Many marine organisms make use of an amorphous form of calcium carbonate, known by the acronym ACC, to build the intricately-shaped mineral structures of their shells and skeletons. The absence of long-range periodic order in ACC allows the organisms to build curved surfaces in a way that would not be possible using crystalline forms of calcium carbonate, which grow preferentially in specific crystallographic directions. ACC itself is highly unstable, but it can be stabilised in vivo long enough for biomineralisation to occur. The mechanisms of stabilisation and subsequent crystallisation have remained key questions in the field.
Using x-ray total scattering and reverse Monte Carlo (RMC) analysis, we have developed the first atomic-scale model of ACC, which will hopefully help answer some of these questions.
During our modelling, a structure evolved that consists of a nanoporous cationic Ca-rich mineral backbone and an anionic mobile carbonate/water component that occupies the framework pores. The existence of nano-scale charge separation my help explain the inherent instability with respect to crystallisation. Likewise the existence of a channel structure also suggests how small charged molecules might be introduced into the material to help delay and eventually control the crystallisation process.