Heptasartorite, Tl7Pb22As55S108, enneasartorite, Tl6Pb32As70S140 and hendekasartorite, Tl2Pb48As82S172, three members of the anion-omission series of ‘sartorites’ from the Lengenbach quarry at Binntal, Wallis, Switzerland

Detailed electron-microprobe investigations and crystal-structure determinations established that ‘sartorite’ represents a group of distinct mineral species, each with unique chemistry and crystal structure. These manifest themselves as the 7-, 9- and 11-fold P2₁/c superstructures of the basic 4.2 Å substructure. Heptasartorite is Tl₇Pb₂₂As₅₅S₁₀₈ [based on 192 atoms per formula unit (apfu), 84Me + 108S] with a = 29.269(2), b = 7.8768(5), c = 20.128(2) Å, β = 102.065(2)° and unit-cell volume V = 4537.8 ų; enneasartorite is Tl₆Pb₃₂As₇₀S₁₄₀ (based on 248 apfu, 108Me + 140S) with a = 37.612(6), b = 7.8777(12), c = 20.071(3) Å, β = 101.930(2)° and V = 5818.6(15) ų; hendekasartorite is Tl₂Pb₄₈As₈₂S₁₇₂ (based on 304 apfu, 132Me + 172S) (empirical ΣMe = 132.48) with a = 31.806(5), b = 7.889(12), c = 28.556(4) Å and β = 99.034(2)° with V = 7076.4(15) ų. Physical and optical properties (grey with metallic lustre, in polished section white with visible bireflectance, red internal reflections; reflectance curves span 28.7–42.5%; Mohs hardness 3–3½) of these phases are very similar so that chemical analysis and/or single-crystal X-ray diffraction is needed to distinguish them. A brief description of complicated As_mS_n crank-shaft chains in the walls of double-ribbons which form the As-based slabs of these structures is given. The three new mineral species differ in their structures by 4.2 Å modular increments, not just by cation substitutions. They represent anion-omission derivatives of the ‘ideal’ PbAs₂S₄ composition with an important role for thallium in charge compensation. The described minerals belong to the late sulfide phases in the Pb–Tl–Ag–As deposit of Lengenbach, Wallis, Switzerland.