Control of Mg²⁺/Ca²⁺ Activity Ratio on the Formation of Crystalline Carbonate Minerals via an Amorphous Precursor

The formation of amorphous calcium carbonate (ACC) and its transformation to crystalline phases plays a key role in the formation of carbonate minerals on Earth's surface environments. Nonetheless, the physicochemical parameters controlling the formation of crystalline CaCO₃ via an amorphous precursor are still under debate. In the present study we examine whether crystalline CaCO₃ formation occurs via an ACC precursor in the pH range from 7.8 to 8.8 and at initial Mg/Ca ratios from 1/3 to 1/8. The obtained results document that the transformation of Mg-rich ACC (Mg-ACC) to a crystalline phase is strictly controlled by the prevailing ratio of the Mg²⁺ to Ca²⁺ activity, a_Mg ²⁺/a_Ca ²⁺, of the reactive solution after Mg-ACC was synthesized: Mg-ACC transformed to (i) Mg-calcite at 5 ≤ a_Mg ²⁺/a_Ca ²⁺ ≤ 8 and to (ii) monohydrocalcite at 8 ≤ a_Mg ²⁺/a_Ca ²⁺ ≤ 12. Our findings suggest that the formation of the crystalline phase induces undersaturation of the reactive solution with respect to the ACC and triggers its dissolution. Thus, the metastability of Mg-ACC in the reactive solution is not determined by its Mg content but is related to the formation kinetics of the less soluble crystalline phase. The experimental results highlight the importance of prevailing physicochemical conditions of the reactive solution on Mg-ACC transformation pathways.