The solubility and complexation of Niobium in hydrothermal fluids

Metal dissolution, complexation, and speciation are the key processes that facilitate metal mobilization and transport in fluids. Niobium (Nb), a kind of critical metal, has traditionally been regarded as a fluid-immobile element; however, it sometimes shows apparent hydrothermal mobility and even mineralization. Studying the solubility and complexation of Nb in fluids is thus crucial for understanding its dissolution, transport, enrichment, and mineralization. In this paper, we reviewed the geological observations on Nb mobility related to magmatic-hydrothermal and metamorphic fluid activities, especially compiled and reprocessed the published data on Nb solubility and related thermodynamic calculation to discuss the complexation and speciation of Nb in fluids. Previous solubility experiments demonstrate that Nb has much higher solubility in F-bearing solutions than in other solutions (Cl⁻, ClO₄⁻, CO₃²⁻, HCO₃⁻, OH⁻, SO₄²⁻, etc.), the maximum of which is up to ∼3 wt% in a 2 mol/kg HF solution. It is revealed that Nb solubility is related to the solution's composition, pH, ionic strength, oxygen fugacity, temperature, and pressure. High solubility could be found in neutral and weakly-basic solutions at near ambient temperature and pressure or in F-bearing fluids at high-temperature and high-pressure conditions. Modeling calculations show that Nb could be soluble and stable in fluids as the mononuclear or polynuclear complexes, such as fluoride complexes, hydroxide complexes, chloride complexes, and hexametalate ions, etc. Thereinto, Nb–OH–F complexes should play a dominant role in Nb hydrothermal mobility and be enriched in medium–high temperature, acidic, and F-bearing fluids. Experiments and modeling calculation have also inferred the existence of the species (e.g., $\text{Nb}{\left(\text{OH}\right)}_{4+y}{F}^{y-}$, $\text{Nb}{\left(\text{OH}\right)}_{3+y}{F}_2^{y-}$, and $\text{Nb}{\left(\text{OH}\right)}_{2+y}{F}_3^{y-}$), in which the F coordination number is no more than 3. Considering that it could have higher F contents than the experimental solutions (F < 4 wt%), we believe that natural mineralized F-rich fluids would facilitate the formation of the species with higher F coordination numbers due to the positive relationship between the F coordination number in the species and F content in fluids. Compared to the Nb–OH–F complexes, Nb–OH complexes are less stable and soluble but predominant in dilute solutions containing low concentrations of ligand anions or in alkaline solutions. In addition, further experiments need to be replenished and the problems need to be addressed are also discussed, which will enhance our understanding of how the Nb cycle and mineralization related to hydrothermal activities happen.