The role of scandium chloride and hydroxide complexes in the formation of scandium deposits: insights from experiments and modeling

Although scandium is commonly concentrated to economic levels by magmatic processes, hydrothermal fluids also play an important role in its concentration. Indeed, the most important source of scandium is currently the Bayan Obo deposit in China, where scandium is extracted from hydrothermally produced aegirine. To know how and why scandium is concentrated by hydrothermal fluids, it is necessary to understand the speciation of scandium in hydrothermal fluids. In a recently published study, we showed that scandium forms stable species with fluoride ions and proposed that such species may have been responsible for the hydrothermal transport of scandium in deposits like Bayan Obo. Chloride ions, which have been shown to form stable complexes with the other rare earth elements (REEs), however, are much more abundant in most hydrothermal fluids than fluoride ions, as are hydroxide ions, particularly at high pH. We, therefore, conducted solubility experiments designed to investigate the stability of scandium chloride and hydroxide complexes in hydrothermal fluids. The experiments investigating the role of chloride species considered the solubility of Sc2O3( s) in the H2O-NaCl-HCl system at 100 degrees, 150 degrees, 300 degrees, and 350 degrees C and saturated water vapor pressure. These experiments showed that scandium concentration is independent of chloride concentration over the range of chlorinity investigated, i.e., up to 3.6 mol Cl- and that scandium, therefore, does not form stable complexes with chloride ions. To evaluate the role of hydroxide species in scandium transport and avoid the effect of chloride ions in the complexation, a second set of experiments was conducted that determined the solubility of Sc2O3( s) in H2O-NaClO4-HClO4 solutions at 100 degrees, 150 degrees, 200 degrees, and 250 degrees C, and saturated water vapor pressure. The results of these experiments showed that the solubility of Sc2O3( s) depends on pH and temperature. Based on the former dependence, two scandium hydroxide complexes, Sc(OH)(2+) and Sc(OH)(3)degrees, were identified at low and higher pH, respectively. The formation constant (log ss) determined for Sc(OH)(2+) is 10.29 +/- 0.07, 10.32 +/- 0.07, 10.35 +/- 0.19, and 10.91 +/- 0.20 at 100 degrees, 150 degrees, 200 degrees, and 250 degrees C, respectively. That of Sc(OH)(3)degrees is 27.22 +/- 0.68, 26.66 +/- 1.35, 27.04 +/- 0.13, and 28.02 +/- 0.14 at the same temperatures, respectively. These results demonstrate that, unlike the case for the other rare earth elements, chloride plays a negligible role in transporting scandium in hydrothermal fluids. Instead, they show that scandium hydroxide complexes can be important in the transport of scandium and could have contributed significantly to the formation of deposits like those at Bayan Obo.