Modelling the effect of minority components in biominerals via biomimetic mineralisation

This work aims to determine whether the different crystallisation pathways of amorphous calcium carbonate (ACC) have an impact on the element and oxygen isotope partitioning of the final crystalline phase. In this study, special emphasis is placed on the sparsely investigated pathway of the so-called pseudomorphic transformation, which is a crystallisation process that preserves the disequilibrium morphology of the amorphous precursor. Since it is shown that the composition of ACC can be preserved under pseudomorphic conditions, the influence of a range of synthesis parameters and conditions on the material properties and chemical composition of ACC was scrutinised. While solid-state transformation retains the chemical composition of doped ACC during crystallisation, the chemical composition is altered during dissolution-reprecipitation pathways. An alternative crystallisation pathway, namely the shape-preserving pseudomorphic transformation, is induced by additives such as poly(acrylic acid), polyaspartic acid and trace amounts of phosphate. Since ACC crystallisation in biominerals occurs in the presence of these or similar additives – especially aspartate-rich domains were found in unusually acidic biomineralisation proteins – a thorough mechanistic understanding of the pseudomorphic transformation is of importance for paleoclimate reconstructions. This work revealed that the pseudomorphic transformation not only preserves the morphology of the amorphous precursor, but also retains the partition coefficients, e.g., in case of Sr-doped ACC. Furthermore, the influence of the pseudomorphic transformation on the oxygen isotope composition demonstrated that this transformation is a quasi-solid to solid phase transformation process, although it takes place in aqueous solutions. Mechanistically, it seems that the presence of certain surface-active additives limits diffusional exchange with the surrounding liquid environment so that pseudomorphic transformation preserves the partition coefficients of ACC even in the final crystalline product. As the pseudomorphic transformation preserves the composition of the amorphous precursor, it is necessary to understand how the formation of ACC and its partitioning coefficients, as well as its material properties, is influenced by the synthesis conditions. For instance, it was recently shown that it is possible to precipitate ACC with distinct short-range order by simply altering the pH slightly. Therefore, it is of high significance to analyse the influence of the synthesis conditions on ACC. In this thesis, three different approaches were conducted to achieve a better understanding of the structural and compositional synthesis dependence of ACC: #1 The influence of the synthesis procedures revealed that material properties such as particle size, level of hydration, crystallisation temperature, and density are sensitive to simple changes in the synthesis conditions, which have much less effect on the chemical composition of ACC. Notably, density measurements indicated that synthesis-dependent microstructures of ACC structures exist. Furthermore, a microfluidic setup allowed for ACC synthesis at an exceptionally low pH (pH 7.5) by using ethanol as anti-solvent. Under these conditions, synthesis of ACC at varying pH revealed a significant increase of barium incorporation by decreasing pH. #2 The influence of the mixing kinetics on the element partition was analysed by precipitating magnesium-, strontium-, and barium-doped ACC under varying flow rates. While less magnesium was incorporated at increasing flow rates, increasing partition coefficients were determined for Sr-doped ACC by increasing flow rates. Notably, no flow rate influence was determined for Ba-doped ACC. These results demonstrate that element partitioning is highly sensitive to changing mixing kinetics, which indicates that prenucleation clusters play a role during ACC formation and control element partitioning to a certain extent. #3 To mimic ACC formation in a natural environment, ACC was synthesised in artificial seawater under varying synthesis conditions as flow rate, temperature, and pH, which resulted in multiple-doped ACC. Besides material properties such as particle size, the chemical composition was also affected by the synthesis conditions. A significant influence of the flow rate and temperature on magnesium and sulphur partitioning was determined. While less magnesium was incorporated by increasing flow rates, increasing sulphur incorporation was detected. Furthermore, both additives were better incorporated at enhanced temperatures. Notably, increasing concentrations of all dopants were determined with increasing pH. In the final chapter, a feasibility study was conducted to assess whether a flow-through synthesis enables the synthesis of the fundamental building blocks of calcareous biominerals, namely calcium carbonate nanograins coated with organic matrices. This study demonstrated that the precipitation of Mg-doped ACC particles coated with negatively charged polyelectrolytes, such as polyacrylates or polystyrene sulfonate, is possible by using a flow-through synthesis.