Structural constraints on the crystallisation of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a biogenic precursor of calcium carbonates forming shells and skeletons of marine organisms which are key components of the whole marine environment. Understanding carbonate formation is an essential prerequisite to quantify the effect climate change and pollution have on marine population, to build biogenic climate proxy archives and to design novel functional materials by biomimetism. Water is a critical component of the structure of ACC and a component controlling the stability of the amorphous phase. Addition of small amounts of magnesium (1–10% of the calcium content) is known to promote the stability of ACC, presumably through stabilization of the hydrogen bonding network. Understanding the hydrogen bonding network in ACC is fundamental to understand the stability of ACC. I successfully developed a synthesis protocol to make stable Amorphous Calcium Carbonate.This study describes the sample synthesis and characterization methods. This is followed by a description of the nucleation and crystallisation of calcium carbonate, along a non-classical pathway. The results are parameterised in terms of kinetics and final product as a function of magnesium doping. The approach used in this study to determine the atomic structure of ACC is to use Monte-Carlo simulations, using the Empirical Potential Structure Refinement program, constrained by X-ray and neutron scattering data. X-ray data are suitable for determining Ca, Mg and O correlations, and neutron data give information on the hydrogen / deuterium (as the interaction of X-rays with hydrogen is too low for us to be able to constrain hydrogen atom positions with only X-rays). The final atomic model includes water molecule positions. This new approach is used to constrain current novel kinetic models, shed light on the role of impurities and provide a basis for understanding biogenic systems in general.