THALLIUM SULFOSALTS, THEIR PHASE-DIAGRAMS AND THE AFFINITY PROBLEM

In spite of the considerable research on sulphide minerals very little thermodynamical informations are available for most of the sulphosalt s. First order thermodynamic approximation, possible only in those cas es when the cation polyhedra of the complex sulphides approximate the cation polyhedra of the simple sulphides, served as a good base for th e investigation of the mutual affinity of sulphides within certain pha se assemblages described by appropriate phase diagram. The used approx imation procedure appears to be the most useful with relatively simple algorithm. It is known under the name ''sulphide sum'' method develop ed by CRAIG and BARTON (1973). In ternary system the magnitude of the free energy of formation (Delta G(i)) of the binary compound i, presen t in this system, defines the extent of affinity of two end members, o ne to another, which build the investigated binary compound. Compariso n of affinities between Ag2S and As2S3 or Tl2S and As2S3 within the Ag 2S-Tl2S-As2S3 system, as well as Ag2S and Sb2S3 or Tl2S and Sb2S3 with in Ag2S-Tl2S-Sb2S3 system gave interesting results. The affinity of As 2S3 to Ag2S is lower than to Tl2S in Ag2S-Tl2S-As2S3. The affinity of Sb2S3 to Ag2S is smaller than to Tl2S in Ag2S-Tl2S-Sb2S3 system. Betwe en three end members, those two, which have more negative Delta G, wil l give more stable binary compounds. The affinity between two end-memb ers, which have the most negative Delta G, is greater than between any of these two end-members to the third end-member, which have less neg ative Delta G, in the investigated ternary system.