As---Sb energetics in argentian sulfosalts

Details As---Sb energetics in argentian sulfosalts As---Sb energetics in argentian sulfosalts

Sep 1995

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995gecoa..59.3573g&link_type=abstract

Geochimica et Cosmochimica Acta, vol. 59, Issue 17, pp.3573-3579

Physics

1

Scientific paper

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 { Mi, Mi, Smt; Ag(Sb, As)S 2 } (350-400°C; evacuated silica tubes) define standard state Gibbs energies of theAs--Sb exchange reactions {Ag 16 As 2 S 11 +Ag 3 SbS 3 =Ag 16 Sb 2 S 11 + Ag 3 AsS 3 , ; Ag 3 AsS 3 , + AgSbS 2 = Ag 3 SbS 3 + AgASS 2 , 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 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 -miargyrite and smithite structures at 350°C determine the relative stabilities of these structures in the As and Sb endmembers to be: ( ) -0.63 kJ/gfw; ( ) 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 AgSbS 2 --AgAsS 2 subsystem. The salient features of this diagram are (1) eutectic behaviour ( T 396°C, X As 0.50), (2) modest increase in the temperature of the miargyrite transition with As substitution ( 380°C in Sb-subsystem; 386.6°C at X As Mi 0.36), (3) a 42.5°C depression of the trechmannite-smithite transition with preferential incorporation of Sb in smithite { smithite ( X As 0.62) -miargyrite ( X As 0.34) + trechmannite (X As 1.00) at 277.5°C}, and (4) widening of the miargyrite-trechmannite gap at lower temperatures. The latter feature is consistent with the inference that the most As-enriched epithermal miargyrites crystallized at temperatures above 195°C.

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