Heptasartorite, Tl7Pb22As55 S108, enneasartorite, Tl6 Pb32 As70 S140 and hendekasartorite, Tl2Pb48As82 S172, three members of the anion-omission series of 'sartorites' from the Lengenbach quarry at Binntal, Wallis, Switzerland

Abstract

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 P 2 1 / c superstructures of the basic 4.2 A substructure. Heptasartorite is Tl 7 Pb 22 As 55 S 108 [based on 192 atoms per formula unit ( apfu ), 84Me + 108S] with a = 29.269(2), b = 7.8768(5), c = 20.128(2) A, β = 102.065(2)° and unit-cell volume V = 4537.8 A 3 ; enneasartorite is Tl 6 Pb 32 As 70 S 140 (based on 248 apfu , 108Me + 140S) with a = 37.612(6), b = 7.8777(12), c = 20.071(3) A, β = 101.930(2)° and V = 5818.6(15) A 3 ; hendekasartorite is Tl 2 Pb 48 As 82 S 172 (based on 304 apfu , 132Me + 172S) (empirical ΣMe = 132.48) with a = 31.806(5), b = 7.889(12), c = 28.556(4) A and β = 99.034(2)° with V = 7076.4(15) A 3 . 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 m S 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 A modular increments, not just by cation substitutions. They represent anion-omission derivatives of the ‘ideal’ PbAs 2 S 4 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.