Growth and structure of CaCO3

Organisms often employ non-classical crystallisation mechanisms to create the remarkable materials that are biominerals. These materials often surpass their synthetic counterparts in terms of physical properties, morphologies and structural organisation. The non-classical mechanisms employed include the controlled formation, transition and release of amorphous precursor material, and the oriented attachment/ nucleation of nano sized particulates. Combined, these strategies are capable of generating hierarchically ordered superstructures. Both of these mechanisms operate under ambient conditions in a physically delimited environment of body fluids, which enables precise regulation of the solution composition.
This thesis describes a range of biomimetic studies which have investigated key aspects in the formation and structural organization of calcium carbonate. Of interest were the influence of additives and physical confinement on the formation and transformation of amorphous calcium carbonate (ACC). The studies revealed that both of these factors play key roles in controlling ACC crystallisation. Additives which inhibit crystallisation in solution can accelerate transformation of ACC in the solid state. This effect was observed for all of the larger molecules examined, while the small molecules retarded crystallisation in both solution and the solid state. Investigation of ACC crystallisation in confinement, in turn, demonstrated that ACC dehydrates prior to crystallizing even in solution, and that nucleation of the first crystal phase in solution must occur by dissolution/ reprecipitation.
Studies were also performed to characterise the “ammonia diffusion method” which is widely used in the precipitation of calcium carbonate. Despite this, virtually nothing is known about the changes in solution conditions which occur during this process. The analysis showed that the supersaturation remains relatively high and constant throughout most of the process, which potentially enables multiple nucleation events to occur in a single experiment. These results were then used to develop a one pot method which offers comparable reaction conditions.
Finally, Bragg coherent diffraction imaging (BCDI) was used to characterise calcite crystals precipitated on self-assembled monolayers (SAM), where these provide a mimic of the organic matrices used to control crystallisation in organisms. Initial observations of the growth and dissolution of calcite by BCDI allowed the visualization of the 3D dislocation network present within a single crystal. Examination of crystals grown on SAMs, in contrast, showed that a build-up strain causes the formation of a single dislocation loop, where this is correlated with the morphological development of the crystal.