Mineralogy of Pt-Pd sulfides: The Redefinition of Braggite and Vysotskite (with Comments on the Extent of Solid Solution in the PtS–PdS Binary)

Abstract

AbstractA re-evaluation of data relating to the crystal structures, phase equilibria, and ideal chemical formulae of the key minerals belonging to the PtS–PdS binary, namely vysotskite, braggite, and cooperite, has led to redefinitions of the ideal chemical formulae for vysotskite and braggite. Results show that vysotskite and braggite are isostructural and crystallize in the space group P42/m. Their crystal structures contain three Me sites and a single S site, giving rise to the general crystal-chemical formula Me(1)2Me(2)2Me(3)4S8. Ordering of Pt and Pd leads to the revised ideal formula of Pd2Pd2Pd4S8 (Z = 1) for vysotskite, which can be simplified to PdS (Z = 8), and a revised ideal formula of Pd2Pt2Pt4S8 (Z = 1) for braggite, which can be simplified to PdPt3S4 (Z = 2). Cooperite, ideally PtS (Z = 8), crystallizes in a related, but different, space group, P42/mmc. Its crystal structure contains a single Me and S site. Both Pt and Pd are disordered over the Me site but with Pt > Pd . Oversimplification of the ideal chemical formulae for all these minerals and failure to consider crystal-structure implications has led to confusion in the literature.Consideration of experimental data combined with that for minerals from world-wide occurrences suggests complete solid solution between vysotskite and braggite, but of a more limited degree between cooperite and braggite, consistent with what would be expected when their respective crystal structures are considered. The maximum Pt possible in braggite should correspond to (Pd0.51Pt0.49)Σ1Pt3S4 (or Pd0.51Pt3.49S4); Pt:Pd = 0.87, which requires Pd 6.29, Pt 78.86, S 14.85, total 100.00 wt.%) and the maximum Pd content, to Pd(Pt0.505Pd0.495)Σ1(Pt1.505Pd0.495)Σ2S4 (or Pd1.99Pt2.01S4); Pt:Pd = 0.50, which requires Pd 28.93, Pt 53.56, S 17.52, total 100.00 wt.%). In light of the complete solid solution between vysotskite and braggite, the 50% rule should be applied to compositions between them. Thus, within the PtS–PdS binary, it is recommended that (1) vysotskite be used for those minerals having compositions ranging from 100 to 56.5 mol.% PdS; (2) braggite, for those having compositions that range from 56.5 to 13 mol.% PdS; and (3) cooperite, for those having <13 mol.%.