Direct Nanoscale Imaging Reveals the Growth of Calcite Crystals via Amorphous Nanoparticles

The formation of calcite (CaCO₃), the most abundant carbonate mineral on Earth and a common biomineral, has been the focus of numerous studies. While recent research underlines the importance of nonclassical crystallization pathways involving amorphous precursors, direct evidence is lacking regarding the actual mechanism of calcite growth via an amorphous phase. Here we show, using <i>in situ</i> atomic force microscopy and complementary techniques, that faceted calcite can grow via a nonclassical particle-mediated colloidal crystal growth mechanism that at the nanoscale mirrors classical ion-mediated growth, and involves a layer-by-layer attachment of amorphous calcium carbonate (ACC) nanoparticles, followed by their restructuring and fusion with the calcite substrate in perfect crystallographic registry. The ACC-to-calcite transformation occurs by an interface-coupled dissolution–reprecipitation mechanism and obliterates or preserves the nanogranular texture of the colloidal growth layer in the absence or presence of organic (macro)­molecules, respectively. These results show that, in addition to classical ion-mediated crystal growth, a particle-mediated growth mechanism involving colloidal epitaxy may operate in the case of an inorganic crystal such as calcite. The gained knowledge may shed light on the mechanism of CaCO₃ biomineralization, and should open new ways for the rational design of novel biomimetic functional nanomaterials