AS-SB ENERGETICS IN ARGENTIAN SULFOSALTS

Experimental brackets on As-Sb partitioning between polybasite-pearceite {Pbp; (Cu, Ag)(16)(Sb, As)(2)S-11} and pyrargyrite-proustite {Ppr; (Cu, Ag)(3)(Sb, As)S-3}, and between pyrargyrite-proustite, and miargyrite and smithite {alpha Mi, beta Mi, Smt; Ag(Sb, As)S-2} (350-400 degrees C; evacuated silica tubes) define standard state Gibbs energies of the As-Sb exchange reactions {Ag16As2S11 + Ag3SbS3 = Ag16Sb2S(11) + Ag3AsS3, Delta ($) over bar G(r)(o) (Pbp-Ppr) = 0.65 +/- 0.60 kJ/gfw; Ag3AsS3 + AgSbS2 = Ag3AsS2, Delta ($) over bar G(r)(o) (Ppr-alpha Mi) = 3.10 +/- 0.50 kJ/gfw), Delta G ($) over bar(r)(o) (Ppr-Smt) = 1.70 +/- 0.50 kJ/gfw} and the nonidealities associated with the As-Sb substitutions in these minerals (measured by symmetric regular-solution parameters for formula units on a one As-Sb site basis; W-AsSb(Pbp) = 4.00 +/- 0.25 kJ/gfw; W-AsSb(Ppr) = 6.00 +/- 0.60 Kj/gfw; W-AsSb(alpha Mi) = W-AsSb(Smt) = 7.00 +/- 0.50 kJ/gfw). The above constraints applied to the miscibility gap between Ag(Sb, As)S-2 solutions with alpha-miargyrite and smithite structures at 350 degrees C determine the relative stabilities of these structures in the As and Sb endmembers to be: (($) over bar G(Sb)(o) (alpha Mi) - ($) over bar G(Sb)(o) (Smt)) similar to -0.63 kJ/gfw, (($) over bar G(As)(o) (alpha Mi) - ($) over bar G(As)(o) (Smt)) similar to 0.77 kJ/gfw. Combining these constraints with the calorimetric data of Bryndzia and Kleppa (1988, 1989) and our melting point determinations we have constructed a phase diagram for the AgSb2 -AgAsS2 subsystem. The salient features of this diagram are (1) entectic behaviour (T similar to 396 degrees C, X(As) similar to 0.50), *2) modest increase in the temperature of the alpha --> beta miargyrite transition with As substitution (similar to 380 degrees C in Sb-subsystem; similar to 386.6 degrees C at X(As)(Mi) similar to 0.36), (3) a 42.5 degrees C depression of the trechmannite-smithite transition with preferential incorporation of Sb in smithite {smithihite (X(As) similar to 1.00) at similar to 277.5 degrees C}, and (4) widening of the miargyrite-trechmannite gap at lower temperatures. The latter feature is consistent withthe inference tha the most As-enriched epithermal miargyrites crystallized at temperatures above 195 degrees C.