A key factor in ensuring the sustainable use of raw materials to meet global demand has been the recovery and reuse of key components. Phosphate, PO4 3-and many transition metals are listed as critical raw materials . Moreover, these elements may cause adverse effects in the environment. Phosphorous is the major culprit in anthropogenic eutrophication. Transition metal ions are toxic pollutants, especially in ground-and wastewater. Therefore, coprecipitation of phosphate and transition metal-bearing phases from various waste streams of agricultural, industrial, or mining origin (tailings, sludge, wastewaters etc.) is a viable extraction and recycling method to yield transition metal phosphates (TMPs) . Such materials could be further upcycled into processed, higher value compounds suitable for applicational use. For example, transition metal phosphates such as Zr- , Sn- , Ni-or Co- phosphates garnered attention as low-cost, chemically stable and tunable proton conductors for fuel cells . However, to effectively harvest metal and phosphate in a form of TMPs and convert them to functional materials, one needs to elucidate their nucleation and crystallization pathways from aqueous solutions. Although, the synthesis and precipitation of Mg or Ca phosphate-bearing phases are relatively well characterized , investigations of the crystallization of transition metal phosphate materials are relatively scarce and focus on mineralogical aspects . Most of the available studies concentrate on the incorporation or adsorption of transition metals and metalloids rather than their complete substitution in phosphate-based compounds . However, TMPs exhibit in their crystallization pathways, growth kinetics, thermodynamic stabilities, or transformations major differences among each other and differ from their related main-group analogues . The traditional view of crystallization and nucleation through a classical model has been widely used to describe chemical systems since its establishment in the 19 th century . This is mainly because of its simplicity and ability to make accurate predictions. The classical crystallization theory (CCT) assumes a layer-by-layer growth of primary crystal nuclei due to the adsorption of growth units/ monomers (atoms, ions, molecules etc.) on the crystal surface/kinks and their following reconfiguration to a new interface. Although CCT works well for model systems , a frequent number of complex interactions could not be explained by this theory due to its simplified assumptions and limitations i.e. capillary assumption or considering only single particle attachment. Theoretical predictions tend to deviate significantly from experimental results in orders of magnitudes in nucleation and reaction rates of the crystalline particles . Different experimental studies and approaches demonstrated empirically multiple possible formation pathways toward the final crystal through the inclusion of prenucleation clusters , liquid droplets, gels, primary amorphous or crystalline nanoparticles, nanoaggregates or mesocrystals . In the past two decades, a large variety of these transitional precursor phases of crystalline substances were discovered in pharmaceuticals , proteins , organic molecules , thin films , biominerals and inorganic materials . As we demonstrate in this work, TMPs follow a non-classical crystallization pathway, which includes evolving amorphous precursors prior to the final formation of a crystal. We holistically elucidate the crystallization behavior of transition metal phosphate phases, from their intermediate amorphous nanophases in solution, through aggregates, proto-crystalline units, to final crystals. The formation processes are in-detail analyzed with various diffractionand spectroscopy-based ex-and in-situ approaches. Ni and Co ions can be both incorporated into the same crystalline structures (i.e. struvite or octahydrate). However, the crystallization stages of these analogous TMPs are essentially different . We also demonstrate that in a dual NixCo1-x metal mixture compared to the pure cases, even more complex pathway is induced. Here, the Ni:Co ratio and the concentration of both metals have a major influence on the reaction kinetics and the crystallization outcome. In this work, we provide insights into how two 3d metal ions, Ni and Co, interact with each other resulting in changes in reaction time and crystallization stages of the phosphate precipitation.