IMA 2015-073 = heptasartorite; IMA 2015-074 = enneasartorite

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Marco E. Ciriotti
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IMA 2015-073 = heptasartorite; IMA 2015-074 = enneasartorite

Messaggio da Marco E. Ciriotti » gio 28 set, 2017 9:56

▪ Topa, D., Makovicky, E., Stoeger, B., Stanley, C. (2017): Heptasartorite, Tl7Pb22As55S108, enneasartorite, Tl6Pb32As70S140 and hendekasartorite, Tl2Pb48As82S172, three members of the anion-omission series of ‘sartorites’ from the Lengenbach quarry at Binntal, Wallis, Switzerland. European Journal of Mineralogy, 29, 701-712.

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 Å3; 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) Å3; 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) 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 AsmSn 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.
Marco E. Ciriotti

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