THERMOCHEMISTRY OF POLYBASITE-PEARCEITE SOLID-SOLUTIONS

A model is developed for the thermodynamic mixing properties of polybasite-pearceite (Ag,Cu)16(Sb,As)2S11 solid solutions. In this model the configurational entropy is formulated for the assumption that Ag and Cu display long-range, nonconvergent, ordering between three crystallographically distinct sites and that As and Sb mix randomly on one type of site. The nonconfigurational Gibbs energy is described using a Taylor series of second degree in composition and ordering variables. Model parameters are calibrated from constraints on 75-degrees-C miscibility gaps derived from unmixing experiments on (Ag,Cu)16(Sb.5As.5)2S11 grains (X(Cu)Pb-Pr greater-than-or-equal-to 0.32) and from 75-350-degrees-C Ag-Cu exchange experiments (evacuated silica tubes; variable mass ratio) between polybasite-pearceites and one- and two-phase subassemblages in the Ag2S-Cu2S subsystem. The resulting model is consistent with the distribution of polybasite-pearceite compositions observed in nature. It is also consistent with the inferences that (1) only slight negative departures from ideality are associated with the Cu for Ag substitution (less-than-or-equal-to -0.13 kJ/gfw for polybasite and less-than-or-equal-to -0.22 kJ/gfw for pearceite on a one Cu + Ag formula basis), (2) positive deviations from ideality due to the As-Sb substitution in polybasite-pearceite (less-than-or-equal-to 1.0 kJ/gfw on a one As-Sb site formula basis) are virtually identical to those in tetrahedrite-tennantite fahlores (Sack and Ebel 1993), and (3) the Gibbs energy of formation of Ag16Sb2S11 polybasite is 4.58 +/- 1.4 kJ/gfw more positive than the equivalent combination of beta-Ag2S and Ag3SbS3 pyrargyrite at 400-degrees-C.