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Mineralogical Magazine; December 2000; v. 64; no. 6; p. 1047-1056
© 2000 Mineralogical Society of Great Britain and Ireland
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Phase relations in the metal-rich portions of the phase system Pt-Ir-Fe-S at 1000°C and 1100°C

E. Makovicky1,* and S. Karup-moller2

1 Department of Mineralogy, Geological Institute, University of Copenhagen, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark
2 Institute for Geology and Geological Engineering, The Danish Technical University, Building 205, DK-2800 Lyngby, Denmark

* E-mail: emilm{at}geo.geol.ku.dk

Phase relations in the S-poor portions of the dry condensed Pt-Ir-Fe-S system were determined at 1000° and 1100°C with a particular emphasis on delineation of the solid solubility fields of the Pt-Ir-Fe alloys. At both temperatures, a broad field of {gamma}(Ir,Fe,Pt) alloy coexists with {gamma}-(Pt,Fe), Pt3Fe and PtFe which dissolve respectively at least 5.1, 29.3 and 24.0 at.% Ir at 1100°C (2.2, 23.6 and <=17.2 at.% Ir at 1000°C). Gaps between the nearly Ir-free Pt-Fe alloys {gamma}(Pt,Fe), Pt3Fe s.s., PtFe s.s. and {gamma}(Fe,Pt) were estimated as 20–23 at.%, 40–42 at.% and 54.2–~57 at.% Fe at 1100°C (18–23, 39.5–42.5 and 59–62 at.% Fe at 1000°C). The first gap agrees with data from natural phases by Cabri et al. (1996). The Fe-rich sulphide melt dissolves only traces of Pt and Ir; Fe1–xS dissolves up to 5.8 at.% Ir at 1100°C and 3.4 at.% Ir at 1000°C. These solubilities and the compositions of coexisting alloys (before low-temperature exsolution) may serve as geothermometers. The S-rich melt dissolves up to 8.2 at.% Pt and 2.5 at.% Ir (measured on a solidified melt) at 1100°C. The Fe-Ir thiospinel dissolves up to 2.2 at.% Pt (for the composition Fe21.6Ir18.9Pt2.2S57.3 at 1100°C). The 25–28 at.% Fe portions of Pt3Fe with <10 at.% Ir partake in all important sulphide (± melt)-alloy assemblages (with PtS, Ir2S3, pyrrhotite, S-rich melt and thiospinel). Therefore, their use in recognizing distinct natural assemblages of genetic importance is of limited value.

KEYWORDS: Pt-Ir-Fe-S phase system, Pt-Fe alloys, isoferroplatinum, tetraferroplatinum, platinum-group metal deposits




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