When the pH is further increased to 8.7, we observe only the instantaneous formation of large, nonparticulate structures (see Figure ). The kinetics of this formation are too rapid to be adequately resolved even at a 5 ms time resolution, making it impossible to describe them using Avrami-like kinetics (Figure A, violet curves) analytically. We have shaded this region of very fast kinetics green in Figure A. Here, ACC nanostructures emerge quasi-instantaneously, forming semi-continuous structures rather than undergoing nucleation and growth, spinodal liquid-liquid phase decomposition. While previous reports have documented ACC formation in the spinodal region, they describe the formation of discrete spherical nanoparticles, differing from the structures presented in this work. This finding is crucial as it highlights the existence of an ideal point in the phase diagram for the formation of highly monodisperse Mg-ACC nanoparticles. This ideal point lies in the binodal region but close to the spinodal line, which marks the boundary between the binodal and spinodal regions.