While the role of Mg 2+ in biomineralization has been extensively discussed in numerous publications over the past decades, this study presents for the first time the influence of Mg 2+ ions on the nucleation and growth of Mg-ACC nanoparticles with exceptionally low size distributions below 5 % under mildly alkaline conditions. We utilized synchrotron-based small-angle X-ray scattering (SAXS, ID02, ESRF) coupled with a stopped-flow device to observe the in situ precipitation of CaCl 2 and Na 2 CO 3 in the presence of MgCl 2 . This approach allows us to gather information about changes in electron density relative to water (the solvent), enabling the determination of particle shape, size, size distribution, and concentration with an unprecedented sub-10 ms time resolution. In situ SAXS has proven to be exceptionally valuable for monitoring nucleation processes, as it is one of the few real bulk, in situ structural characterization methods with inherent statistical significance. This technique has resulted in numerous publications on ACC formation and bioinspired nanoparticle growth in general. Remarkably, we found that nucleation kinetics accelerate dramatically with a slight increase in pH or a reduction in Mg 2+ concentration. We delineate three distinct nucleation or phase separation regions: i) no phase separation at low pH, ii) phase separation through nucleation and growth (binodal region) at intermediate pH, yielding dispersed Mg-ACC nanoparticles with a narrow size distribution, and iii) quasi-spontaneous phase separation at higher pH, leading to large, polydisperse aggregates (spinodal region).