CaREE-fluorcarbonates: A variety of morphologies, compositions and nanostructures with insights into REE partitioning and mobility

A variety of CaREE-fluorcarbonates with differing morphologies and compositions have been studied: i) synchysite-(Ce) forming hexagonal prisms and rosette-like aggregates; ii) bastnäsite-(Ce) forming blocky aggregates; iii) bastnäsite-(Nd) forming desert rose-like intergrowths and iv) combinations of synchysite-(Ce) and bastnäsite-(Nd) forming more complex micro and nanostructures. These minerals are retained to form from a hydrothermal fluid in a temperature range between 350 and 100 °C. After an investigation spanning from the microscale to the atomic scale, which employed optical stereomicroscopy, scanning and transmission electron microscopy, microanalysis and Raman spectroscopy, a possible scenario for their formation has been depicted. It is assumed that the different CaREE-fluorcarbonates crystallized sequentially under decreasing temperature and fluid fractionation, with synchysite-(Ce) forming first, followed by bastnäsite-(Ce) and finally bastnäsite-(Nd). The different morphologies, in this case, besides being influenced by the dominant REE in the fluid, were also due to varying temperatures and cooling rates. Our study highlights that nature may efficiently fractionate REE from one another, like Y from La, Ce and Nd; Ce from Nd and La. Additionally, Th, a radioactive and toxic element, seems to enter the structure of CaREE-fluorcarbonates during the final stages of crystallization. The understanding of these natural processes may suggest routes for improving current metallurgical separation processes.