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 (original) (raw)

Original paper

Topa, Dan; Makovicky, Emil; Stoeger, Berthold; Stanley, Chris

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 11fold P 21/c superstructures of the basic 4.2 Å substructure. Heptasartorite is Tl7 Pb22 As55 S108 [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 Å3 ; enneasartorite is Tl6 Pb32 As70 S140 (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) Å3; hendekasartorite is Tl2 Pb48 As82 S172 (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) Å3. 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 Asm Sn crank-shaft chains in the walls of doubleribbons 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' PbAs2 S4 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.

Keywords

anion-omission series • pb–as sulfosalts • hendekasartorite • enneasartorite • thallium substitution • sartorite homologous series • heptasartorite