Thermochemistry of Ag-Cu exchange equilibria between proustite, sinnerite, and pearceite: Constraints on Ag-Cu and As-Sb mixing in pyrargyrite-proustite
Harlov, Daniel E.
European Journal of Mineralogy Volume 11 Number 4 (1999), p. 709 - 720
published: Jul 16, 1999
manuscript accepted: Feb 12, 1999
manuscript received: Apr 22, 1998
ArtNo. ESP147051104010, Price: 29.00 €
Abstract Experimental Ag-Cu exchange equilibria (evacuated silica tubes; variable mass ratio) involving proustite (Ag,Cu)3AsS3, smithite AgAsS2, pearceite (Ag,Cu)16As2S11, lautite CuAsS, and sinnerite Cu6As4S9 are reported over the temperature range 150-350 °C. Lautite and sinnerite are found to take in negligible amounts of Ag (below the detection limit of the microprobe), even at 350 °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)S3 and polybasite-pearceite (Ag,Cu)16(Sb,As)2S11, 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°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 (WPyr-PrsCuAg = 21.0; WPyr-PrsCuAg = 11 ± 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 (WPyr-PrsCuAg = 6.00 ± 0.60 kJ/gfw). The reciprocal energy, ΔḠrec, 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 °C.