In Situ Raman Spectra Of Freshwater Cultured Pearls And The Phase Transformation Of Its Inorganic Cystals

In this paper, Chinese freshwater cultured pearls from Zhejiang Province is studied emphatically, at the same time eggshells of Pomacea canaliculata is analyzed to compare with pearls. Basic structure and chemical composition of pearls are understood by scanning electron microscopy , resonance Raman spectra, X-ray diffraction and FTIR spectra. Finally, phase transformation of calcium carbonate polymorphs are studied preliminarily by DSC.Main conclusions of this work are drawn as follows: (1) In situ resonance Raman spectra of Chinese freshwater pearls are collected and analyzed systematically with three different excitation wavenumbers at λ=514nm, 633nm and 785nm. Meanwhile, the Raman spectra of eggshells of Pomacea canaliculata have also been detected under the same experimental conditions in order to compare with pearls' Raman spectra. We get the following results: Frequency dispersions of Raman shifts by organic pigments are obviously observed in Raman spectra of pearls. In contrast, Raman spectra of organic pigments in eggshells ofPomacea canaliculata have no frequency dispersion phenomena, it is considered that the organic pigments in pearls are polyacetylenic materials but not carotenoids which was proposed by the previous researchers, and the organic pigments in eggshells of Pomacea canaliculata are carotenoids; The conjugated C=C number of polyacetylenic material in pearls is calculated to be about 10 and 16, and the conjugated C=C number of carotenoid in eggshells of Pomacea canaliculata is about 13 based on the Raman shifts resulted by conjugated C=C double bonds(2) The relative concerntration of this organic material is closely associated with the color of the pearls. Raman band's relative intensity of polyacetylenic material is smaller in lighter color sample. The corresponding Raman band is not detected in the white pearls.(3) Different quality pearls are studied by X-ray diffraction and in situ Raman spectra. The results show that frequency dispersions of Raman shifts in inorganic material are not probed and high quality pearls mainly contain atagonite; however low quality pearls mainly contain vaterite. So the quality of pearls is closely associated with its calcium carbonate polymorphs.(4) DSC is used to initially study the phase transformation of aragonite and vaterite in pearls, the results are quite different from natural aragonite and synthetic vaterite. The phase transformation from natural aragonite into calcite is an endothermal process, but this process of aragonite inpearls is exthermal, and the tempreture is lower than mineral aragonite for about 90℃; though the phase transformation process of biomineral vaterite into calcite is exthermal which is the same as the synthetic vaterite, the temperture is lower than it for about 100℃. It is presumed that the grains of calcium carbonate in pearls are up to nono-meter so that there is excessive free energy in lattice which is the main reason of this phenomenon.