Pizgrischite, (Cu,Fe)Cu14 PbBi17S35, a new sulfosalt from the Swiss Alps: Description, crystal structure and occurrence

Nicolas Meisser, Kurt Schenk, Peter Berlepsch, Joël Brugger, Michel Bonin, Alan J. Criddle, Philippe Thélin, François Bussy

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Abstract

Pizgrischite, (Cu,Fe)Cu14 PbBi17S35, 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 (Cu15.24 Fe0.80 Pb0.60) ∑16.64 (Bi17.07S b0.17)17.24 (S35.09 Se0.03) ∑35.12, in accordance with the formula derived from the single-crystal refinement of the structure, (Cu,Fe)Cu14Pb Bi17S35. 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)°, V5881.24 Å3, Z = 4. The strongest seven X-ray powder-diffraction lines [d in Å (1)(hkl)] are: 5.364(40) (6̄04), 4.080(50)(8̄05), 3.120(40)(118), 3.104(68)(3̄18), 2.759(53) (9̄11), 2.752(44)(910) and 1.956(100)(020). The crystal structure is an expanded monoclinic derivative of kupčikite. 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.

Original languageEnglish
Pages (from-to)1229-1245
Number of pages17
JournalCanadian Mineralogist
Volume45
Issue number5
DOIs
Publication statusPublished - 1 Oct 2007
Externally publishedYes

Keywords

  • Alps
  • Crystal structure
  • Kupčikite
  • New mineral species
  • Pizgrischite
  • Sulfosalt
  • Switzerland

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