Abstract:
An updated thermodynamic database for Ag2S-Cu2S-ZnS-Sb2S3-As2S3 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 [(Ag1-x,Cux)16(Sb1-y,Asy)2S11] + ZnS sphalerite assemblage with respect to assemblages containing (Ag,Cu)2S sulfides coexisting with (Cu, Ag)10Zn2(Sb,As)4S13 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 αAg,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°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 Ag10Zn2Sb4S13 and Cu10Zn2Sb4S13 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: 14Ag10Zn2Sb4S13+12Ag16As2S11=12Ag16Sb2S11+14Ag10Zn2As4S 13Sb-fahlorepearceitepolybasiteAs-fahlore and Ag3SbS3+12Ag16As2S11=12Ag16Sb2S11+Ag3AsS3pyrargyritepearceitep olybasiteproustite 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 Cu10Fe2Sb4S13 and Ag10Fe2Sb4S13 fahlores (-63.92 and +11.46 kJ/gfw at 200°C and -75.73 and -3.31 kJ/gfw at 400°C).