Mechanistic insights into the fluorite-fluocerite-bastnasite transformation

Fluocerite is a rare earth element (REE) fluoride mineral with chemical formula (REE)F₃. It is found in nature as an accessory mineral in magmatic-hydrothermal REE ore deposits, including alkaline complexes and carbonatites, where is associated with REE fluorocarbonates (i.e. bastnasite, parisite, synchysite), REE-bearing fluorite, cerianite, monazite and xenotime. Due to its relatively scarcity in these deposits, fluocerite kinetics and mechanisms of crystallisation, its role in REE fractionation and as phase for the evolution of magmatic-hydrothermal REE mineralizing systems have not received a lot of attention in the past years. Recently instead, fluocerite is gaining interest as it was suggested to be a precursor phase of bastnasite. Bastnasite is one of the most important minerals for the extraction of REE, indispensable in modern world because of their wide range of hi-tech industry and in clean energy applications. For this reason, illuminating the mechanisms of fluocerite crystallization and its role in the evolution of REE deposits could significantly enhance our understanding of the genesis of REE fluorocarbonates. 
The present study has two main objectives: 1) To study the kinetics and mechanisms of the fluocerite formation at temperatures ranging from ambient to low hydrothermal (up to 200 °C) 2) To demonstrate in situ that fluocerite reacting in the presence of a CO₃-rich solution can transform into REE fluorocarbonates in hydrothermal conditions. 
For the first purpose, fluocerite was synthesized by reacting pure fluorite (CaF₂) powder with REE-bearing solutions at different temperatures. To achieve the second objective, synthetic fluocerite was reacted with Na₂CO₃ solutions at hydrothermal conditions up to 200 °C. Samples of solids were taken at specific time intervals to follow the ongoing of the crystallisation reactions. The nature of crystallising solids, their quantification and growth morphology were determined with a combination of powder X-ray diffraction (XRD) and scanning electron microscopy with energy dispersive microscopy (SEM-EDS).
Our results showed that the fluocerite crystallisation rate decreases proportionally with the REE atomic number and increases with increasing T. The activation energy of crystallisation is similar irrespective of the REE used in the synthesis (~75 KJ/mol) while the activation energy of nucleation increases with the REE atomic number (80 - 96 KJ/mol). All fluocerites reacted with the CO₃-bearing solutions transformed into bastnasite at all temperature, also forming cerianite in the Ce-bearing experiments.