We can thus outline the general formation mechanism of Mg-ACC nanoparticles within the window of spatiotemporal resolution of this experiment as follows: Initially, a nucleation burst forms Ca 2+ /Mg 2+ /CO precursors in the size range of 20--40 nm that are characterized by a diffuse particle solvent interface and a lower electron density compared to that of the final Mg-ACC nanoparticles. Earlier studies have identified comparable precursors as a transient, hydrated proto-structured ACC, likely originating from a dense liquid phase or prenucleation cluster (PNC) enriched liquid formed through liquid-liquid phase separation. The proto-structured Mg-ACC particles condense with concomitant loss of water to form Mg-ACC nanoparticle seeds with a defined water interface, as indicated by their early stage fractal dimension of the nanoparticles (D F ≈ 3, Figure SI 9), corresponding to that of a sphere with homogenous electron density distribution. Subsequently, the Mg-ACC nanoparticles grow through a monomer addition mechanism.