Micro-structure, composition and implications of Vaterite in bio-minerals: The case of freshwater cultured pearls.

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

Cambridge, UK

Conferencia; Frontiers in Mineral Sciences 2007; 2007

Fitzwilliam College and the Department of Earth Sciences, University of Cambridge

Tissue- and bead- nucleated freshwater cultured pearls from China and Japan, grown in mussels of the genus Hyriopsis, were studied for their micro-structure and chemical composition. Freshwater cultured pearls are formed in the same way as the mussel shell by continuous growth of aragonite platelets embedded in an organic matrix [1]. Growth of the pearl is initiated by introducing a small piece of mussel mantle epithelial layer into a protected space in the mussel’s body (e.g. the gonad) and progresses in regular concentric rings until the pearl is extracted after about 6-7 years. Resonance Raman Spectroscopy of polished cross-sections of pearls revealed that ca. 50% of the samples contained vaterite (the most unstable polymorph of calcium carbonate) in addition to the aragonite matrix. The vaterite areas of good-quality pearls are relatively small (1-1.5 mm diameter), spherical to irregular and always in close vicinity 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 [2, 3]. To further study the micro-structure of the vaterite zones, cross-sections were etched with Mutvei’s solution [4]. This etching method softly dissolves the calcium carbonate with acetic acid, while the organic matrix is stabilized with glutaraldehyde and a dye colors the pearls with different intensity that highlights the organic regions. SEM analyses of the etched surfaces show that growth rings transect the vaterite areas, implying that vaterite and aragonite grew simultaneously, and that vaterite was not 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. Sr concentration in vaterite (250 ppm) is about the half of that found in aragonite, while for Mn, Zn and Ba no significant differences were found. 1. Urmos J, Sharma SK, Mackenzie FT (1991) American Mineralogist 76: 641-646. 2. Ma HY, Lee IS (2006) Materials Science and Engineering C 26: 721 – 723. 3. Qiao L, Feng Q-L, Li Z (2006) Cristal Growth and Design 1-5. 4. Mutvei H, Dunca E, Timm H, Slepukhina T (1996) International Symposium on Biomineralization. Monaco p.65-72