MICROSTRUCTURE OF MAGNESIUM-BEARING CARBONATES PRECIPITATING FROM SHALLOW FRESHWATER LAKES

Carbonate formation in lakes has been intensely studied for decades, yet the mechanisms of their nucleation and the structures (Mg/Ca ordering schemes) of precipitating particles with compositions between those of calcite and dolomite are poorly understood. We used HRTEM and STEM imaging and EDS mapping to study carbonate particles from Lake Balaton and Lake Fertő (Neusiedlersee). Both are extremely shallow, carbonate-forming, alkaline, moderately evaporative, large lakes in Central Europe.

In both lakes micron-sized Mg-bearing calcite particles were attached to nm-scale smectite flakes, which suggested the possibility that clays could act as templates for the heterogeneous nucleation of calcite. We performed laboratory experiments in order to compare homogeneous calcite nucleation with precipitation in the presence of either montmorillonite or kaolinite. Calcite precipitation was induced in the presence of clays at lower supersaturations than in the bulk solution. Molecular dynamics simulations further suggested that montmorillonite is highly efficient in anchoring ionic clusters to the clay surface, presumably initiating calcite nucleation. The templated nucleation might affect the morphologies of the growing calcite crystals, resulting in distinctly elongated particle shapes in Lake Balaton.

In Lake Balaton Mg-bearing calcite particles precipitate with 2 to 17 mol% MgCO₃, with the variation of compositions reflecting the seasonal and geographic changes in lakewater chemistry (Mg/Ca mol ratio ranging from 1 and 4, and typically increasing from west to east). In addition, anomalous, Ca-rich dolomite is also present as a minor phase. In contrast, carbonates in Lake Fertő (where dissolved Mg/Ca mol ratio of the water can be as large as 8) show a continuous compositional range from low-Mg calcite to stoichiometric dolomite. Both HRTEM and HAADF images show that grains with high Mg contents typically display ordering of Mg and Ca, occurring in few-nm large, dolomite-like domains. Thus, these shallow, mildly evaporative lakes provide a case for primary nanoscale dolomite formation in modern freshwater environments.