Figure 4

LC-PolScope analysis of nacre/prismatic transition. LC-PolScope ("Abrio™-Imaging") microscopy in transmission of a 60 μm section in orientation A. The birefringent retardance values (left, 0–273 nm from dark blue to dark red) and the orientation of the slow axis vector (right, red 0° / 180°, light blue 90° / 270°) are calculated for each pixel and color coded in the images. The two arrows in the left image indicate two differently oriented calcite prisms. Note that the marked light blue prism with a retardance value of ~109 nm appears orange in the orientation image (right), which corresponds to an azimutal angle of 17°. The marked red prism with a retardance value of ~267 nm appears purple in the orientation image (right), which corresponds to an azimutal angle of 129°. This analysis shows that the prisms consist of different domains, which can be distinguished according to their homogeneous crystallographic orientation. Not all of them are homogeneous. The tapering nacre layer gradually changes as a function of distance from the prismatic layer. Due to the blurred polarization signal, no structure is visible. Arrows in the left image indicate pixels with medium (ligh blue, 109 nm/17°) and high (dark red, 267 nm/129°) birefringent retardance contrast. P, prismatic layer; N, nacre.