Uncovering the Role of Bicarbonate in Calcium Carbonate Formation at Near‐Neutral pH

Mechanistic pathways relevant to mineralization are not well‐understood fundamentally, let alone in the context of their biological and geological environments. Through quantitative analysis of ion association at near‐neutral pH, we identify the involvement of HCO3− ions in CaCO3 nucleation. Incorporation of HCO3− ions into the structure of amorphous intermediates is corroborated by solid‐state nuclear magnetic resonance spectroscopy, complemented by quantum mechanical calculations and molecular dynamics simulations. We identify the roles of HCO3− ions as being through (i) competition for ion association during the formation of ion pairs and ion clusters prior to nucleation and (ii) incorporation as a significant structural component of amorphous mineral particles. The roles of HCO3− ions as active soluble species and structural constituents in CaCO3 formation are of fundamental importance and provide a basis for a better understanding of physiological and geological mineralization. An important role of bicarbonate at near‐neutral pH during CaCO3 formation was found. Its structural incorporation in amorphous intermediates, revealed by a combination of potentiometric titrations, NMR, computer simulations, and other techniques, can be quantitatively explained by a model based on the binding of bicarbonate ions to pre‐nucleation clusters. The findings seem to be crucial for a better understanding of, e.g., CaCO3 biomineralization.