2, I 2 approximates the intensity of the water peak at ~20 nm -1 . Thus, the trend shows that the maximum of this broad peak at ~20 nm -1 decreases at the early stages of struvite crystallisation in correlation with the growth of the diffraction peaks (Fig. , inset II). The actual position of the peak at ~20 nm -1 is effectively constant, with d = 0.311 ± 1.0E-4 nm (insets I and II in Fig. ). Furthermore, the scattering curves remain unchanged for q > ~25 nm -1 (Fig. , inset II). This all indicates that the O-O network (correlated with the peak at ~20 nm -1 ) becomes perturbed due to the presence of evolving ionic species/precursor species, and solids as struvite evolves. We interpret this perturbation in terms of an excluded volume effect, significant enough to affect the global structure of the medium, i.e., the number density of species causing the "displacement" is large. At the same time, the second peak of water remains unaffected (Fig. , inset II), indicating that the local tetrahedral structure around the solvent is unaltered. Still, at larger distances, other species perturb the original water structure. This is in line with the PHREEQC calculations, indicating that the majority, ~54% (see Methods), of available ions remain part of the solution and do not precipitate as struvite. Classical thermodynamic models assume that unconsumed ions primarily form solvated ions. However, it is more likely that post-nucleation solutions contain also more complex species, e.g., in the form of coordination compounds, which may explain the concurrent increase in ξ OZ and the decrease of I 1 . This hypothesis suggests the presence of populations of nucleated entities (i.e., more complex than solvated ions) causing OZ fluctuations. As already mentioned, at 10 s, nucleation begins and leads to a short-lasting increase in both ξ OZ and I 1 . Again, this event can be interpreted as a liquid-liquid phase separation where one of the separated components comprises pre-critical entities This potentially broad speciation of dissolved species leads to OZ solution heterogeneities , which manifest themselves similarly to phase separation effects, e.g. in oil and water emulsions . Fig. shows the scattering patterns of some aqueous solutions considered in this work. The scattering pattern of water exhibits a nearly flat plateau I(q) ∝ q 0 at 0.0169 cm -1 , which is the expected value at 20 °C56 . Strictly speaking, the water profile for ~0.3 nm -1 < q < ~6 nm -1 is not in fact a perfectly "straight line". It contains a weak structure factor feature caused by an interplay of attractive and repulsive intermolecular interactions . The ionic aqueous solutions, on the other hand, all show a pronounced local intensity minimum, for 5 < q < 6 nm -1 (Fig. , and the inset), indicating prevalent OZ fluctuations, whose magnitude (proportional to I 2 in Eq. 2) appears to be correlated with concentrations (e.g., 0.1 M vs 0.05 M DAP in Fig. ). Our measurements on ionic aqueous solution in Fig. indicate that they all contain some structural heterogeneities in the form of OZ fluctuations. When the two reactant solutions (Eq. 1, Fig. , Mg 2+ and DAP) are mixed, the temporary miscibility gap between two liquids leads to an increase in the "concentration" of OZ heterogeneities, inducing? the liquid-liquid phase separation , resulting in the nucleation of an amorphous struvite phase and its further crystallisation. The inset shows in detail the scattering range 2 < q < 10 nm -1 in where OZ fluctuations are clearly manifested,