Thermochemistry of Ag 2SCu 2S sulfide solutions: Constraints derived from coexisting Sb 2S 3- and As 2S 3-bearing sulfosalts

Abstract

We report experimental brackets (150–350°C, evacuated silica tubes) on (1) AgCu partitioning between hexagonally close packed (hcp)-, face centered cubic (fcc)-, and body centered cubic (bcc)-(Ag, Cu) 2S sulfides and the sulfosalts polybasite-pearceite [(Ag 1− x Cu x ) 16(Sb, As) 2S 11, 0.10 > x > 0.57] and high-skinnerite [(Cu 1− x , Ag x ) 3SbS 3, x < 0.26] and (2) Ag Cu ratios of (Ag, Cu 2S sulfides coexisting with polybasite and skinnerite, and polybasite and pyrargyrite [(Ag 1− x , Cu x ) 3SbS 3, x < 0.21]. Based on these brackets on AgCu partitioning and activity-composition models for polybasite-pearceite and high-skinnerite (Harlov and Sack, 1994, 1995) we infer that (1) both hcp- and bcc-(Ag, Cu) 2S sulfides exhibit positive deviations from ideality sufficient to produce metastable unmixing at slightly less than 90°C and (2) fcc-(Ag, Cu ) 2S sulfides display more moderate positive deviations from ideal activity-composition relations, characterized by a regular-solution-type parameter, W AgCu fcc = 5.4 ± 0.2 kJ/mol, for an assumed site multiplicity of AgCu mixing of 2. The inferred activity-composition relations for fcc- and bcc-(Ag, Cu) 2S are consistent with brackets on the T − X Cu systematics of the transition between these phases (Skinner, 1966) and the calorimetric data of Grønvold and Westrum (1986) for Ag 2S provided that the effective site multiplicity of AgCu mixing in bcc-(Ag, Cu) 2S decreases linearly from about 3 in Cu-poor to about 2 in Cu-rich compositions ( X Cu ∼ 0.315). Our constraints are also consistent with the calorimetric data for Cu 2S (Grønvold and Westrum, 1987) and Skinner's (1966) brackets on the T − X Cu systematics of the transition between fcc- and hcp-(Cu, Ag) 2S provided that the effective site multiplicity of AgCu mixing in hcp-(Ag, Cu) 2S approximates 3. From our brackets on Ag Cu ratios of (Ag, Cu) 2S sulfides coexisting with polybasite and skinnerite, and polybasite and pyrargyrite and calibrations for the AgCu exchange reactions we calculate average values of 5.30 ± 1.14 and −31.1 ± 0.7 kJ/mol for the Gibbs energies of formation of hypothetical Cu 16Sb 2S 11 and Ag 16Sb 2S 11 polybasite components from the simple sulfides over the temperature range 150–350°C.