Impact of Mg ²⁺ and pH on amorphous calcium carbonate nanoparticle formation: Implications for biomineralization and ocean acidification

Crystallization by amorphous calcium carbonate (ACC) particle attachment (CPA) is a prevalent biomineralization mechanism among calcifying organisms. A narrow, controlled size distribution of ACC nanoparticles is essential for macroscopic crystal formation via CPA. Using in situ synchrotron small-angle X-ray scattering, we demonstrate that synthetic magnesium-stabilized ACC (Mg-ACC) nanoparticles form with an exceptionally narrow size distribution near the spinodal line during liquid-liquid phase separation. We monitored ACC formation kinetics at pH 8.4 to 8.9 and Mg[Formula: see text] contents of 50 to 80%, observing a 2-order magnitude rise in nucleation kinetics for a 0.1 pH increase and a 6-order magnitude rise for a 10% Mg[Formula: see text] decrease. Within the binodal region, faster nucleation kinetics result in more monodisperse particles, narrowing the particle size distribution by factors of 2 for a pH increase of merely 0.1 and by a factor of 3 for a 10% Mg[Formula: see text] decrease. While the influence of Mg[Formula: see text] on calcite biomineralization is well studied, its effect on Mg-ACC formation and particle size distribution-an essential parameter in CPA-based biomineralization pathways-remained unexplored. These findings highlight the delicate interplay of pH and Mg[Formula: see text] in controlling the kinetics and thermodynamics of Mg-ACC formation, significantly impacting particle size distribution.