Internally consistent database for sulfides and sulfosalts in the system Ag 2S–Cu 2S–ZnS–Sb 2S 3–As 2S 3

Abstract

An updated thermodynamic database for Ag 2S–Cu 2S–ZnS–Sb 2S 3–As 2S 3 sulfides and sulfosalts applicable to temperatures above 119°C is developed to calculate phase relations for polybasite-pearceite- and fahlore-bearing assemblages. It is based on pre-existing and new constraints on activity-composition, Ag–Cu and As–Sb partitioning, and other relations, and on experiments (200–300°C, evacuated silica tubes) conducted to define the stability of the polybasite-pearceite [(Ag 1− x ,Cu x ) 16(Sb 1− y ,As y) 2S 11] + ZnS sphalerite assemblage with respect to assemblages containing (Ag,Cu) 2S sulfides coexisting with (Cu, Ag) 10Zn 2(Sb,As) 4S 13 fahlore sulfosalts. It was found that the thermodynamics of mixing of bcc– and hcp–(Ag,Cu) 2S solutions, which are fast-ion conductors, may be described by using site multiplicities of metals α Ag,Cu > 2 and temperature-dependent regular solution parameters. We obtained estimates for the Gibbs energies of formation for Ag 16Sb 2S 11 and Cu 16Sb 2S 11 polybasite endmembers from the simple sulfides (Ag 2S, Cu 2S, and Sb 2S 3) of −30.79 and −4.07 kJ/gfw at 200°C, and −32.04 and −0.59 kJ/gfw at 400°C, respectively, that are about one half kJ/gfw more positive and about 6 kJ/gfw more negative than those estimated by Harlov and Sack (1995b). The corresponding estimates for formation energies of Ag 10Zn 2Sb 4S 13 and Cu 10Zn 2Sb 4S 13 fahlores (−20.29 and −105.29 kJ/gfw at 200°C and −23.72 and −105.76 kJ/gfw at 400°C) are comparable to, and roughly 110 kJ/gfw more positive than, the corresponding estimates of Ebel and Sack (1994). We also determined that the Gibbs energies of the As–Sb exchange reactions: 1 4 Ag 10 Zn 2 Sb 4 S 13 + 1 2 Ag 16 As 2 S 11 = 1 2 Ag 16 Sb 2 S 11 + 1 4 Ag 10 Zn 2 As 4 S 13 Sb–fahlore pearceite polybasite As–fahlore and Ag 3 SbS 3 + 1 2 Ag 16 As 2 S 11 = 1 2 Ag 16 Sb 2 S 11 + Ag 3 AsS 3 pyrargyrite pearceite polybasite proustite are, respectively, 8.75 and 0.40 kJ/gfw in the range 150–350°C, and these predictions are consistent with As–Sb partitioning relations observed in nature and produced in laboratory studies. Finally, we obtain estimates for the Gibbs energies of formation of Cu 10Fe 2Sb 4S 13 and Ag 10Fe 2Sb 4S 13 fahlores (−63.92 and +11.46 kJ/gfw at 200°C and −75.73 and −3.31 kJ/gfw at 400°C).