Calcium carbonate is an abundant substance that can be created in several mineral forms by the reaction of dissolved carbon dioxide in water with calcium ions. Through biomineralization, organisms can harness and control this process to form various functional materials that can act as anything from shells through to lenses. The early stages of calcium carbonate formation have recently attracted attention as stable prenucleation clusters have been observed, contrary to classical models. Here we show, using computer simulations combined with the analysis of experimental data, that these mineral clusters are made of an ionic polymer, composed of alternating calcium and carbonate ions, with a dynamic topology consisting of chains, branches and rings. The existence of a disordered, flexible and strongly hydrated precursor provides a basis for explaining the formation of other liquid-like amorphous states of calcium carbonate, in addition to the non-classical behaviour during growth of amorphous calcium carbonate.

The performance of existing force-field models for the calcium carbonate -water system has been critically assessed with particular reference to the thermodynamic consequences. It is demonstrated that all currently available parametrizations fail to describe the calcite-aragonite phase transition, and the free energies of solvation for the calcium cation are also considerably in error leading to a poor description of the dissolution enthalpy for calcite. A new force-field, based on rigid carbonate ions, has been developed that corrects these deficiencies and accurately describes the thermodynamics of the aqueous calcium carbonate system within molecular dynamics simulations. Not only does this new model lead to quantitative changes in the properties of the calcite (101 j 4) surface in contact with water, but also significant qualitative differences. With this more accurate model it is found that calcium ions do not adsorb at the pristine basal plane of calcite, while carbonate ions only weakly bind. Carbonate diffusion across the surface is found to occur only when the anion is solvent separated from the underlying surface, with there being an equal tendency to readsorb or migrate into the bulk liquid.

Nucleation of gas hydrates remains a poorly understood phenomenon, despite its importance as a critical step in understanding the performance and mode of action of low dosage hydrate inhibitors. We present here a detailed analysis of the structural and mechanistic processes by which gas hydrates nucleate in a molecular dynamics simulation of dissolved methane at a methane/water interface. It was found that hydrate initially nucleates into a phase consistent with none of the common bulk crystal structures, but containing structural units of all of them. The process of water cage formation has been found to correlate strongly with the collective arrangement of methane molecules.