All SAXS time series exhibit a rapid increase in intensity within the first milliseconds to seconds, reaching an intensity plateau at low q-values and an intensity decay towards high q-values with a power-law slope between 2.0 and 3.5. This pattern suggests the presence of sparse, transient aggregates of calcium and carbonate, exhibiting iso-structural scattering similar to that of a densely coiled polymer. Consecutively, their density and size consistently increase until distinct form-factor oscillations become visible with a q -4 intensity power-law decay. This indicates the formation of well-defined Mg-ACC nanoparticles with a sharp solvent-particle interface. We further observe a shift in form factor minimum towards lower q-values with time, indicative of nanoparticle growth.