Abstract
BackgroundMollusc shells are commonly investigated using high-resolution imaging techniques based on cryo-fixation. Less detailed information is available regarding the light-optical properties. Sea shells of Haliotis pulcherina were embedded for polishing in defined orientations in order to investigate the interface between prismatic calcite and nacreous aragonite by standard materialographic methods. A polished thin section of the interface was prepared with a defined thickness of 60 μm for quantitative birefringence analysis using polarized light and LC-PolScope microscopy. Scanning electron microscopy images were obtained for comparison. In order to study structural-mechanical relationships, nanoindentation experiments were performed.ResultsIncident light microscopy revealed a super-structure in semi-transparent regions of the polished cross-section under a defined angle. This super-structure is not visible in transmitted birefringence analysis due to the blurred polarization of small nacre platelets and numerous organic interfaces. The relative orientation and homogeneity of calcite prisms was directly identified, some of them with their optical axes exactly normal to the imaging plane. Co-oriented "prism colonies" were identified by polarized light analyses. The nacreous super-structure was also visualized by secondary electron imaging under defined angles. The domains of the super-structure were interpreted to consist of crystallographically aligned platelet stacks. Nanoindentation experiments showed that mechanical properties changed with the same periodicity as the domain size.ConclusionsIn this study, we have demonstrated that insights into the growth mechanisms of nacre can be obtained by conventional light-optical methods. For example, we observed super-structures formed by co-oriented nacre platelets as previously identified using X-ray Photo-electron Emission Microscopy (X-PEEM) [Gilbert et al., Journal of the American Chemical Society 2008, 130:17519–17527]. Polarized optical microscopy revealed unprecedented super-structures in the calcitic shell part. This bears, in principle, the potential for in vivo studies, which might be useful for investigating the growth modes of nacre and other shell types.
Highlights
Mollusc shells are commonly investigated using high-resolution imaging techniques based on cryo-fixation
Shells were embedded in two different orientations: For orientation A, the polished section was horizontal to the coronal plane of the shell (Figure 1, left), which results in an inclination angle between the normal of each nacre platelet and the normal of the polished surface
The lamellar structure of the nacreous part in the μm range is only visible in orientation A (Figure 2a,c), whereas the prismatic calcite part is characterized by structures in the sub-mm range which are best visualized in orientation B (Figure 2b,d)
Summary
Mollusc shells are commonly investigated using high-resolution imaging techniques based on cryo-fixation. Sea shells of Haliotis pulcherina were embedded for polishing in defined orientations in order to investigate the interface between prismatic calcite and nacreous aragonite by standard materialographic methods. In order to study structural-mechanical relationships, nanoindentation experiments were performed. The mollusk shell is one of the most ancient examples of such a hierarchical material [7,8,9,10,11]. Known structures are nacre, and prismatic and crossed-lamellar shell types [15,16]. Many aspects regarding the optical and mechanical properties of mollusk shells are well understood [12,17]. The most studied shell type is nacre [18,19,20,21]
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