Crystallisation of REE carbonates from aqueous solutions

Low temperature aqueous synthesis of Rare Earth Element (REE) carbonates show extensive variability in the resulting minerals. Precipitated mineral phases and crystallisation rates vary depending, in part, on the REE used. Indeed, much of the work to date on REE aqueous geochemistry focuses on the individual behaviour of discrete REEs. 

We present a low temperature aqueous geochemical investigation of REE carbonate crystallisation pathways, which takes into consideration the influence of multiple REEs in solution. This serves to mimic more realistic conditions that are found in natural geological settings propitious to REE mineralisation. Our experiments focus on the behaviour of La, Ce, Nd, Dy carbonates at 30^oC.

Concordant with previous studies, our results suggest that the crystallisation process of REE carbonates begins with the formation of an amorphous phase that transitions into a crystalline phase after a lag time that depends on the element and the proportions in the mixture. 

This lag time is REE specific and is shorter for lighter REE compared to their heavier counterparts. In particular, the presence of another REE in the system affects the crystallisation timings and the morphology of the resulting crystals. For example, samples of mixed La/Nd carbonates begin their phase transition at lag times in between that of the two end-members (i.e. La and Nd) carbonate compositions. Furthermore, we find that the resulting growth rates and crystal habits are unique to the ratio of the REE mixture, with the underlying ionic potential of the mixture linked to the growth rates. In addition, observations throughout the crystallisation process also show that growth begins with flocculation of nanoparticles followed by crystal growth via Ostwald ripening.

REEs are sought after due to their unique properties and are integral to modern technologies such as lasers, catalytic converters, batteries, electro-magnets and wind turbines. Considering how the crystallisation behaviour with REE mixtures differs from that of discrete REE in solution, this work gives insights into the fundamental chemistry of REEs in aqueous solutions - relevant for studies of REE mineralisation and materials processing.