.

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