Speciation of Niobium in hydrothermal fluids

Niobium (Nb) is classified as a ‘ critical mineral ’ due to its irreplaceable applications in superalloys and superconductors but limited supply chain. This study employs high-energy resolution fluorescence-detected X-ray absorption spectroscopy (HERFD-XAS) at the Nb K -edge to investigate Nb(V) mobility in fluids containing various ligands, namely fluoride [F - ], chloride [Cl - ], carbonate [CO 3– ], and hydroxide [NaOH], under hydrothermal conditions (25–413 ˚C; 800 bar). Ab initio simulations of X-ray absorption near-edge structure (XANES) spectra were combined with shell-by-shell modelling of extended X-ray absorption fine structure (EXAFS) data to elucidate the nature and geometry of the aqueous Nb(V) complexes. Key findings are that Nb(V) predominantly forms eight-coordinated hydroxyfluoride complexes (e.g., [NbF 6 O 2 ] moiety) in acidic F-rich solutions, whereas high-chloride acidic conditions favour six-coordinated chlorohydroxy complexes (e.g., [NbCl 4 O 2 ] moiety). Competitive ligand interactions between F - and Cl - are modulated by temperature and pH, with higher temperature or low pH promoting Cl-dominated coordination. Carbonate solutions exhibit retrograde Nb solubility and promote lower solubility than acidic F/Cl solutions above 282 ˚C, with spectral features suggesting the presence of [NbO 6 ]-like moieties in polynuclear clusters and/or nanoparticles. Comparative analysis with tantalum (Ta) highlights distinct geochemical behaviours: Ta dissolution depends strictly on F concentration, whereas Nb demonstrates greater coordination flexibility in Cl-bearing solution systems. These findings challenge conventional views that dismiss the role of chloride in Nb transport. Differences in Nb and Ta complexation provide a geochemical discriminator, leading to Nb/Ta ratios that are low near intrusions, increase outwards in intrusion-related hydrothermal systems, and are highest in metamorphic fluids.