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

GeoForschungsZentrum Potsdam, Potsdam, Federal Republic of Germany

Experimental Ag-Cu exchange equilibria (evacuated silica tubes; variable mass ratio) involving proustite (Ag,Cu) ₃ AsS ₃ , smithite AgAsS ₂ , pearceite (Ag,Cu) ₁₆ As ₂ S ₁₁ , lautite CuAsS, and sinnerite Cu ₆ As ₄ S ₉ are reported 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 pearceite and previously published As-Sb exchange data between pyrargyrite-proustite (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 properties of the pyrargyrite-proustite solid solution over the temperature range 120-400 degrees C. Explicit provision for site distortion in proustite resulting from the Cu+ for Ag+ substitution (or Ag+ for Cu+ substitution) is made 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 (super Pyr-Prs) _CuAg = 21.0; W (super Pyr-Prs) _AgCu = 11+ or -0.5 kJ/gfw) as opposed to As-Sb mixing along the proustite-pyrargyrite join, which has been found to be both more symmetric and more ideal (W (super Pyr-Prs) _AsSb = 6.00+ or -0.60 kJ/gfw). The reciprocal energy, Delta G _rec , between proustite and pyrargyrite is found to approximate 0. Ag-Cu exchange experiments between pearceite grains and equimolar powder mixtures of proustite and sinnerite are used to define equilibrium compositions for a three-phase region involving proustite, pearceite and sinnerite over the temperature range 150-350 degrees C.

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