Formation condition of monohydrocalcite from CaCl2–MgCl2–Na2CO3 solutions

Monohydrocalcite (MHC: CaCO3·H2O) is a rare mineral in geological settings. Its rare occurrence indicates that MHC forms in unique environmental conditions in nature. Both laboratory studies and natural observations have indicated that the formation of MHC requires magnesium in the solution in spite of the incompatibility of magnesium into the MHC structure. In this study, systematic syntheses from CaCl2–MgCl2–Na2CO3 solutions were conducted to elucidate MHC formation conditions and to clarify the role of magnesium in MHC formation. For this study, MHC was formed from mixing solutions of which the ratio of the total carbonate to calcium was higher than 1 in the presence of a certain amount of magnesium. The pH of the reacted solutions after the MHC formation was 9.8–11.0. The solid-phase magnesium contents in the formed precipitates were widely varied. MHC exhibits low crystallinity and small particle size when the ratio of magnesium to calcium in the solid was higher than 0.4. After MHC formation, ion activity products of the reacted solutions with respect to MgCO3·xH2O stoichiometry took almost constant values. The ion activity products were significantly higher than the solubility product of anhydrous MgCO3, but they were close to the solubility of nesquehonite, which indicates that MHC formation of requires paragenesis of the hydrous magnesium carbonate. To produce the hydrous magnesium carbonate simultaneously with MHC, the initial solution must contain magnesium and more CO32- than Ca2+, which is consistent with the formation conditions of MHC from the initial solution compositions. Magnesium ion possesses a high hydration energy. The hydrous magnesium carbonates surrounding MHC probably play a protective role for MHC in preventing its dehydration to anhydrous calcium carbonate. The solution chemistries of saline lakes of which the MHC formation was observed in nature are also at equilibrium or are oversaturated with respect to hydrous magnesium carbonates and MHC. Rare occurrences of MHC in nature are explainable by the paragenesis requirements of hydrous magnesium carbonates and by the metastability of MHC.