Thermochemistry of Ag-Cu exchange equilibria between proustite, sinnerite,and pearceite: Constraints on Ag-Cu and As-Sb mixing in pyrargyrite-proustite

Experimental Ag-Cu exchange equilibria (evacuated silica tubes; variable ma ss ratio) involving proustite (Ag,Cu)(3)AsS3, smithite AgAsS2, pearceite (A g,Cu)(16)AS(2)S(11), lautite CuAsS, and sinnerite Cu(6)AS(4)S(9) are report ed over the temperature range 150-350 degrees C. Lautite and sinnerite are found to take in negligible amounts of Ag (below the detection limit of the microprobe), even at 350 degrees C. Similarly smithite is found to take in < 1 % Cu. Utilizing new Ag-Cu exchange data between proustite and pearceit e and previously published As-Sb exchange data between pyrargyrite-proustit e (Ag,Cu)(3)(Sb,As)S-3 and polybasite-pearceite (Ag,Cu)(16)(Sb,As)(2)S-11, a model is calibrated which reflects both the Ag-Cu and As-Sb mixing proper ties of the pyrargyrite-proustite solid solution over the temperature range 120-400 degrees C. Explicit provision for site distortion in proustite res ulting from the Cu+ for Ag+ substitution (or Ag+ for Cu+ substitution) is m ade by assuming that the standard state properties of the Ag-proustite and Cu-proustite end members are linearly dependent on the Cu/(Cu+Ag) ratio. Cu substitution in pyrargyrite-proustite is both asymmetric and substantially non-ideal (W-CuAg(Pyr-Prs) = 21.0; W-AgCu(Pyr-Prs) = 11 +/- 0.5 kJ/gfw) as opposed to As Sb mixing along the proustite-pyrargyrite join, which has be en found to be both more symmetric and more ideal (W-AsSb(Pyr-Prs) = 6.00 /- 0.60 kJ/gfw). The reciprocal energy, Delta (G) over bar(rec) between pro ustite and pyrargyrite is found to approximate 0. Ag-Cu exchange experiment s between pearceite grains and equimolar powder mixtures of proustite and s innerite are used to define equilibrium compositions for a three-phase regi on involving proustite, pearceite and sinnerite over the temperature range 150-350 degrees C.