Nucleation and growth of Mg-bearing calcite in a shallow, calcareous lake

Both inorganic and microbial processes play important roles in carbonate mineral precipitation in freshwater lakes. Identifying the individual factors that determine particle size, shape, composition, and the spatial relationships of carbonates with other constituents of the sediment are important for understanding sediment formation and nutrient cycling, and for providing input for lake management planning. We studied the formation of Mg-bearing calcite in Lake Balaton, a large, shallow, Mg-rich, calcareous lake in Hungary, by filtering particles from the water and analyzing them using X-ray powder diffraction and electron microscopy techniques. Mg-bearing calcite particles from Lake Balaton have distinct and remarkably consistent morphologies, independent of seasonal and annual variations in water temperature and composition. They are typically elongated parallel to the [001] crystallographic direction, are about 4 to 8 μm long, and even though appear to be composed of many smaller units, electron diffraction patterns indicate they are perfect single crystals. The Mg content of calcite increases from west to east, reflecting a gradient in lakewater composition. The calcite crystals are invariably attached to nm-scale flakes of smectite, suggesting that the clay mineral serves as a nucleation site. The templated nucleation may direct the growth of calcite parallel to the clay flakes, resulting in its elongated shape and single crystalline character. While in deep, oligotrophic lakes calcite typically nucleates on picoplankton cells, we rarely observe this phenomenon in Lake Balaton. Because of the shallowness of the lake, sediments are stirred up by even moderate winds and the movement of organisms, making nm-scale smectite flakes readily available for templating calcite nucleation. Thus, while calcite precipitation is an indirect consequence of biological activity (photosynthesis), the physical properties of the particles are primarily determined by inorganic factors.