The Formation of Magnesium Oxychloride Phases in the Systems MgO‐MgCl 2 ‐H 2 O and NaOH‐MgCl 2 ‐H 2 O

The reactions leading to the formation of crystalline Mg 3 (OH) 5 Cl·4H 2 O (phase 5), Mg 2 (OH) 3 ,Cl·4H 2 O (phase 3), and Mg(OH) 2 are compared for the systems MgO‐MgCl 2 ‐H 2 O and NaOH‐MgCl 2 ‐H 2 O. The crystalline phases were determined by X‐ray diffraction analysis. The concentration of the total magnesium and chloride in the solution and the pH of the solution determine the reaction product(s) in both systems. The influence of MgO reactivity and the molar ratio of reactants on the formation and stability of reaction products is discussed and the mechanism of the formation of phases 3 and 5 is explained. In the system MgO‐MgCl 2 ‐H 2 O, MgO serves only to increase the concentration of total magnesium and the pH of the MgCl 2 solution.