Micro‐Raman spectroscopy of pigments contained in different calcium carbonate polymorphs from freshwater cultured pearls

Abstract

The metastable calcium carbonate polymorph vaterite has recently been found to occur commonly in freshwater cultured pearls from Japan and China. Organic pigment molecules in these vaterite regions of freshwater cultured pearls were analysed by resonance and near-resonance micro-Raman spectroscopy, in single spectra and in mapping modes and are compared with their aragonitic counterparts. Four different excitation wavelengths (487.9, 514.5, 532.2 and 632.8 nm) were used, resulting in differential increase of resolution for the pigments. Although vaterite areas were always lightly coloured (light yellow, light brown or white), ten different pigments were identified and a single colour is formed by a combination of three and up to nine polyenes of different lengths. The molecules were identified as unmethylated polyenes with effective chain lengths between 8 and 12 conjugated double bonds and FWHMs of 10 ± 2 cm −1 . Additionally, and for the first time in pearls, methylated polyenes were detected together with unmethylated pigments in aragonite. Polyenes with chain lengths shorter than 8 or longer than 12 were not found in vaterite, whereas both shorter and longer pigment chains were observed in aragonite from the same pearls. Pigments have lower concentrations in vaterite and are distributed more homogeneously, while they have higher concentrations in aragonite and are arranged along the annual growth lines of the pearls.

Raman spectroscopy of synthetic, geological and biological vaterite: a Raman spectroscopic study

“…Higher magnesium concentrations within vaterite are found in the organic-rich growth rings. This suggests that considerable amounts of Mg may be present as Mg organic species [31,44,45] rather than in the crystal lattice of vaterite. This may also explain the fact that synthetic vaterite reaches Mg saturation roughly at around 2000 ppm [45] (Materials and methods), while much higher values are observed in biominerals.…”

Experimental and theoretical studies on corals. I. Toward understanding the origin of color in precious red corals from Raman and IR spectroscopies and DFT calculations

“…These signals were described as fingerprints of mixtures of linear polyenes containing various numbers of double bonds. [19,20] Recently, Soldati et al [41] used micro-Raman spectroscopy to study polyenic pigments in two calcium carbonate polymorphs, aragonite and vaterite, from freshwater cultured pearls. Four different laser excitation wavelengths enabled selective enhancement or suppression of different bands, presumably originating from unmethylated polyenic pigments containing different number of conjugated double bonds.…”

Conjugated polyenes as chemical probes of life signature: use of Raman spectroscopy to differentiate polyenic pigments

“…Non-carotenoid linear conjugated polyenes, such as polyenals (or unsubstituted polyacetylenes) and laetiporic acids, are not as diverse as carotenoids, and they are restricted to a few taxonomic groups. For example, polyenals containing from six to nine conjugated double bonds, named psittacofulvins, were identified from parrots [12]; polyenals containing from six to 14 double bonds were described for pearls [20,21], mollusc shells [22][23][24] and octocorals [25][26][27][28][29][30]; and laetiporic acids [13] containing 10-12 conjugated double bonds were found in fungi. Synthetic polyenals present antioxidant, antitumoural and anti-inflammatory activity [31], and laetiporic acids are potential food colorants [13].…”