On the densification and hydration of CaCO3 particles by Q-switched laser pulses in water

•Dynamic synthesis and quenching of calcite II and calcium carbonate hydrates.•The habit planes of calcite II nanoparticles for {h k l}-specific coalescence.•Commensurate superstructure of defective calcite II due to displacive transformation.•UV–visible absorptions of the colloidal suspension containing dense calcite and hydrates.

Calcite powders subjected to Q-switched laser pulses in water were characterized by X-ray/electron diffraction and optical spectroscopy to have a significant internal compressive stress (up to ca. 1.5 GPa) with accompanied transformation into defective calcite II and hydrates. The defective calcite II particles were (0 1 0), (0 0 1), (0 1¯ 1), (0 1 3) and (0 1¯ 3) faceted with 2×(0 2 0)II commensurate superstructure and tended to hydrate epitaxially as monohydrocalcite co-existing with ikaite (CaCO3·6H2O) with extensive cleavages and amorphous calcium carbonate with porous structure. The colloidal suspension containing the densified calcite polymorphs and hydrates showed two UV–visible absorptions corresponding to a minimum band gap of ca. 5 and 3 eV, respectively.