THE SOLID SOLUTIONS OF REBULITE AND JANKOVICITE IN THE PHASE SYSTEM Tl2S-As2S3-Sb2S3

Syntheses along the Tl-5(As,Sb)(13)S-22 compositional line in the Tl2S-As2S3-Sb2S3 phase system showed that the compositional range of rebulite extends from Tl5As9.5Sb3.5S22 to Tl5As7.75Sb5.25S22. The Sb-rich end-member is in equilibrium with jankovicite of the ideal composition Tl5Sb7.5As5.5S22, which is considered to be the As-rich end-member of the jankovicite solid solution. The crystal structure analysis of specimens from the As and Sb end-members of rebulite show that the Sb/As substitution is present in Sb3, Sb4, Sb5, As1 and As2 structural sites. Of them, Sb3 is always Sb dominated whereas other four vary from As- to Sb-dominated over the range of the solid solution. The change of the structural topology from jankovicite to rebulite, closely related but not identical structures, is explained through the necessity to accommodate the smaller volumes of the As coordination polyhedra, and is accomplished through unit-cell twinning over the periodic (001)(reb) twin boundaries. The As end-member of the rebulite solid solution is in equilibrium with a phase of composition close to Tl(2.4)Sb(0.68)gAs(7.18)S(13), interpreted as imhofite.