Atomic Defects and Their Relationship to Aragonite-Calcite Transformation in Portlandite Nanocrystal Carbonation

The defects in calcium carbonate polymorphs forming when portlandite nanocrystals are exposed to high relative humidity were directly observed and analyzed under high-resolution transmission electron microscopy. The findings provided significant insight into aragonite to calcite phase transformation mechanisms. The aragonite lattice exhibited stacking faults, the insertion of extra atomic planes into its normal sequence, and vacancies. These defects were shown to generate minor lattice translation of around 1/3d(111aragonite) or nanovoids 2d(111aragonite) or 3/2d(020aragonite) in size. The presence of both extra atoms and nanovoids suggests the involvement of crystallization/dissolution processes. Calcite phase nucleation transformation mechanisms were observed as a result of the rearrangement of CO32- anions in areas characterized by the disorder resulting from aragonite lattice dislocations. A series of dislocations, stacking faults, and modulated microstructures were identified in the neoformed calcite nanocrystals, more intensely after shorter than after longer exposure times.