The role of fluocerite in the genesis of bastnäsite: mechanistic insights and transformation pathways.

Fluocerite is a rare earth element (REE) fluoride found as an accessory mineral in magmatic-hydrothermal REE ore deposits, including alkaline complexes and carbonatites, where it is often associated with REE-fluorocarbonates. This study investigates the crystallisation kinetics, mechanisms and energetics of fluocerite (REEF 3 ) and its role as a precursor phase of bastnäsite, one of the key minerals used for the extraction of REE. Fluocerite was synthesized by reacting pure fluorite (CaF 2 ) with La-, Ce- and Nd-bearing solutions at temperatures ranging from ambient to low hydrothermal (30-90 °C). The synthetic fluocerites were then placed in contact with Na 2 CO 3 solutions at temperatures up to 200 °C. A combined approach using powder X-ray diffraction and scanning electron microscopy with energy dispersive spectroscopy was used to determine the nature and quantify the crystallising solids. Our findings reveal a temperature-dependent fluorite-fluocerite transformation, with completion observed within 1 hour at 90 °C and extending to around 30 days at 30 °C. The rate of crystallisation decreases proportionally with the atomic number of the rare earth elements. On the other hand, the carbonation reaction of fluocerite exhibits a significantly slower rate, by ∼3 orders of magnitude, in comparison to the fluorite-fluocerite transformation, regardless of temperature conditions. The synthetic La, Ce and Nd fluocerites transformed into bastnäsite at all temperatures, also forming cerianite (CeO 2 ) in the Ce-bearing experiments and metastable kozoite, NdCO 3 OH (orth) , in the Nd-bearing experiments. Activation energies of fluocerite nucleation increase proportionally with the ionic radii of the REE (81 ± 6 (La); 84 ± 5 (Ce), 96 ± 10 (Nd) kJ mol -1 ), while the activation energies associated with fluocerite crystallisation are slightly higher for La (90 ± 12 kJ mol -1 ) and similar for Ce and Nd (76 ± 12 and 72 ± 8 kJ mol -1 , respectively).