THE EFFECTS OF TEMPERATURE AND PRESSURE ON GOLD-CHLORIDE SPECIATION IN HYDROTHERMAL FLUIDS: A RAMAN SPECTROSCOPIC STUDY

Abstract

Raman spectra have been obtained for gold-chloride solutions at elevated temperature and pressure, with gold concentrations of 0.005 to 0.04 M and varying pH. A hydrothermal cell has been developed that allows Raman analyses up to 300°C and 2 kbar. Using simple Au (III) chloride solutions at low pH and up to 300°C, the expected transformation to Au (I) with increasing temperature was not found, even in solutions prepared under reducing conditions. The Au (III) complex [AuCl4]- breaks down and precipitates gold at temperatures above 250°C, but the exact temperature appears to be related to oxygen fugacity. The Raman spectra showed consistent trends in band parameters with increasing temperature, indicating minor changes in bond lengths but no change in speciation. In the range investigated, pressure had a minimal effect on both speciation and band frequencies. However, on heating the Au (III) complex [AuCl4]- in the presence of metallic gold, transformation to the Au (I) form [AuCl2]- does occur. The single Raman band for the complex [AuCl2]- was recorded at ~326/cm at 250°C. A previously described band for this species was probably an artifact of the peak fitting process. The importance of comparing trends in band parameters to determine optimal peak fitting is highlighted. Solutions at pH values of 5.75 and 6.5, which had different mixed chloro-hydroxy complexes at ambient temperature, showed changes in speciation with increasing temperature. In the lower pH solution (OH)- groups were replaced by Cl- ligands on heating, resulting in a transformation from [AuCl3(OH)]- to [AuCl4]-. In the higher pH solution, there was initially an increase in the number of (OH)- ligands, from [AuCl2(OH)2]- to a mixture with [AuCl(OH)3]- at 75 to 100°C, but with increasing temperature, this trend was reversed and [AuCl4]- became dominant. No effect of pressure on the chloro-hydroxy speciation was observed.