EXPERIMENTAL INVESTIGATION OF THE HYDROTHERMAL GEOCHEMISTRY OF PLATINUM AND PALLADIUM .3. THE SOLUBILITY OF AG-PD ALLOY PLUS AGCL IN NACL HCL SOLUTIONS AT 300-DEGREES-C/
Ch. Gammons et al., EXPERIMENTAL INVESTIGATION OF THE HYDROTHERMAL GEOCHEMISTRY OF PLATINUM AND PALLADIUM .3. THE SOLUBILITY OF AG-PD ALLOY PLUS AGCL IN NACL HCL SOLUTIONS AT 300-DEGREES-C/, Geochimica et cosmochimica acta, 57(11), 1993, pp. 2469-2479
The solubility of the assemblage AgCl(s) + Ag-Pd alloy was measured in
NaCl/HCl solutions at 300-degrees-C using the silica tube method. Oxi
dation state was controlled by the solid phases, according to the reac
tion Ag(alloy) + 1/4O2(g) + Cl- + H+ = AgCl(s) + 1/2H2O. (A1) The SIGM
ACl concentrations of the solutions ranged from 0.2 to 4.0 m, with cal
culated pH300-degrees-C = 0.47 to 1.67. The surfaces of the alloys aft
er the experiments were corroded and showed a substantial depletion in
Ag relative to the initial compositions (either Ag.5Pd.5 or Ag.7Pd.3)
. SIGMAAg solubilities ranged from 0.026 to 0.39 m, whereas SIGMAPd so
lubilities were 3 to 4 orders of magnitude lower (max. of 10(-3.41) m)
. The results are in excellent agreement with previous studies of the
solubility of AgCl (SEWARD, 1976; ZOTOV et al., 1986) and Pd-sulfide p
hases (GAMMONS et al., 1992) in hydrothermal brines. For the following
reaction: Pd(alloy) + 2H+ + 4Cl- + 1/2O2(g) PdCl42- + H2O, (A2) a val
ue of log K300-degrees-c = 12.81 +/- .18 was obtained, which compares
with a value of 12.65 +/- 0.5 obtained by GAMMONS et al. (1992). The a
greement is strong evidence in favor of the validity of both experimen
tal investigations. The results of the present study indicate that Ag
will be far more soluble than Pd over most of the range of pH, f(O2),
and SIGMACl concentration found in nature. Nonetheless, Pd-rich alloys
may form, owing to the strong thermodynamic partitioning of Pd into t
he solid phase. Scavenging of dissolved palladium by pre-existing Au-A
g alloy is a viable precipitation mechanism and may limit PGE mobility
to extremely low levels in many natural hydrothermal systems. Because
this is the first detailed study of its type, the methods and results
presented in this study have important applications to future experim
ental work in the hydrothermal geochemistry of precious metal alloys.