Solid-State Phase Transformation and Self-Assembly of Amorphous Nanoparticles into Higher-Order Mineral Structures

Digging into nonclassical pathways to crystallization to unearth design principles for fabricating advanced functionalized materials shapes the future of materials science. Nature has long since been exploiting such nonclassical pathways to crystallization to build inorganic-organic hybrid materials that fulfill support, mastication, defense, attack, or optical functions. Especially, various biomineralizing taxa such as stony corals deposit metastable, magnesium-rich, amorphous calcium carbonate nanoparticles that further transform into higher-order mineral structures. Here we examine whether a similar process can be duplicate in abiogenic conditions using synthetic, amorphous calcium magnesium carbonate nanoparticles. Applying a combination of ultrahigh-resolution imaging, and, in situ, solidstate nuclear magnetic resonance (NMR) spectroscopy, we reveal the underlying mechanism of the phase transformation of these synthetic amorphous nanoparticles into crystals. When soaked in water, these synthetic amorphous nanoparticles are coated by a rigid hydration layer of bound water molecules. In addition, fast chemical exchanges occur between hydrogens from the nanoparticles and those from the free water molecules of the surrounding aqueous medium. At some stage, crystallization spontaneously occurs, and we provide spectroscopic evidence for a solid-state phase transformation of the starting amorphous nanoparticles into crystals. Depending on their initial chemical composition, and especially on the amount of magnesium, the starting amorphous nanoparticles can aggregate and form ordered mineral structures through crystal growth by particle attachment, or rather dissolve and reprecipitate into another crystalline phase. The former scenario offers promising prospects for exerting some control over such non-classical pathway to crystallization to design mineral structures that could not be achieved through a classical layer-by-layer growth.