Pigments in Vaterite in Freshwater Cultured Pearls: a Micro Raman Spectrometry Study.

SOLDATI, ANALIA L.; JACOB, DORRIT E.; WEHRMEISTER, URSULA; HÄGER, TOBIAS; HOFMEISTER, WOLFGANG

Bangkok and Kanchanaburi, Thailand

Conferencia; The 2nd International Gem & Jewelry Conference (GIT 2009); 2009

Different polymorphs of calcium carbonate have been reported to occur in freshwater cultured pearls from China and Japan. Vaterite, a meta-stable polymorph, can be found as lightly colored (light yellow, light brown or white) independent of the colour of the pearl, well defined areas near the centre of the pearls, but also as spots in the surface of good quality pearls. In this study we investigated the nature of the vaterite-related pigments by Micro Raman Spectrometry and compare them to the well known aragonite-related natural pigments in the same pearls. Using different excitation energies we increased resolution to identify the pigments of polyenic origin and were able to decipher the different polyene lengths in a mixture. Vaterite and aragonite pigments were identified as mixtures of unmethylated polyenes with effective chain lengths between 8 and 12 conjugated double bonds and FWHMs of 10 ± 2 cm-1. As expected from the lighter colour, the pigment molecules are less concentrated in vaterite and are distributed more homogeneously than in aragonite, which has higher concentrations and where the pigments are clearly arranged along the annual growth lines of the pearls. In addition, pigment mixtures present in aragonite are richer in components than those related to vaterite areas. Nevertheless, a single spot of vaterite is formed by combination of three and up to nine components of different lengths. Polyene chains shorter than 8 or longer than 12 were not found in vaterite, whereas both cases occur in aragonite areas from the same pearls.