The solubility of Ag 2 S in near-neutral aqueous sulfide solutions at 25 to 300°C

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The solubility of Ag 2 S (acanthite/argentite) was measured in vapor-saturated aqueous sulfide solutions at 25-300°C, total sulfide = 0.2-1.4 m , and pH 25° = 5.8-7.3. Under these conditions, Ag 2 S was found to dissolve according to the following reaction: 1/2Ag 2 S(s)+1/2H 2 S(aq)+HS-=Ag(HS) - 2 for which the following log K 's were obtained: -3.82 ± 0.10 (25°C), -3.26 ± 0.10 (100°C), -2.91 ± 0.10 (150°C), -2.55 ± 0.10 (200°C), -2.32 ± 0.10 (250°C), and -2.11 ± 0.10 (300°C). These data are in good agreement with previous work at 20°C ( and , 1966), and 100-180°C ( et al. , 1969), but not with the dinuclear complex stoichiometries recently obtained by et al. (1987). Although (1976) has shown that considerable silver can be dissolved as chloride complexes, our data indicate that Ag(HS) 2 - is the dominant silver species in hydrothermal fluids with near-neutral to alkaline pH, relatively low oxidation state, high total sulfide, and T < 300°C ( e.g. , Broadlands, N.Z.; , 1986). Silver transported as Ag(HS) 2 - is precipitated in response to a decrease in aqueous sulfide concentration, a change in pH away from the p K 1 for H 2 S, or, in the native silver field, a decrease in oxidation state. Cooling is a less important depositional mechanism, particularly for solutions whose pH is buffered by feldspar alteration reactions.

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