posted on 2021-06-29, 23:03authored byMichael
L. Whittaker, Wenhao Sun, Danielle O. Duggins, Gerbrand Ceder, Derk Joester
Metastable
carbonates play important roles in geochemistry, biomineralization
and serve as model systems for nonclassical theories of nucleation
and growth. Balcite (Ca0.5Ba0.5CO3) is a remarkable carbonate phase that is isostructural with a high-temperature
modification of calcite (CaCO3), yet can be synthesized
at ambient conditions. Here, we investigate crystallization pathways
in the Ba–Ca–CO3–H2O 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 (BaCO3) phases.
Density functional theory calculations show that balcite is an unstable
solid solution (Ca1–xBaxCO3, R3̅m) 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.