Processes of biomineralization in freshwater cultured pearls

JACOB, DORRIT E.; SOLDATI, ANALIA L.; WEHRMEISTER, URSULA

Cologne, Germany

Conferencia; Goldschimdt Conference 2007 "Atoms to Planets"; 2007

German Mineralogical Society

Freshwater cultured pearls formed by the mussel Hyriopsis represent excellent examples to study processes of biomineralization in molluscs. They are grown by inserting a piece of epithelial tissue from the mantle lobe into the mussel where processes identical to those forming the mussel shell occur. Resonance Raman Spectroscopy of polished pearl crosssections revealed that ca. 50% of the samples contain vaterite in addition to aragonite. Vaterite is the thermodynamically most unstable polymorph of CaCO3 and is often discussed as a precursor phase in the mineralization of aragonite or calcite by organisms (Weiss et al., 2002). Vaterite forms relatively small areas in high quality pearls (1-1.5 mm diameter) which are spherical to irregular and always in close proximity to the center of the pearl. However, in low-quality pearls, they can be much larger, sometimes comprising the major part of the pearl, including the surface (Ma and Lee, 2006; Qiao et al., 2006). To further study the micro-structure of the vaterite zones, cross-sections were etched with Mutvei’s solution (Schoene et al., 2005). This etching method dissolves the calcium carbonate surface with acetic acid, while the organic matrix is stabilized with glutaraldehyde and Comassie Blue colours the organic material in intensities which correspond to the concentration of the organics. SEM analyses of the etched surfaces show that growth rings transect the vaterite areas, implying that vaterite and aragonite grew simultaneously, and that vaterite may not have been an initial template for aragonite growth. LA-ICP-MS and electron microprobe were used to quantify minor and trace elemental differences between the calcium carbonate polymorphs. Vaterite areas have about 1400 ppm Mg and 1000 ppm Na, versus around 30 ppm Mg and 1650 ppm Na found in aragonite. The Sr concentration in vaterite (250 ppm) is about half of that found in aragonite, while for Mn, Zn and Ba no significant differences were detected.   References Ma H.Y. and Lee I.S. (2006) Materials Science and Engineering C 26: 721 – 723. Qiao, L., Feng, Q.-L. and Li, Z. (2006) Crystal Growth and Design 7(2): 275-279. Schoene, B.R.; Dunca, E.; Fiebig, J. and Pfeiffer, M. (2005) Palaeogeography, Palaeoclimatology, Palaeoecology; 228: 149-166. Weiss I.M., Tuross N., Addadi L., Weiner S. (2002) J Experimental Zoology 293: 478-491.