We summarize our results in Figure , showing an increase of the kinetic constant k with increasing precipitation pH for all tested Mg ratios (Figure ). This directly translates to a decrease in the size of the early precursors formed (Figure ), a decrease in the final mean size of Mg-ACC nanoparticles (Figure ), and a decrease or narrowing in RSD (Figure ). Moreover, regardless of the synthesis parameter, we observed a clear exponential dependence of the kinetic constant on the Mg-ACC nanoparticle and precursor particle size (Figure ), manifesting in smaller sizes for faster kinetics. Additionally, an exponential decrease in relative size distribution with increasing early kinetics is evident from Figure , showing more monodisperse nanoparticles for faster kinetics. This indicates that the final nanoparticle size, as well as the nanoparticle size distribution, is determined in the early stages of particle formation, where the kinetic constant is determined. We conceptualize these findings following notions of liquidliquid phase separation and classical nucleation theory. Figure A shows the CaCO 3 /H 2 O phase diagram in temperature-concentration coordinates proposed elsewhere, showing a miscibility gap and a lower critical solution temperature. The green line in Figure A represents an isothermal cut through the stability fields of the solution crossing, with increasing concentration, metastable one-phase, and two-phase (binodal) regions, and an unstable two-phase spinodal region. Their boundaries are defined by equality of chemical potentials for the binodal lines and by d 2 G/dc 2 i = 0, the second derivative of the Gibbs free energy with concentration, for the spinodal lines, where c i is the concentration of ions (anions or cations),