EXPLORING CONTROLS FOR POLYMORPHISM IN VATERITE

Vaterite is the least stable, least abundant in geologic environments, and more often of biogenic origin, as compared to the more familiar polymorphs of calcium carbonate, calcite and aragonite. Biogenic vaterite can be found in or mediated by a wide range of organisms, such as coral, mollusks, fish (otoliths), birds (eggs), fungus, and bacteria. Aragonite is most common in fish otoliths, however the occurrences of vaterite and calcite in fish otoliths are more common than generally realized, because quantitative methods (XRD, Raman spectroscopy, polarized light optical microscopy, or neutron diffraction) are required for exact conformation but are not always available or used. We have been studying vaterite in otoliths by various materials science probes to help constrain the structural systematics of vaterite and understand the impacts of calcium carbonate polymorph abundance and their microstructure on studies of microchemistry and fish stock assessments used by fisheries biologists. Despite many studies of synthetic, biogenic and mineral samples, the exact crystal structure and possibility of polymorphism in vaterite lack consensus. Here we review and explore the various controls (synthesis route, temperature, pressure, composition, etc.) for possible polymorphism in vaterite. A portion of this research used resources at the High Flux Isotope Reactor and Spallation Neutron Source, DOE Office of Science User Facilities operated by the Oak Ridge National Laboratory.