Abstract
Amorphous magnesium carbonate (AMC: MgCO3·nH2O) is a metastable phase with respect to crystalline magnesium carbonate. AMC has been suggested to be an important mineral, a major process of CO2 fixation in alkaline saline lakes. Understanding the role of AMC for carbon fixation in alkaline lakes requires knowledge of the solubility of AMC under the full range of relevant environmental conditions, but its solubility is poorly known for low water temperatures. In this study, the solubility of AMC at 7 °C was measured in laboratory experiments and compared with chemical analyses of water samples from Olgoy Lake, a natural alkaline saline lake in Mongolia. Laboratory measurements using both supersaturation and undersaturation methods provided very similar solubility products of AMC (log Ksp = −5.20 ± 0.02) at 7 °C, which was markedly higher than the solubility product at 25 °C (log Ksp = −5.59). The temperature dependence of the solubility of AMC is much greater than that of monohydrocalcite, another important authigenic calcium carbonate in alkaline lakes. Analyses of water from Olgoy Lake consistently showed a significant temperature dependency of ion activity products with respect to magnesium carbonate. The relatively high solubility of AMC at low temperatures implies that the formation of AMC requires relatively high concentrations of dissolved components in lakes. Our results suggest that the formation of AMC is favored more in summer than in winter.
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