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

Abstract 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 HCO 3 − ions in CaCO 3 nucleation. Incorporation of HCO 3 − 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 HCO 3 − 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 HCO 3 − ions as active soluble species and structural constituents in CaCO 3 formation are of fundamental importance and provide a basis for a better understanding of physiological and geological mineralization.