An experimental study of the aqueous solubility and speciation of Y(III) fluoride at temperatures up to 250 °C

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 (log Ksp) 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 (log β) 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.