Experimental determination of the stabilities of Au 2S (cr) at 25 °C and [formula omitted] at 25–250 °C

Abstract

The solubility of (i) Au (cr), (ii) mixtures of crystalline Ag 2S–Ag 3AuS 2, and (iii) Ag 3AuS 2 (uytenbogaardtite)–AgAuS (petrovskaite) was measured at 25 °C, 1 bar, whereas the solubility of AgAuS (cr) was measured at 91–250 °C and P sat. These measurements were performed in aqueous sulphide solutions containing ∼0.012–0.12 m of total reduced sulfur. At 25 °C sealed glass ampoules were employed, and Ti autoclaves allowing sampling of experimental solutions were used at higher temperatures. It was found that under investigated experimental conditions AgAuS dissolves incongruently with the formation of Ag 3AuS 2. The values of the following equilibrium reaction constants were determined: Au (cr) + H 2 S 0 (aq) + HS - = Au ( HS ) 2 - + 0.5 H 2 (g) ( i ) K ( i ) = K ( Au ( HS ) 2 - ) Ag 3 AuS 2 ( cr ) + HS - + 0.5 H 2 S 0 ( aq ) = 1.5 Ag 2 S ( cr ) + Au ( HS ) 2 - ( ii ) K ( ii ) 1.5 AgAuS ( cr ) + HS - + 0.5 H 2 S 0 ( aq ) = 0.5 Ag 3 AuS 2 ( cr ) + Au ( HS ) 2 - ( iii ) K ( iii ) It was found that at 25 °C, 1 bar log K (i) = −5.62 ± 0.14, log K (ii) = −4.49 ± 0.26 and log K (iii) = −4.03 ± 0.15. Solubility data collected at higher temperatures resulted in the following values for log K (iii): −2.12 ± 0.25, −1.32 ± 0.12, and −1.04 ± 0.08 at 91, 150, and 250 °C, respectively (the latter value corresponds to the reaction 1.97 AgAuS (cr) + HS - + 0.5 H 2 S 0 (aq) = 0.735 Ag 2.68 Au 1.32 S 2 (cr) + Au (HS) 2 - because Ag 3AuS 2(cr) undergoes a phase transition at 183 °C). These values are in good agreement with log K (iii) = −2.81 ± 0.10 obtained at 80 °C by Zotov et al. (Zotov, A.V., Baranova, N.N., Bannykh, L.N., 1996. Solubility of the gold sulfides Au 2S and AgAuS in solutions containing hydrogen sulfide at 25–80 °C and pressures of 1 and 500 bar. Geochem. Int. 34, 216–221). The values of K (ii) and K (iii) determined in the present study were combined with the thermodynamic properties of Ag–Au sulphides reported by Osadchii and Rappo (Osadchii, E.G., Rappo, O.A., 2004. Determination of standard thermodynamic properties of sulfides in the Ag–Au–S system by means of a solid-state galvanic cell. Am. Miner. 89, 1405–1410), and thermodynamic properties for aqueous species and hydrogen from SUPCRT92 database to generate the values of K ( Au (HS) 2 - ) . The results of the three experimental series performed at 25 °C, P sat (involving metallic gold (i) and Ag–Au sulphides (ii) and (iii)) yield the average value of log K ( Au (HS) 2 - ) = - 5.38 ± 0.25 . Log K ( Au (HS) 2 - ) values derived at higher temperatures are −3.85 ± 0.20, −3.13 ± 0.29, −2.22 ± 0.20, and −1.96 ± 0.25 at 80, 91, 150 and 250 °C, respectively.At 150 and 250 °C these log K ( Au (HS) 2 - ) values are in close agreement with the data of Shenberger and Barnes (Shenberger, D.M., Barnes, H.L., 1989.Solubility of gold in aqueous solutions from 150 to 350 °C. Geochim.Cosmochim.Acta 53, 269–278).The log K ( Au (HS) 2 - ) value obtained at 25 °C, 1 bar combined with Au 2S (cr) solubility products ( Au 2 S (cr) + 3 HS - + H + = 2 Au (HS) 2 - and Au 2 S (cr) + HS - + H + = 2 AuH S 0 (aq) ) measured at 25 °C by Renders and Seward (Renders, P.J., Seward, T.M., 1989.The stability of hydrosulphido- and sulphido-complexes of Au(I) and Ag(I) at 25 °C. Geochim.Cosmochim.Acta 53, 245–253) yields Δ f G ( 298.15 K ) o ( Au 2 S (cr) ) = 19.41 ± 1.43 kJ / mol and log K (AuHS 0) = −10.98 ± 0.30 for the reaction Au (cr) + H 2 S 0 (aq) = AuH S 0 (aq) + 0.5 H 2 (g) K ( AuHS 0 ) .