Vaterite

Vaterite is a polymorph of calcium carbonate (CaCO3). Alongside calcite and aragonite, it represents one of the three crystalline forms of CaCO3. At ambient conditions, vaterite is metastable and gradually converts to calcite over time.

Occurrence and formation: In nature, vaterite is rare. It has been reported in some carbonate rocks and marine biominerals, usually as a transient or locally stable phase. It can form in laboratory or industrial precipitations under high supersaturation or in the presence of specific impurities or organic molecules.

Structure and morphology: The crystal structure of vaterite is less well defined than that of calcite or aragonite; many studies describe it as hexagonal or pseudo-hexagonal. In practice, vaterite forms as very small crystals that tend to assemble into spherical, fibrous, or platy aggregates.

Stability and transformation: Under typical environmental conditions, vaterite slowly transforms to calcite, and sometimes to aragonite depending on chemistry. Transformation rates depend on temperature, pH, salinity, impurities, and the presence of organic molecules.

Applications and relevance: In materials science, vaterite is used as a synthetic precursor to calcium carbonate materials and as a component in nanoparticle systems for drug delivery due to its high porosity and dissolvability. In biology, some organisms deposit vaterite temporarily in biominerals or under abnormal environmental conditions, though it is not a common long-term phase in most natural carbonate structures.