Targeted Crystallization of Rare Earth Carbonate Polymorphs at Hydrothermal Conditions via Mineral Replacement Reactions.

The interaction between rare earth element (REE)-rich (La, Pr, Nd, Dy) aqueous solutions, dolomite (CaMg(CO 3 ) 2 ), and aragonite (CaCO 3 ) at low temperature hydrothermal conditions (25-220 °C) is studied. The experiments result in the solvent-mediated surface precipitation and subsequent pseudomorphic mineral replacement of the dolomite and aragonite seeds by newly formed REE-carbonates. The host grains are replaced from periphery inward. The newly formed REE-bearing carbonates in La-, Pr-, and Nd-doped systems follow the crystallization sequence: lanthanite [REE 2 (CO 3 ) 3 ·8H 2 O] → kozoite [orthorhombic REECO 3 (OH)] → hydroxylbastnasite [hexagonal REECO 3 (OH)]. The interaction of Dy-bearing solutions with dolomite results only in the crystallization of kozoite [orthorhombic DyCO 3 (OH)]. However, experiments with aragonite reveal a two-step crystallization pathway: tengerite [Dy 2 (CO 3 ) 3 ·2-3(H 2 O)] → kozoite [orthorhombic DyCO 3 (OH)]. The temperature, the dissolution rate of the host mineral, and the ionic radii of the REE 3+ in question are found to control the kinetics of the replacement reaction, the polymorph selection, and the crystallization pathways toward bastnasite. The findings allow to gain a more in-depth understanding of the formation REE-bearing carbonates, particularly the mineral bastnasite, which is the main source of REEs for industry. This knowledge can be used to improve REE separation, exploration, exploitation methods, as well to produce carbonate minerals with tailored structures.