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) Å3 (from the crystal-structure determination). The strongest powder-diffraction lines [dobs in Å(Iobs)(hkl)] are 15.63(100)(001), 3.143(90)(21 , 1 4), 3.531(80)(300), 2.911(70)(422, 2 ), 2.520(60)(2 3), 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, Tl5.95(Ag2.56Cu6.46) 9.02(As8.23Sb0.48) 8.71S21, was derived. The simplified formula is Tl2AgCu2As3S7. The calculated density Dcalc is 5.38 g/cm3. In reflected light, gabrielite is white, has common red internal reflections, shows weak anisotropy and moderate reflectivity [%Rmax 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)3IV(Cu,Ag)6III[(As,Sb)S3]3[(As,Sb)2S4]3; the empirical formula obtained from the microprobe data is Tl5.95(Ag2.56Cu6.46) 9.02(As8.23Sb0.48) 8.71S21. The name of the mineral honors Walter Gabriel, of Basel, Switzerland, well-known mineral photographer and expert of Lengenbach minerals.
Canadian Mineralogist, 2006, Vol 44, p. 141-158
gabrielite; Schweiz; sulfosalte; nye mineral arter; Switzerland; Lengenbach; thallium; sulfosalts; new mineral species; The Faculty of Science