Reaction Pathways toward the Formation of Bastnäsite: Replacement of Calcite by Rare Earth Carbonates
Adrienn Maria Szucs, Alexandra Stavropoulou, Claire O’Donnell, Seana Davis, Juan Diego Rodriguez‐Blanco
Abstract
The interaction of rare earth bearing (La, Nd, Dy) aqueous solutions with calcite crystals at ambient and hydrothermal conditions (25–220 °C) results in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of calcite by rare earth carbonates. The newly formed rare earth carbonates follow the crystallization sequence lanthanite [REE2(CO3)3·8H2O] → kozoite [orthorhombic REECO3(OH)] → hydroxylbastnäsite [hexagonal REECO3(OH)]. The specific rare earth involved in these processes and the temperature have strong control of the polymorph selection, crystallization pathways, and kinetics of mineral replacement. Kozoite-(La, Nd) grows oriented onto the calcite surface, forming an epitaxy. This phase forms elongated crystals on [100], with the {011} and {01̅1} as major forms. The epitaxial relationship is (104) [010]cc∥(001) [100]koz and is strongly dependent on the ionic radius of the rare earth in the structure of kozoite.