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GABRIELITE, Tl₂AgCu₂As₃S₇, A NEW SPECIES OF THALLIUM SULFOSALT FROM LENGENBACH, BINNTAL, SWITZERLAND

Stefan Graeser^1,, Dan Topa², Toni Bali-uni³ and Emil Makovicky³

¹ Natural History Museum Basel, CH-4001 Basel, and Mineralogical Institute, University of Basel, CH-4056 Basel, Switzerland
² Department of Geology and Mineralogy, University of Salzburg, Hellbruner Str. 34/III, A-5020 Salzburg, Austria
³ Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark

E-mail address: stefan.graeser{at}unibas.ch

Gabrielite, Tl₂AgCu₂As₃S₇, is a new species of sulfosalt mineral occurring in the famous Lengenbach locality, at Binntal, Canton Valais, Switzerland. It was discovered in association with numerous other As sulfosalts, such as hutchinsonite, hatchite, edenharterite, trechmannite, tennantite, and abundant realgar and rathite, generally occurring as idiomorphic crystals in cavities of a Triassic dolomitic rock. The whole region was transformed during the Alpine orogeny to the greenschist to garnet-amphibolite facies of metamorphism. Gabrielite occurs as pseudohexagonal crystals; its true symmetry is triclinic, space group P, Z = 6, a 12.138(3), b 12.196(3), c 15.944(4) Å, 78.537(5), ß 84.715(4), 60.470(4)°, V 2013(1) ų (from the crystal-structure determination). The strongest powder-diffraction lines [d_obs in Å(I_obs)(hkl)] are 15.63(100)(001), 3.143(90)(21, 14), 3.531(80)(300), 2.911(70)(422, 2), 2.520(60)(23), and 2.978(60)(324). Gabrielite is opaque with a grey to black color, metallic luster and blackish red streak. Individual crystals reach up to approximately 0.4 mm, and aggregates may exceed 1 mm across. Euhedral crystals are short prismatic to platy parallel to (001), with excellent cleavage along this face. The microhardness VHN,18 kg/mm² (load 10 g), corresponds to a Mohs hardness of 1–2. The chemical composition (electron microprobe, mean result of 11 analyses, in wt%) is Tl 37.30, Cu 12.50, Ag 8.47, As 18.90, Sb 1.79, S 20.64, total 99.70 wt%, from which the empirical formula, Tl_5.95(Ag_2.56Cu_6.46) _9.02(As_8.23Sb_0.48) _8.71S₂₁, was derived. The simplified formula is Tl₂AgCu₂As₃S₇. The calculated density D_calc is 5.38 g/cm³. In reflected light, gabrielite is white, has common red internal reflections, shows weak anisotropy and moderate reflectivity [%R_max in air (COM wavelengths in nm)]: 30.53(470), 29.10(546), 27.94(589), 26.35(650). The crystal structure is layered parallel to (001), which explains the habit and cleavage. The twin law with (100) as a twin plane is due to the local symmetry properties of the structural layers. A presence of metal–metal (Tl–Tl and Tl–Cu) and metal–semimetal (Tl–As) bonds in the structure explains the low hardness and metallic character. Substitutions of Cu for Ag in the tetrahedral coordinations, Ag for Cu in the trigonal–planar coordinations, and Sb for As in isolated AsS₃ coordination polyhedra and As₄S₈ rings lead to the structural formula Tl₆(Ag,Cu)₃^IV(Cu,Ag)₆^III[(As,Sb)S₃]₃[(As,Sb)₂S₄]₃; the empirical formula obtained from the microprobe data is Tl_5.95(Ag_2.56Cu_6.46) _9.02(As_8.23Sb_0.48) _8.71S₂₁. The name of the mineral honors Walter Gabriel, of Basel, Switzerland, well-known mineral photographer and expert of Lengenbach minerals.

Keywords: gabrielite, new mineral species, sulfosalt, thallium, Lengenbach, Switzerland.

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