Study of Platinum-Group Element (PGE) distribution in minerals shows that "light' PGE (LPGE: Ru, Rh, Pd) are found more often in sulphides (pyrrhotite, pentlandite) in comparison with "heavy" PGE (HPGE: Os, Ir, Pt). Natural Co-Ni-(Fe)-
sulfoarsenides could contain all six PGE. On the ground of litterature data PGE-content in sulfoarsenides is connected with Co:Ni ratio: Co-members of cobaltite-gersdorfite serie are generally enriched in LPGE (i.e. up to 10.5 wt.% of Rh) while Ni-varieties "prefer" HPGE (9.5 wt.% of Os).
Taking into account this difference the results of experimental study of "PGE"AsS-(Co,Ni,Fe)AsS systems (Evstigneeva and Tarkian, 1996) were analysed for testify the hypothesis that LPGE- and HPGE-content in sulfoarsenides depends on Co and Ni concentration. Another problem to consider is the connection of PGE-content in sulfoarsenides with the deviation of As:S from 1:1.
In contrast to natural sulfoarsenides the synthesized compounds are characterized by more high PGE-concentration: up to 5.53 wt% of Pt, 29.73 wt.% of Os, 10.29 wt.% of Pd, 21.57 wt.% of Ru, 27.6 wt.% of Rh and 21.5 wt.% of Ir. However no supposed connection (LPGE in Co-sulfoarsenides and HPGE in Ni-sulfoarsenides was observed. The average contents of Pt, Pd and Os in sulfoarsenides dont exceed 15 mol.% "PGE"AsS. Pt and Ru are found in synthesized cobaltites. Pd and Os are established in the samples of whole series. The magority of synthesized phases is enriched in As (As:S ratio is deviated from 1:1 in favour of As). Near complete series "PGE"AsS-(Co,Ni)AsS were synthesized with Ir and Rh. The most PGE-enriched phases in these series have As:S ratio close to 1:1.
The results received could be explained using two possibilities: 1) synthesized sulfoarsenides are heterogeneous because very fine grains of "PGE"-arsenides are intergrown with the main phase, and 2) there is the close connection "As:S ratio -
sulpfur fugacity". These two points of view are discussed. The conditions of formation of natural sulfoarsenides are considered using the experimental data on the stability of Rh-, Ir- and Pt- sulfoarsenides at 550ø C under controlled sulphur fugacity. Corresponding these data they are stable in the monoclinic pyrrhotite-pyrite field, but the reaction of transformation sperrylite-platarsite is carried out under a little higher sulphur fugacity. The role of iron in the formation of PGE-bearing sulfoarsenides is also discussed.
Evstigneeva, T.L. & Tarkian, M., Mineralogy & Petrology (1996, in press).