Three Oxford Chemistry groups collaborate on amorphous calcium carbonate

Geometrically frustrated interactions drive structural complexity in amorphous calcium carbonate has been published by Nature Chemistry.

Amorphous calcium carbonate (ACC) is one of Nature’s important building materials. It lacks an ordered atomic structure, making it amorphous, or non-crystalline. This complex structure means it is valuable to marine organisms that use it to form structures such as shells. However, the best way to understand the structure of ACC, and why it forms so readily, are longstanding open questions.

This new study combines experimental data with advanced computer simulation techniques to answer precisely these questions. The work involves DPhil student Thomas Nicholas and former DPhil student Adam Stones, and is a collaboration between the Aarts, Deringer, and Goodwin groups in Oxford Chemistry working together with two USA teams.

To better understand the structure and resulting metastability of ACC, the first aim was to generate a high-quality atomistic model of ACC, which was done by using state-of-the-art interatomic potentials to guide interpretation of diffuse X-ray scattering data. The main features of the model are a disordered network of calcium cations, linked by carbonate anions of opposite charge, and interspersed water channels, visible in the image above (a “blue cheese” model, in the authors’ words).

With this new structural model in hand, the team extracted the effective interactions between the calcium ions using a recently developed “inversion” method. It was found that the effective interaction between the calcium ions has two attractive minima, arising from the two different ways in which carbonate anions can bridge calcium cations. These competing length scales lead to geometrically frustrated ordering – no simple way for the ions to pack into an extended structure – explaining the amorphous nature and the relative stability of ACC.

The paper was published this afternoon and is available via Open Access at