Solubility of Amorphous Magnesium Carbonate at Low Temperatures: Implications for Carbonate Mineral Formation in Alkaline Lakes

Amorphous magnesium carbonate (AMC: MgCO3 center dot nH(2)O) is a metastable phase with respect to crystallinemagnesium 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 lakesrequires knowledge of the solubility of AMC under the full range ofrelevant environmental conditions, but its solubility is poorly knownfor low water temperatures. In this study, the solubility of AMC at7 degrees C was measured in laboratory experiments and compared withchemical analyses of water samples from Olgoy Lake, a natural alkalinesaline lake in Mongolia. Laboratory measurements using both supersaturationand undersaturation methods provided very similar solubility productsof AMC (log K (sp) = -5.20 +/- 0.02) at 7 degrees C, which was markedly higher than the solubilityproduct at 25 degrees C (log K (sp) =-5.59). The temperature dependence of the solubility of AMCis much greater than that of monohydrocalcite, another important authigeniccalcium carbonate in alkaline lakes. Analyses of water from OlgoyLake consistently showed a significant temperature dependency of ionactivity products with respect to magnesium carbonate. The relativelyhigh solubility of AMC at low temperatures implies that the formationof AMC requires relatively high concentrations of dissolved componentsin lakes. Our results suggest that the formation of AMC is favoredmore in summer than in winter.