Modelling calcium carbonate biomineralisation processes

The structure-directing influence of the organic dicarboxylates malonate, succinate, glutarate and adipate as templating species on the hydrothermal formation of CaCO3 was investigated at different temperatures (60, 80, 90, 120, 150 and 190 °C) and with a range of molar ratios of [Ca2+]/[templating species] (20, 14.3, 10, 7.7, 5, 1, 0.5 and 0.33). In the presence of the dicarboxylates, one, two or three polymorphs of CaCO3 – calcite, aragonite and vaterite – could be formed, depending on the reaction conditions. In addition changes in crystal morphology were observed for the CaCO3 polymorphs depending on the concentration of the template. In contrast, synthesis under ambient conditions of temperature and pressure resulted only in calcite formation, although template-dependent morphological changes were again observed. Crystalline products were all characterized by powder X-ray patterns and SEM (Scanning Electron Microscopy) micrographs. The ambient reactions with the chelating, dinucleating carboxylato ligands H3heidi and H5hpdta produce more profound changes in calcite morphology. With H3heidi rounded calcite crystals with shapes similar to that of otoliths are formed and with H5hpdta the formation of microtrumpets of constructed from bundles of nanocrystals of calcite is observed. The possible mode of action of these ligands on calcite formation is discussed in the context of known coordination chemistry with other metal ions.