The effect of Mg 2+ at constant pH on the phase separation behavior of Mg-ACC nanoparticles is indicated by the dot line arrow in Figure A and C. A significant decrease in phase separation kinetics is observed with increasing Mg 2+ concentration. We predict this change to originate from a shift in the Gibbs free energy curve towards higher concentration (pink arrow Figure A) with increasing Mg 2+ due to lower solubility of MgCO 3 (K S =10 -7.8 ) compared to CaCO 3 (K S =10 -9 ), following the dependency of the free energy of a stable state as a function of solubility product ∆G = -RT lnK S . Consequently, this shift in the free energy landscape of Mg-ACC formation shifts the binodal and spinodal lines to higher concentrations. As a result, at a constant pH, the driving force for phase separation is reduced, resulting in larger particles with a broader size distribution. Additionally, shifting the binodal and spinodal lines to the right with increasing Mg 2+ concentration can allow a higher pH while remaining in a regime where nanoparticles form by nucleation and growth. Whereas we cannot rule out that the formation of Mg-ACC nanoparticles may proceed via a direct nucleation and growth process from solution without prior LLPS, our findings -independent of the formation process-have important implications for particle attachment-based biomineralization processes that require very narrow size distribution. Adjusting the Mg 2+ concentration at a given pH makes it possible to fine-tune the phase separation kinetics for nanoparticle formation in close vicinity to the spinodal line but within the binodal region, where the most monodisperse Mg-ACC nanoparticles are formed under the given conditions.