The ikaite-to-vaterite transformation: new evidence from diffraction and imaging

Vaterite is one of three polymorphs of calcium carbonate (CaCO₃) found in nature, the others being calcite and aragonite. Here the formation of vaterite from decomposition of ikaite (CaCO₃·6H₂O) was investigated using synchrotron powder diffraction and scanning electron microscopy. The crystallite sizes of vaterite (∼40 nm) were found to be much smaller than those of the precursor ikaite (∼0.5–1.0 µm) as a result of vaterite nucleating as ikaite dehydrates. The rate of decomposition to vaterite increases with temperature, indicating kinetic control of this transformation. It is postulated that the structural arrangement of the hydration sphere around Ca²⁺ in ikaite determines the orientation of Ca²⁺ and CO₃²⁻ ions such that vaterite nucleates upon dehydration. This implies that the dehydration of a precursor hydrated phase such as ikaite is required for vaterite nucleation.