Spectroscopic characterisation of biological vaterite: relations to synthetic and geological vaterites

The pair aragonite and calcite are some of the most intensively studied polymorphous minerals. These CaCO3 polymorphs are most commonly observed in biological minerals produced by marine molluscs, whereas in freshwater molluscs mostly aragonite and vaterite, the third CaCO3 polymorph is identified (e.g. Wehrmeister et al., 2007). Vaterite is the thermodynamically most unstable CaCO3 polymorph and is often discussed as a precursor phase in the mineralization of aragonite or calcite by organisms. Apart from these biological parageneses, vaterite is also known as rare small polycrystalline aggregates from geological occurrences. In laboratory crystallisation experiments, vaterite can be stabilized either kinetically or with the help of organic macromolecules (e.g. Falini et al., 2005).