Dynamic Barriers to Crystallization of Calcium Barium Carbonates

Metastable carbonates play important roles in geochemistry, biomineralization and serve as model systems for nonclassical theories of nucleation and growth. Balcite (Ca<sub>0.5</sub>Ba<sub>0.5</sub>CO<sub>3</sub>) is a remarkable carbonate phase that is isostructural with a high-temperature modification of calcite (CaCO<sub>3</sub>), yet can be synthesized at ambient conditions. Here, we investigate crystallization pathways in the Ba-Ca-CO<sub>3</sub>-H<sub>2</sub>O system, with a focus on the transformation of amorphous calcium barium carbonate (ACBC) to balcite and subsequent decomposition into the equilibrium calcite (CaCO3) and witherite (BaCO<sub>3</sub>) phases. Density functional theory calculations show that balcite is an unstable solid solution (Ca<sub>1-x</sub>BaxCO<sub>3</sub>, R<sub>3¯</sub> m) in the range 0.17 < x < 0.5, but is accessible through the amorphous ACBC precursor for x ≲ 0.5, and predict its decomposition into calcite and witherite. We confirm this pathway experimentally but found demixing to proceed slowly and remain incomplete even after 9 months. Nucleation kinetics of balcite from ACBC was assessed using a microfluidic assay, where increasing barium content led to a surprising increase in the balcite nucleation rate, despite decreasing thermodynamic driving force. We attribute crystallization rates that dramatically accelerate with time to changes in interfacial structure and composition during coarsening of the amorphous precipitate. By carefully quantifying the thermodynamic and kinetic contributions in the multistep crystallization of a metastable carbonate, we produce insights that allow us to better interpret the formation and persistence of metastable minerals in natural and synthetic environments.