Empirically Testing Vaterite Structural Models Using Neutron Diffraction and Thermal Analysis

Abstract

Otoliths, calcium carbonate (CaCO 3 ) ear bones, are among the most commonly used age and growth structures of fishes. Most fish otoliths are comprised of the most dense CaCO 3 polymorph, aragonite. Sturgeon otoliths, in contrast, have been characterized as the rare and structurally enigmatic polymorph, vaterite-a metastable polymorph of CaCO 3 . Vaterite is an important material ranging from biomedical to personal care applications although its crystal structure is highly debated. We characterized the structure of Lake Sturgeon otoliths using thermal analysis and neutron powder diffraction, which is used non-destructively. We confirmed that while Lake Sturgeon otoliths are primarily composed of vaterite, they also contain the denser CaCO 3 polymorph, calcite. For the vaterite fraction, neutron diffraction data provide enhanced discrimination of the carbonate group compared to x-ray diffraction data, owing to the different relative neutron scattering lengths, and thus offer the opportunity to uniquely test the more than one dozen crystal structural models that have been proposed for vaterite. Of those, space group P6 5 22 model, a = 7.1443(4)Å, c = 25.350(4)Å, V = 1121.5(2) Å 3 provides the best fit to the neutron powder diffraction data, and allows for a structure refinement using rigid carbonate groups.Vaterite, a metastable polymorph of calcium carbonate (CaCO 3 ) 1 is of substantial interest as a naturally occurring biomaterial, and for its use as an additive in various consumer products ranging from paper and coatings to plastic and elastomer reinforcement to food supplements, oral hygiene aids, and cosmetics 2 . For instance, vaterite's greater ability to dissolve in body fluids compared to other polymorphs of CaCO 3 makes it desirable for manufacturing nanocapsules for delivering drugs at the cellular level 3 . Despite the realized and potential importance of vaterite to humans, some of the most basic facts about this compound such as its crystalline structure have challenged researchers. Currently, more than a dozen crystal structure models have been proposed with little weight favoring one over the others 1 . Many basic properties of vaterite are unresolved due in part to the rarity of naturally occurring vaterite. Naturally occurring vaterite structures include human gallstones and portions of fish ear bones, or otoliths, from primitive fishes such as sturgeons 4 or sporadically from more modern fishes such as salmon and trout, and are potentially formed in response to physiological stress, especially thermal stress 5 and captive rearing 6,7 . In fact, only a few reliable sources of biogenic vaterite exist for study including sea squirt spicules 8,9 and otoliths of sturgeons, but only rarely those of other fish or animals. The ability of making biogenic vaterite has apparently been retained through evolution in some fish such as salmonids because these fishes sometimes have vaterite patches in their otoliths 4,10 , but sturgeons are really the sole reliable source of vaterite from fish otoliths. Unlike...

Otoliths of sub‐adult Lake Sturgeon Acipenser fulvescens contain aragonite and vaterite calcium carbonate polymorphs

“…Calcite, aragonite and vaterite are the crystalline polymorphs of CaCO 3 and make up the primary composition of otoliths (Campana, ; Carlström, ; Gauldie, ; Pracheil et al ., ). Importantly, crystal densities and lattice structures vary with each polymorph and, thus, the concentration per unit volume of Ca also varies, with aragonite being the densest and vaterite being the least dense (Chakoumakos et al ., ). At present, studies investigating otolith chemistry assume CaCO 3 polymorph composition is homogeneous across the entire otolith, which can potentially confound inferences that are made on life‐history events when interpreting these chemical signatures (Melancon et al ., ; Pracheil et al ., ; Tzeng et al ., ).…”

“…Quantitative phase analysis by the Rietveld method (Rietveld, ) was done using the Fullprof Suite software package (Figure d; Rodríguez‐Carvajal, ), using reference structural data for aragonite from the Inorganic Crystal Structure Database (http://www2.fiz-karlsruhe.de/icsd_home.html) and the structural data of vaterite proposed by Wang and Becker () and empirically validated for adult Lake Sturgeon otoliths by Chakoumakos et al . ().…”

Flume study on the effects of microbial induced calcium carbonate precipitation (MICP) on the erosional behaviour of fine sand

“… Presence of CaCO 3 phases other than calcite, e.g. vaterite, with an equivalent density to silica sand (2.65g/cm 3 ) (Chakoumakos et al, 2016). While their presence could not be captured by XRCT, these would have been included in the SEM and CaCO 3 content analyses.…”

Morphology and composition of Goldeye (Hiodontidae; Hiodon alosoides) otoliths

“…We determined otolith polymorph composition from neutron diffraction data using the Rietveld method (Rietveld, 1969) with GSAS II software (Toby & Von Dreele, 2013). Structural data for aragonite and calcite were obtained from the Inorganic Crystal Structure Database (http://www2.fizkarlsrudhe.de/icsd_home.html) and structural data for vaterite were proposed by Wang and Becker (2009) and empirically validated with Lake Sturgeon ( Acipenser fulvescens Rafinesque, 1817) otoliths by Chakoumakos et al (2016). The reported uncertainties in the fractions when more than one polymorph was present are from the least‐squares fitting used in the Rietveld method.…”