Fabrication of aragonite rosette superstructure through the weak interaction between nonionic polymers and Ca2+

Formation and properties of spindle-shaped aragonite mesocrystals from Mg-bearing solutions

The formation of aragonite under ambient conditions is typically linked to Mg-rich aqueous environments. The grains that form in such environments show peculiar properties such as aggregate-like appearance and mesocrystalline character. We tested the effect of dissolved Mg2+ ions on the formation of aragonite mesocrystals by synthesizing aragonite with an automatic titrator at constant pH and at different dissolved Mg : Ca ratios, and by studying the properties of the precipitated material with various scanning transmission electron microscopy (STEM) techniques. At all studied Mg : Ca ratios the firstly condensed carbonate phase was Mg-bearing amorphous calcium carbonate (Mg-ACC) that transformed into aragonite during the synthesis experiments. The aragonite grains had typically aggregate-like appearance and spindle shapes, with the external morphologies of the spindles unaffected by variation in solution chemistry. The alignment of the nanocrystals within the aggregates was crystallographically highly coherent, the [001] directions of nanocrystals showing only a small misorientation with respect to one another; however, both parallel and twin assembly of neighbouring crystals occurred. An increase in the dissolved Mg concentration decreased the crystallographic coherence between the aragonite nanocrystals, suggesting an important role of Mg2+ ions in the assembly of aragonite spindles. Whereas the mesoscale-ordered arrangement of nanocrystals implies a particle-mediated assembly, the observed differences in particle size and composition between the amorphous precursor and the crystalline end-product suggest that the crystallization includes at least partial dissolution and re-precipitation. These findings provide insight into the formation of aragonite and could contribute to the understanding of important aspects of the formation of mesocrystals and hierarchically structured biogenic minerals.

Influence of Inorganic Solution Components on Lithium Carbonate Crystal Growth

Lithium-bearing brines are an increasingly attractive source of Li for extraction. One extraction mechanism is the removal of Li from the fluid phase through the precipitation of zabuyelite (Li₂CO₃). The chemistry of the brine plays an important role in this process because ions in solution can compete for the components of the Li-carbonate phase. Here we explore the effect of different brine components on the precipitation of zabuyelite using experiments and computational simulations. Crystals formed in all solutions showed morphological evidence for potential transformation from a precursor phase. Our study indicates that Ca²⁺ and SO₄ ^2- are incorporated into the precipitated zabuyelite crystals. Sulfate also interacts directly with specific surfaces on the growing crystal and is expected to form ion pairs with Li⁺ in solution. Similarly, Na⁺ appears to form ion pairs in solution with the carbonate ion, slowing nucleation of zabuyelite in the experiments. K⁺ and Cl^- may interact with the growing zabuyelite crystals but do not appear to affect zabuyelite nucleation and growth times. These experiments highlight the importance of understanding the solution chemistry on zabuyelite formation in order to predict the efficiency of extraction processes and the purity of the solids.