Structure and provenance of freshwater cultured pearls

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

Budapest, Hungry

Congreso; IMA2010, 20th General Meeting of the International Mineralogical Association; 2010

International Mineralogical Association

Freshwater pearls, cultured in bivalves of the genus Hiriopsis cumingii have been increasing constantly in quality and size over the last years. Main producing countries are China and Japan, but other countries, e.g. Thailand, are also known to culture freshwater pearls. We review here the micro- and nanostructure of pearls that lead to their extra-ordinary material properties including the occurrence of vaterite which can cause considerable loss of quality and value [1,2]. Vaterite in freshwater cultured pearls has higher concentrations of organic material and higher manganese contents than aragonite. Most extreme values of ca. 3 wt% Mn are found in so-called lacklustre pearls [3] which consist entirely of vaterite. Freshwater bivalve shells incorporate Mn seasonally connected with algal blooms and it can therefore be suspected that water quality plays a significant role in the formation of vaterite in pearls. Environments for pearl culturing are very different in China, where pearl farms of different sizes are located along the Yangtse Rivers, and in Japan, where freshwater are cultured traditionally in Lake Biwa and Lake Kasumigaura. These differences are mirrored by the trace element chemical composition of the pearls. Trace element concentrations in the pearls were measured in situ by LA-ICP-MS. This method allows simultaneous measurement of ca. up to forty minor and trace elements with sub-ppm detection limits [4]. Laser craters of 100 μm or less are created on the surfaces of the samples and are hardly visible by eye. Additionally, Sr isotopic compositions were measured in situ in the pearls by LA-MC-ICP-MS to explore the potential of this geochemical tool for provenance determination. Best discrimination between the different sample localities was achieved with Ba/Sr ratios vs B concentrations [5]. While Chinese freshwater pearls have higher and more variable Ba/Sr ratios, those from the Japanese lakes each occupy rather confined fields at lower Ba/Sr ratios. The field for pearls from Lake Biwa (Japan) overlaps with the Chinese sample population but Biwa pearls trend to lower boron concentrations than Chinese samples. Sr isotopic ratios range between 0.70759 and 0.71237. Lake Biwa samples show the most radiogenic compositions, while those from Lake Kasumigaura have the lowest 87Sr/86Sr ratios.[1] Jacob, D.E. et al. (2008) Geochim. Cosmochim. Ac., 72(22), 5401-5415. [2] Wehrmeister, U. et al. (2007) J. Gemmol., 31, 399-416. [3] Qiao, L. et al. (2006) Cryst. Growth Des., 7, 275- 279. [4] Jacob, D.E. (2006) Geostand. Geoanal. Res., 30(3), 221-235. [5] Jacob,