Non-destructive Analytical Study of Raman Spectra Variations and Mechanisms of Calcite and Aragonite in Modern and Fossilized Oysters.

Oyster fossils are some of the most common bivalve mollusk fossils found all over the world, they are different from other fossils because the oyster is still alive in the present day, and the body structure of modern oyster is almost the same as that of ancient one. Therefore, we designed a control experiment comparing the Raman spectra of minerals from both modern oysters and fossil oysters to explore the mechanism of oyster's fossilization process, which is considered to be helpful for investigating biological evolution or paleoenvironment. The oyster fossil sample was found in Nagi-Cho, Okayama Prefecture, Japan. We focused on the variations of band position and full width half-maximum of ν₁ Raman band (symmetric stretching mode) of calcite (CaCO₃) from modern and fossil oysters and the mineral conversion between calcite and aragonite (CaCO₃) around the adductor muscle inside the oyster. Compared to modern oysters, the ν1 band at around 1086 cm^-1 of calcite from oyster fossils shifted to a high wavenumber region, and the possible reason for this phenomenon is considered an elemental substitution between Ca²⁺ and Mg²⁺. As for aragonite around adductor muscle in fossil oysters, it has been found by Raman spectra that most of the aragonite has been converted into calcite because calcite has a relatively more stable structure.