Micro-Raman investigation of pigments and carbonate phases in corals and molluscan shells

We report the results of a micro-Raman investigation, in resonance conditions, on the pigments and on the nature of the biogenic carbonate in different species of corals and marine shells of a wide variety of colours. Corallium rubrum (Anthozoa) and Stylaster roseus (Hydrozoa) show calcite and aragonite skeletons, respectively. The shells are mostly aragonitic, with the exception of bivalves of Pectinidae family which reveal a calcite/aragonite distribution. The main Raman features of the pigments (carotenoids or polyenes) show Raman dispersion (upshift of the C=C (ν1) and C–C (ν2) stretching vibration frequencies of the polyenic chain with decreasing excitation wavelength). Partially or completely demethylated polyenes with different conjugation lengths may be responsible for the colours of Corallium rubrum and of most of the marine shells. Stylaster roseus and the dark brown colours of Gibbula sp (Gastropod class) show typical bands of carotenoids, similar to those found in canthaxanthin. Overtones and combination bands are described in terms of the four fundamental vibrations of polyenes. A constant ratio between ν1 and ν2 frequencies, nearly independent of the Raman excitation wavelength, has been determined from a large set of data for corals and shells. A simple relationship between the main Raman frequencies and the effective conjugation length N of the polyenic chains is suggested.