where the CO 2 - 3 concentration is too low, and the solutions remain as one metastable liquid phase even after mixing. In the case of Mg-ACC formation at a ratio of 50:50, a 0.1 pH increase to 8.4 moves the system from a one-liquid phase into the two-phase region of liquid-liquid miscibility gap, where thermodynamic equilibrium is then reached by the phase separation into a calcium-rich and calcium-poor phase of equal chemical potentials. Thus, based on the thermodynamic notions introduced above, going deeper into the binodal region reduces the critical energy barrier for nucleation, resulting in faster phase separation kinetics and the formation of smaller, more numerous nanoparticle seeds. As Mg-ACC nanoparticles grow by the uptake of smaller molecular clusters, a larger number of smaller initial particles will lead to a larger number of smaller final particles. This is evident from the reduction in particle size with increasing pH and the increase in nanoparticle number density (Figure SI 5-7 F). Additionally, we find a decrease in RSD from 20 % to 5 % with a 0.1 pH increment between pH 8.4 and 8.5 in agreement with the above-introduced consideration, proposing a more narrow nice distribution for faster reaction kinetics under more alkaline conditions.