Since the TMPs crystallization pathways are multi-step, consequently, the reaction kinetics of the TMPs precipitation is primarily dependent on the stability of the amorphous precursors and their agglomerates, and to a lesser extent on the final crystallization step. However, this "final" transformation ought to be also considered. Looking at the solubility product i.e., the thermodynamic driving force for precipitation, the Ksp of M-struvite is magnitudes higher compared to the one of M-phosphate octahydrate at ambient conditions . However, Mstruvite still demonstrates the faster reaction kinetics compared to M-phosphate octahydrate. This could be explained by crystallographic differences in the transition metal phosphate structures. Based on the cryo-TEM and in-situ scattering data, we could indeed extract the information about phase, morphology and key differences in the evolving frameworks of the amorphous phases and their transformation to crystalline solids depending on the involved transition metals. While we focused in the previous part of discussion on the stability and the effects of the amorphous phases, we consider further the evolution of the occurring crystal structures, namely M-struvite and M-phosphate octahydrate. From a crystallographic point of view, as Co-phosphate octahydrate exhibits large dhkl of the (110) and (002) plane, due to its layered structure (Figure Figure , q110 or q002 < 10 nm -1 ), these crystal planes exhibit resulting high load densities and growth velocities. This is the reason why these planes have a large probability to form and scatter with high intensity (as is concluded from Bravais-Friedel-Donnay-Harker model) . As it can be seen in Figure , in M-struvite the intensity of the (101), (002), and (011) reflections increases immediately, while the (110) and (020) reflections of M-phosphate octahydrate grow gradually due to their layered nature. The fast formation kinetics of M-struvite compared to M-phosphate octahydrate are probably related to its simple crystallographic structure of isolated building units, mainly the isolated phosphate tetrahedron and isolated metal octahedron coordinated by crystal water . However, the Co-phosphate octahydrate structure is more complex depicted for example in the occurrence of two different metal sites. In the first site, the transition metal is surrounded by four water molecules and two oxygen atoms, resulting in the octahedron Co(H2O)4O2 . In the second site, the transition metal is coordinated by two water molecules and four oxygen atoms, leading to Co(H2O)2O4. The (110) reflection of M-phosphate octahydrate represents the crystal plane constructed by a regular arrangement of octahedral Co(H2O)4O2 or Co(H2O)2O4 units with a dspacing of d110 = 7.83 Å, whereas the (020) reflection lies in the plane of linked Co octahedrons (both types) with tetrahedral coordinated PO4 phosphate units with a d-spacing of d020 = 6.67 Å. Firstly, the crystal structure is most likely to build up in one c-layer by the arrangement of disordered super units of linked Co-octahedrons and PO4 tetrahedrons resulting in the presence of the ( ) and (020) reflections. Here 00l reflections would not occur as the layers are internally disordered and grow in single layers. At extended reaction time, additional layers begin to form in the c-direction. Here, inside the c-layer the Co(H2O)4O2 and PO4 selfassemble and reach a higher degree of ordering resulting in the appearance of (200) [d200 = 4.84 Å], (001) [d001 = 4.52 Å] and other reflections. A visualization of the lattice planes in the crystal structure of Co-phosphate octahydrate is depicted in the ESI (ESI: Figure ). The actual accomplishment of the transformation in the solid, from a disordered, amorphous to an ordered, crystalline state in the solid compounds is still not well-understood, either in phosphates or in other mineral systems. For instance, this issue of a stochastic 'jump' of a dense agglomerate to a primary crystal is also debated in other material classes such as the conversion of amorphous calcium carbonate (ACC) to a crystalline carbonate or in the calcium sulfate system . However, we assume that the amorphous highly agglomerated solid exhibits at least a certain short-range order. Indicators for this assumption could be the fast transformation to a crystal within seconds, while the process of agglomeration and densification of the primary building blocks required more time, from minutes to hours. By slightly ordering the disordered c-layers in M-phosphate octahydrate or isolated building units (NH4, PO4, MO6) in M-struvite, the previously amorphous solid could establish a threedimensional periodic crystalline order in the compound. Thus, the primary nucleating crystal would form.