This acceleration in kinetics, corresponding to a lowering of the free energy barrier of nucleation, leads to the formation of smaller nanoparticles with a narrower size distribution. However, if the driving force for nucleation becomes too large (too high pH or too low [Mg 2+ ]), we enter the spinodal region, where only polydisperse larger structures form quasi-instantaneously, faster than the time resolution of our experiment ( 30 ms). These findings illuminate the potential role of magnesium ions in particle assembly-based biomineralization pathways, prevalent in numerous calcifying species. When considering the particle-based mechanism within the context of classical colloidal crystal formation, there is a size distribution limit of 5 % above which the formation of colloidal crystals results in sluggish particle ag-gregates. In our case, such a narrow distribution is achievable only for nanoparticles formed near the spinodal line.