The last decade has seen a revolution in our understanding of crystal growth mechanisms, and it is now clear that crystallisation often proceeds via the aggregation of precursor units, rather than the classical ion-by-ion addition mechanism. This can generate crystals with hierarchical structures and complex morphologies; additives such as block-copolymers can facilitate this, but are not essential. Assembly can be non-oriented, leading to polycrystalline particles with no preferred orientation, or oriented, leading to “ mesocrystals” where the crystallites are arranged in crystalline register and the product particles behave as single crystals (e.g. by XRD). A continuum of structures also exists between these two end-members Notably, both mesocrystals and polycrystalline particles can also be generated from the coherent crystallisation of arrays of amorphous precursor particles.
Similar nanoscale processes are also involved in the formation of biominerals, and a growing body of evidence, based on ultrastructural studies of biominerals including the aragonite tablets in nacre, sea urchin spines and the calcitic red coral, suggests that despite their single crystal character they have been constructed from nanosized units, and are therefore “mesocrystals”. Clearly, there is considerable synergy between the fields of biomineralisation and nanomaterial synthesis.