Calcium Carbonate Mineralization in the Presence of Magnesium and Otoconial Proteins: Implications for Dolomitization and Biomineralization
Open AccessSolution based crystallization process begets mineralization in a variety of natural settings ranging from hydrothermal veins to physiological fluids, and can lead to the formation of gigantic crystals as well as nanoparticles. Dolomitization and biomineralization are two typical examples of solution based crystallization that are not well understood. One of the unsolved issues concerns the differences in the behavior of cationic and anioic particles during crystal growth and the factors that control such differences. Classical crystallization models successfully depict the dependence of growth kinetics on thermodynamic driving force but give no predictions to the roles of solution chemistry. We approached this problem by studying multicomponent systems (e.g. calcite) with individual solution chemistry parameter isolated, including pH, ionic strength and stoichiometry using in situ atomic force microscopy (AFM). We proposed a new step growth model based upon the independent incorporation of cations and anions.Dolomitization: We investigated the evolution of calcite spiral hillock morphology with different Ca to Mg ratios and supersaturation levels using in situ AFM at fixed pH, ionic strength, and Ca2+/CO32-. Our results revealed three types of morphologies and suggest that this effect depends upon multiple parameters including Mg concentration in solution, calcite step speed, and the extensiveness of cation substitution in calcite lattice. We proposed that the morphological variation may be understood by a model considering (1) the life time and flux size of Mg ions at kinks in comparison to step kinetics, and (2) the diffusion and alignment of point defects created by the substitution of Mg for Ca in the crystal lattice.Otoconia biomineralization: We also studied the roles of four proteins associated with the formation of otoconia: Fetuin A, osteopontin, otoconin 90 and Otolin-1. In situ AFM and SEM were performed to obtain insights into the effects of proteins on calcite step growth kinetics and crystal morphology. We then used in situ optical microscopy to observe calcite nucleation on protein films to determine the interfacial energy that controls the free energy barrier to heterogeneous nucleation. Zeta potential measurements provided the negative surface charge each protein carries to be scaled with the solution-substrate interfacial energy.
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