Ph microelectrode

Two solutions were prepared in ultrapure water (Milli‐Q). In solution A, urea (Sigma–Aldrich), calcium chloride dihydrate (Sigma–Aldrich) and hydrochloric acid (Sigma–Aldrich) were dissolved. Solution B contained urease/phosphate (from Canavalia ensiformis (Jack bean) type III, Sigma–Aldrich, 31 430 u g⁻¹, containing phosphate 0.17 mg/u). For experiments in the cuvette, one mL of B was added to one mL of A under constant stirring using a magnetic stirrer (HI‐190M) with a Spinfin® stirrer bar. Experiments were performed at room temperature (20±2 °C). The pH was monitored using a pH microelectrode (Mettler Toledo and Pico Data logger software) and images were taken using a PixeLINK® camera and analyzed using ImageJ or MATLAB. Turbidity measurements were obtained using a VWR UV‐3100PC spectrophotometer. The precipitate was filtered, rinsed with doubly deionized water and air dried. Raman spectroscopy was performed on a Thermo ScientificTM DXR^TM Raman microscope using a green laser (λ=532 nm), at 5 mW. Scanning electron microscopic (SEM) images were recorded by a Hitachi S‐4700 field emission scanning electron microscope, at 10 kV. Optical microscope images were produced from an aqueous sample of solution collected after oscillations ceased using a Leica TCS SP8 Confocal Microscope. For more details, see the supplementary information.