Bi-Sb Energetics in Sulfosalts and Sulfides

S. Ghosal1 and R. O. Sack2
1 Department of Earth and Atmospheric Sciences, Purdue University, West Lafayette, IN, 47907-1397, USA
2 Department of Geological Sciences, Box 351310, University of Washington, Seattle, WA, 98195-1310, USA

Abstract: Experimental brackets (300–450°C on Sb-Bi partitioning between stibnite-bismuthinites (Sb,Bi)2S3 and sulfosalts in the AgSbS2-AgBiS2 binary subsystem (α-Ag(Sb,Bi)S2, β-Ag5(Sb,Bi)4I(Sb,Bi)IIS10) and extant constraints are used to define mixing properties and standard state Gibbs energies of Sb-Bi exchange reactions. They are also used to construct a phase diagram for Ag(Sb,Bi)S2 sulfosalts. We infer that the non-ideality associated with Sb-Bi mixing is largest in minerals of the β-Ag5(Sb,Bi)4I(Sb,Bi)IIS10 series, and is sufficient to produce miscibility gaps between an ordered intermediate species Ag5(Sb)4I(Bi)IIS10 and Sb- and Bi-end-members at T < 240°C (measured in terms of symmetric regular-solution-type parameters 14WBi−SbIβ=WBi−SbIIβ∼8.5 kJ/gfw). The non-ideality associated with the Sb-Bi substitution in stibnite-bismuthinite and α-Ag(Sb,Bi)S2 is ≈ 70% that in the Ag5(Sb,Bi)4I(Sb,Bi)IIS10 series (WBi−SbBS≈12.0 kJ/gfw; WBi−Sbα≈6.0 kJ/gfw). It is insufficient to produce exsolution at temperatures of ore deposition (T > Tc ≈ 88°C), but most likely is responsible for a preponderance in molar Sb/Bi ratios towards end-member compositions. Finally, positive Gibbs energies of the Sb-Bi exchange reactions 12Bi2S3+AgSbS2=AgBiS2+12Sb2S3(ΔG¯Bi−SbBS−α≈2.70kJ∕gfw) and 12Bi2S3+15Ag5Sb5S10=15Ag5Bi5S10+12Sb2S3(ΔG¯Bi−Sb0BS−β≈4.89kJ∕gfw) indicate that Bi is more compatible in stibnite-bismuthinite sulfides than in Ag(Sb,Bi)S2 sulfosalts.

Keywords: bismuthinite-stibnites • Ag(Sb,Bi)S2 sulfosalts • Sb-Bi energetics • ordering and exchange reactions

Mineralogical Magazine; October 1999 v. 63; no. 5; p. 723-733; DOI: 10.1180/002646199548871
© 1999, The Mineralogical Society
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