CONTRIBUTIONS TO THE CRYSTAL-CHEMISTRY OF THALLIUM SULFOSALTS .3. THECRYSTAL-STRUCTURE OF LORANDITE (TLASS2) AND ITS RELATION TO WEISSBERGITE (TLSBS2)

Crystal structure of lorandite TLAsS(2) was refined from single crysta l data using 793 independent reflections (final R = 0.071). ZEAMANN & ZEMANN'S (1959) and FLEET's (1973) model was confirmed and further ref ined; current refinement yields average Tl-S and As-S distances equal to 3.30 Angstrom and 2.28 Angstrom, respectively. Structure of lorandi te is related to the SnS archetype but it differs from all the other S nS-related sulphosalt structures by its distinct layer stacking. Confi gurations of the (100) layer surfaces of lorandite are very similar to those on (010) layer surfaces of weissbergite (TlSbS2). These surface s contain Tl-S double strands that alternate with strips that comprise As-S (resp. Sb-S) chain fragments; their mesh is rectangular for lora ndite but oblique for weissbergite. Microprobe and powder diffraction data on the system TlAsS2-TlSbS2 show miscibility gaps between 54 and 85 mol% TlAsS2 and (probably) also between 24 and 34 mol% TLAsS(2) for syntheses at 250 degrees C. Lorandite and As-poor weissbergite are se parated by a triclinic ''weissbergite-like phase'' TlAs0.34-0.54Sb0.66 -0.46S2 with a (probably) ordered As and Sb distribution.