Identification of Vaterite in cultured pearls by non-destructive methods

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

Hanoi, Vietnam

Workshop; The Fourth International Workshop on Geo- and Material-Science on Mineral Resources of Vietnam; 2008

Vaterite not only occurs at the surface, but also in the internal parts of freshwater cultured pearls from Hyriopsis cumingii and Hyriopsis cumingii hybridized with Hyriopsis schlegeli. Strontium and Sodium concentrations are significantly lower in vaterite compared with adjacent aragonite whereas the Mg concentration is up to two orders of magnitude higher. These large differences in Mg/Na ratios allow identification of these two polymorphs even without Raman spectroscopy. In addition, vaterite containing areas could be identified by different intensities zones in 2D and 3D µ-CT maps. This contrast in the µ-CT tomograms may be caused rather by higher concentrations of organic molecules in the vaterite containing area than by the difference between the densities of aragonite and vaterite. However, it is a fast and non-destructive method to identify the area in the pearl in which vaterite occurs, before Ra-man spectroscopy is used to map the exact vaterite distribution. The occurrence of vaterite close to the centres of many pearls and commonly close to the remains of organic material derived possibly from implanted tissue implies that vaterite plays a significant role in the biomineralization process.