Other
Scientific paper
May 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997gecoa..61.1971g&link_type=abstract
Geochimica et Cosmochimica Acta, Volume 61, Issue 10, p. 1971-1983.
Other
5
Scientific paper
The disproportionation of aqueous Au(I) chloride complexes at elevated temperature has been investigated experimentally using the solubility method. At 300°C, the dominant gold species in aqueous HCl solutions is AuCl2-. Upon cooling, this aurous complex partially decomposes according to the following reaction: 3AuC12-=2Au(s)+AuC14-+2C1 (A1) Log KA1 values were obtained at 100°C (4.42 ± 0.22), 150°C (2.86 ± 0.12), and 200°C (1.45 ± 0.19). The results are in excellent agreement with the earlier potentiometric study of Nikolaeva et al. (1972) at 25 80°C. The combined data were used to obtain the following polynomial: log KA1 = -13.55 + 8593/T-700610/T2 (T =Kelvin, valid from 25 to 200°C). The rate of reaction (A1) at 25°C was investigated by monitoring the production of AuCl4- after quench using a UV spectrophotometer. The rates were very slow for the first 5 10 min, but then rapidly increased to values that remained approximately constant with further reaction progress. The measured reaction rates fell in the range 2.1 · 10-8 to 3.7 · 10-6 moles AuCl4- · kg H2O-1 · minute-1. In general, faster rates were obtained for samples with high initial AuCl2- concentrations. Addition of gold foils caused an abrupt increase in rate, indicating that the reaction is catalyzed by the native metal. Gold crystals formed during the disproportionation reaction at 25°C show a variety of morphologies, including examples with anomalous fivefold symmetry. Our results indicate that the stability of AuCl2- relative to AuCl4- increases quickly with temperature. At 25°C, AuCl4- is unlikely to be of geochemical importance, with the possible exception of oxidized, acidic solutions that are also rich in chloride ion. In contrast, AuCl2- may be the dominant form of dissolved gold in brines with near-neutral pH (e.g., seawater), as well as hydrothermal fluids that are both saline and oxidized. Cooling or dilution of solutions saturated with AuCl2- could result in deposition of Au via a disproportionation reaction, as in our experiments.
Gammons Christopher H.
Williams-Jones Anthony E.
Yu Yunmei
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