(111),(11-1) ) in accordance with the pH profile. The inset in Fig. provides time-resolved diffraction patterns confirming that struvite is the only crystalline phase forming in solution at all times. The diffraction data indicate that struvite started to crystallise after 10 s, which is corroborated by the pH trend. Starting a t = 0 s, pH starts to drop from an initial ~7.9 due to the addition of a neutral solution (MgCl 2 , pH ~7.0) and the release of protons that originated from the precipitation. The crystal growth process continued for ~60 s and beyond, following the evolution of the pH profile. The experimentally determined equilibrium (i.e. final) pH is in close agreement (∆pH = ±0.05) with the predicted value obtained from PHREEQC calculations (Supplementary Material: Fig. ). To evaluate what happened during the first minute after mixing, we sampled and cryo-quenched the solution after 5 s and 60 s of mixing. Fig. shows the morphology of particles present after 5 s. At low magnification (Fig. , left), one can observe aggregates of particles (see arrows) predominantly embedded into thicker vitrified ice (purple false colour).