To retrieve quantitative structural information such as Mg-ACC nanoparticle size and size distribution, we employed an analytical scattering model (see SI 2.1) to fit the time-dependent SAXS data. In brief, to model the scattering of the Mg-ACC nanoparticles, we used a fractal form factor model with an exponential size cut-off, which can reproduce the scattering behavior of a solid sphere for a fractal dimension approaching 3. This allows for an analytical description of early, transient structures of low-density and solid ACC nanoparticles within the same analytical model by varying the fractal dimension. To describe the distribution in Mg-ACC nanoparticle sizes, we use a Gaussian distribution. The intensity modulation in the mid-q regime originating from particle interaction is modeled using a hard sphere repulsion structure factor accounting for non-interpenetrability, paired with a square well potential structure factor to account for attraction between Mg-ACC nanoparticles. The increase at very low q where hard sphere and square well potentials do not exhibit substantial effects on the scattering behavior, was described with a simple power law scattering model using the scattering behavior of a critical density fluctuation described by Ornstein and Zernicke. Using this description, we model the time-dependent scattering behavior of the growth of Mg-ACC nanoparticles for all parameters tested (see Figure SI 10-14). Consequently, we can determine the fractal dimension of the emerging precursors to be between 2.2 and 2.6.