Raman spectroscopy was used to study vaterite samples of biological, geological and synthetic origin. The Raman band positions and the full width at half-maximum (FWHM) of the lattice modes and the internal modes of the carbonate ion of all specimens show no significant differences between vaterites of different origin. With increasing Mg concentrations, synthetic vaterite samples show increasing FWHM in the region of the lattice modes and the three ν 1 bands, whereas no change in luminescence was detected. In contrast, in situ measurements of vaterite areas in freshwater cultured pearls (FWCPs) by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) together with the Raman spectra obtained at the same points show that the luminescence intensity in biological samples is affected by the magnesium content. The Raman spectroscopic features of vaterite and parisite-(Ce) are compared, and their similarities suggest that the structure of vaterite contains at least three crystallographically independent carbonate groups and similar carbonate group layers. A band at 263 cm −1 is observed for the first time in this study, whereas it could be demonstrated that bands previously reported at 130 and 190 cm −1 do not belong to vaterite.

Raman spectroscopy is a powerful tool in identifying different calcium carbonate polymorphs. Here, the method is applied to cultured pearls from freshwater (genus Hyriopsis) and marine bivalve species (Pinctada maxima) as well as to shells of Diplodon chilensis patagonicus bivalves. Raman spectra for vaterite, detected for the first time in an adult shell, and amorphous calcium carbonate (ACC) are discussed. Results for ACC are compared with those of synthetically produced ACC and with the Raman spectroscopic features of stable biogenic ACC from the crustacean Porcellio scaber. Decomposition of the most intense signal of all calcium carbonate polymorphs -the ν 1 symmetric stretching mode of the carbonate ion -leads to the identification of two polymorphs within the ACC areas: a mixure of an amorphous and a crystalline fraction. The amorphous phase is characterised by a broad peak in the region of the lattice modes, which is composed of two distinct lattice modes with very high full-widths at half-maximum (FWHMs). The FWHMs of most of the crystalline fractions (in the range of 6.3-10.7 cm −1 ) are too high for well-crystallised materials and support reports of nanocrystalline calcium carbonate polymorph clusters in ACC. Crystallinity indices of different samples are calculated and found to be useful to describe roughly the state of crystallisation in the ACC areas.

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Vaterite and aragonite polymorphs in freshwater cultured pearls from mussels of the genus Hyriopsis (Unionidae) were structurally and compositionally characterized by Raman spectroscopy, Micro computer tomography, high resolution field emission scanning electron microscopy, electron microprobe analysis and laser ablation inductively coupled plasma mass spectrometry. The appearance of vaterite in pearls is related to the initial stages of biomineralization, although we demonstrate that vaterite can not be a precursor to aragonite. It is not related to a particular crystal habit and therefore does not have a structural functionality in the pearls. Larger contents of elements typically bound to organic molecules, such as P and S in vaterite, as well as larger total organic contents in vaterite as opposed to aragonite in conjunction with larger concentrations of Mn2+ and Mg2+, imply a stabilizing role of organic macromolecules and X2+ ions for biological vaterite. Distribution coefficients between aragonite and vaterite for provenance-independent elements, such as Mn and Mg (0.27 and 0.04, respectively) agree very well with those observed in fish otoliths.

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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.

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