Abstract
Fish otoliths, or ear bones, are comprised of the CaCO3 polymorphs (aragonite, calcite and vaterite), which can occur either alone or in combination. The polymorph phase abundance in an otolith depends on, as yet, unexplained genetic and environmental factors. Most fish otoliths are comprised of the densest CaCO3 polymorph, aragonite. Sturgeon otoliths, on the other hand, contain significant amounts of the rare and the structurally enigmatic polymorph, vaterite. Sturgeon otoliths are frequently comprised of agglomerations of small microcrystalline vaterite spherulites (<300 μm in diameter), that range in shape from nearly perfect spheres to oblate spheroids. These spherulites are similar to the synthetic vaterite microspheres employed in laser trapping applications. Vaterite spherulites from both hatchery-reared (juvenile) and wild (adult) Lake Sturgeon exhibit extreme crystallographic texture as evidenced by X-ray diffraction patterns and their reconstructed pole-figures determined here. The vaterite crystallites making up the spherulites have excellent registry in both the axial and equatorial directions. Whether synthesized or natural, the texture manifested in these spherulites suggests that vaterite nucleates and grows similarly in vivo otolith formation as well as from laboratory synthesis. The uniaxial optical character of the vaterite spherulites, confirmed by these diffraction experiments and combined with their large birefringence, makes them well suited for laser trapping applications.
Highlights
Diffraction patterns of several vaterite spherulites larger than 100 μm from Lake Winnebago Lake Sturgeon showed the crystallographic preferred orientation to degrade for these larger spherulite sizes, which is generally consistent with the observation by Parkin et al.[5] that the optical retardation of synthetic vaterite spherulites increases with diameter but reaches a plateau beyond a diameter of ~10 microns
X-ray diffraction of juvenile and adult vaterite-rich Lake Sturgeon otoliths, constituting single spherulites, exhibit pronounced crystallographic preferred orientation, despite that the vaterite spherulites from Lake Sturgeon otoliths exhibit daily growth layers (Fig. 2)
The uniaxial optical character of vaterite spherulites is consistent with a sheaf-like texture, and this single crystal like form enables them to efficiently couple with laser light in laser trapping applications[7,8]
Summary
Both spherulites from both hatchery-reared (juvenile) and wild Lake Sturgeon (adult) exhibit extreme crystallographic texture. Keep in mind that these spherulites were arbitrarily mounted for the diffraction data collections. Diffraction patterns of several vaterite spherulites larger than 100 μm from Lake Winnebago Lake Sturgeon showed the crystallographic preferred orientation to degrade for these larger spherulite sizes, which is generally consistent with the observation by Parkin et al.[5] that the optical retardation of synthetic vaterite spherulites increases with diameter but reaches a plateau beyond a diameter of ~10 microns.
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