An experimental study of the aqueous solubility and speciation of Y(III) fluoride at temperatures up to 250°C
2013
Loges, Anselm | Migdisov, Artas A. | Wagner, Thomas | Williams-Jones, Anthony E. | Markl, G.
The solubility of Yttrium (III) fluoride as a function of fluoride activity was investigated experimentally at 100, 150, 200 and 250°C and vapor-saturated water pressure. Data obtained from the experiments were used to determine the solubility product of YF₃(s), the fluoride speciation of Y and the stability constants of the corresponding complexes. The dominant Y-fluoride species at the temperatures investigated is the di-fluoride complex (YF₂ ⁺). Consequently, fluoride speciation of Y differs substantially from that of the lanthanides (Ln), which dominantly form mono-fluoride complexes (LnF²⁺) at these temperatures. The logarithm of the solubility product (logKₛₚ) of YF₃(s) is −20.8±0.50, −21.5±0.46, −22.4±0.56 and −24.3±0.12 at 100, 150, 200 and 250°C, respectively. The logarithm of the formation constant (logβ) of YF₂ ⁺ is 8.3±0.77, 10.7±0.43, 12.1±0.31 and 13.3±0.16 at the same temperatures, respectively. Differences in the speciation of Y from that of the lanthanide, holmium (Ho), quantitatively explain the fractionation between these geochemical twin elements, which has been reported for many fluoride-rich hydrothermal systems. Our results emphasize the usefulness of the Y/Ho ratio as a geochemical indicator in hydrothermal systems.
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