Polarized Raman Spectroscopy Reveals Unaligned Orientation in Pearls

Polarized Raman spectroscopy is a useful technique in studying orientation of molecular vibrations of crystals, which, however, has been applied insufficiently in crystallography, mineralogy, or geology. This present study is devoted to measure the transversal orientation of high-quality cultivated pearls using polarized Raman spectroscopy. The pearls were cultivated by Pinctada fucata in Japan, consisting of aragonite dominantly. On the pearl surface, the aragonite micro-crystals show irregularly hexagonal and pentagonal shapes, orienting their c axes towards the pearl surface. A number of aragonite micro-crystals makes up a lamina with a noncircular contour, and a batch of laminae stacks up with a decreasing area from bottom to top to form a target pattern. So there is an abundant of target patterns spreading over the pearl surface. Using a marker on the pearl surface as a reference, angular variation of aragonite is quantifiable by measuring mode intensities of aragonite and depolarization ratio. Polarized Raman images are collected on the pearl surface, which show spatial differences in orientation of aragonite micro-crystals on the mesoscopic scale. This transversal heterogeneity exists not only between different laminae but also between aragonite micro-crystals on the same lamina. We suggest that the development of the stress around the aragonite micro-grains lead to the angular variations of aragonite. The orientation of biominerals has been studied using transmission electron microscope (TEM) and electron back-scattered diffraction (EBSD). The present study indicates another sensitive technique of polarized Raman spectroscopy to evaluate molecular orientation in biominerals.