e amount of added magnesium is referred to as the molar ratio Mg:P, where Mg is the magnesium amount in all suspended MgO or dissolved MgCl2, and P is the amount of phosphorous in H2NH4PO4. The molar ratio also represents the degree of initial supersaturation, meaning that experiments carried out at the same molar ratio are also carried out at the same initial super- saturation. Experiments were conducted out at molar ratios Mg:P of 0.5, 0.75 and 1. All MgCl2 runs were controlled at a constant pH during the entire process by monitoring the pH and titrating a 2.0 mol L 1 sodium hydroxide solution into the system to com- pensate for fluctuations in pH. The pH values (range 6.6–8.2, also shown in Fig. 9) were set to the equilibrium pH values of the corresponding precipitation runs with MgO carried out at the same Mg:P ratios.