PNNL

Abstract

Metal carbonates are ubiquitous in the near-surface mineral record, are a major product of biomineralizing organisms, and serve as an important target for strategies aimed at capturing and storing anthropogenic CO2 emissions. However, pathways of carbonate mineralization typically diverge from classical predictions due to the involvement of disordered precursors, such as the dense liquid phase (DLP), which have been implicated in biological production of CaCO3. Yet little is known about DLP formation or solidification processes, DLP composition, or the role of biomolecules in DLP formation and transformation. Using in situ methods, we show that a viscous bicarbonate-rich DLP forms by liquid-liquid phase separation (LLPS), is comprised of 1Ca2+:2HCO3-:7±2H2O, and transforms into hollow hydrated amorphous CaCO3 (ACC) particles accompanied by the release of CO2 and H2O. Acidic proteins and polymers extend DLP lifetimes but otherwise leave the pathway and chemistry unchanged. Molecular simulations suggest that DLP forms via direct condensation of solvated Ca2+•(HCO3-)2 complexes that react due to proximity effects in the confinement of DLP droplets. Our findings provide new insight into LLPS-mediated CaCO3 nucleation from both chemical and physical perspectives, advancing the ability to direct carbonate mineralization and elucidating an often-proposed pathway by which Nature orchestrates the complex process of biomineralization.