Internally consistent database for sulfides and sulfosalts in the system Ag2S-Cu2S-ZnS-Sb2S3-As2S3

摘要

An updated thermodynamic database for Ag2S-Cu2S-ZnS-Sb2S3-As2S3 sulfides and sulfosalts applicable to temperatures above 119 degreesC 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 degreesC, evacuated silica tubes) conducted to define the stability of the polybasite-pearceite [(Ag1-x,Cu-x)(16)(Sb1-y,As-y)(2)S-11] + ZnS sphalerite assemblage with respect to assemblages containing (Ag,Cu)(2)S sulfides coexisting with (Cu, Ag)(10)Zn-2(Sb,As)(4)S-13 fahlore sulfosalts. It was found that the thermodynamics of mixing of bcc- and hcp-(Ag,Cu)(2)S solutions, which are fast-ion conductors, may be described by using site multiplicities of metals alpha (Ag,Cu) > 2 and temperature-dependent regular solution parameters. We obtained estimates for the Gibbs energies of formation for Ag16Sb2S11 and Cu16Sb2S11 polybasite endmembers from the simple sulfides (Ag2S, Cu2S, and Sb2S3) of -30.79 and -4.07 kJ/gfw at 200 degreesC, and -32.04 and -0.59 kJ/gfw at 400 degreesC, 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 Ag10Zn2Sb4S13 and Cu10Zn2Sb4S13 fahlores (-20.29 and -105.29 kJ/gfw at 200 degreesC and -23.72 and -105.76 kJ/gfw at 400 degreesC) 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: 1/4Ag(10)Zn(2)Sb(4)S(13) + 1/2Ag(16)As(2)S(11) = 1/2Ag(16)Sb(2)S(11) + 1/4Ag(10)Zn(2)As(4)S(13) Sb-fahlore pearceite polybasite As-fahlore and Ag3SbS3 + 1/2Ag(16)As(2)S(11) = 1/2Ag(16)Sb(2)S(11) + Ag3AsS3 pyrargyrite pearceite polybasite proustite are, respectively, 8.75 and 0.40 kJ/gfw in the range 150-350 degreesC, 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 Cu10Fe2Sb4S13 and Ag10Fe2Sb4S13 fahlores (-63.92 and +11.46 kJ/gfw at 200 degreesC and -75.73 and -3.31 kJ/gfw at 400 degreesC). Copyright (C) 2000 Elsevier Science Ltd. [References: 29]