EFFECTS OF ORGANIC COMPOUNDS ON THE PRECIPITATION AND STABILIZATION OF VATERITE

Vaterite is a metastable calcium carbonate (CaCO₃) polymorph associated primarily with biologically mediated structures, and, more rarely, cold, alkaline environments. Although vaterite is a relatively common biomineral in invertebrates, there are only a handful of extant vertebrates, such as sturgeon and paddlefish, that are associated with the precipitation of vaterite. These fish produce earbones (termed otoliths) comprised of both calcite and vaterite. The accretionary structure of otoliths — which preserve the history of fish growth and environmental habit — suggest that vaterite can remain stable through the lifetime of these fish (>50 years). Experimentally grown vaterite, however, is extraordinarily unstable in fluid solutions and converts readily to calcite (i.e. in hours to days) under a range of temperature and pH conditions. Because vaterite has been shown to be stabilized by acidic compounds, we hypothesize that otolith-associated vaterite is stabilized by endolymph fluids within the ear structure.

Here, we explore the potential stabilization of vaterite by organic compounds commonly found in endolymph fluids. Vaterite was precipitated in the laboratory under a variety of environmental conditions and reagent concentrations. Precipitates were assessed on the bases of crystal volume, weight, crystal size distribution, ratio of vaterite to calcite, and the conversion rates of vaterite when left in the mother solution. Measurements were conducted using a combination of X-ray diffraction, petrographic microscopy, and digital image analyses. Crystallization experiments were then repeated under the same conditions but with the addition of a variety of organic compounds that commonly occur in fish endolymph (e.g., amino acids, hyaluronic acid, and various proteoglycans). Crystals precipitated in the presence of these organic additives were assessed using the same criteria as those in the control group and subsequently used to determine how discrete organic compounds affect the precipitation and stabilization of vaterite.