Constraints on Biogenic Emplacement of Crystalline Calcium Carbonate and Dolomite

Abstract:

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.

Water is a critical component of the structure of ACC and the key component controlling the stability of the amorphous state. Addition of small amounts of magnesium (1-5% 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.

Our approach is to use Monte-Carlo simulations constrained by X-ray and neutron scattering data to determine hydrogen bonding networks in ACC as a function of magnesium doping.

We have already successfully developed a synthesis protocol to make ACC, and have collected X-ray data, which is suitable for determining Ca, Mg and O correlations, and have collected neutron data, which gives 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 X-ray and neutron data are used to constrain reverse Monte-Carlo modelling of the ACC structure using the Empirical Potential Structure Refinement program, in order to yield a complete structural model for ACC including water molecule positions.

We will present details of our sample synthesis and characterization methods, X-ray and neutron scattering data, and reverse Monte-Carlo simulations results, together with a discussion of the role of hydrogen bonding in ACC stability.