In Situ Study Of Carotenoids In Nacre And Its Microstructure Characteristics

Nacre is one of the general construction units of mollusk shells, Thoroughly studying on it can provide insight not only into the pearl-culture, but also into fabrication of advanced organic-inorganic composite materials. In this paper, the nacre of Hyriopsis cumingii (Lea) which is one of the main freshwater pearl-forming mollusk in China is studied emphatically, as compared with the nacre of three species of saltwater pearl-forming mollusks, Pinctada martensii (DunkerX Pinctada maxima (Jameson )and Peteria penguin (Roding). The basic characteristics of nacreous and prismatic layers are understood by powder X-ray diffraction analysis ^ trace elements analysis ^ amino acid composition of protein and differential thermal analysis. Based on these results, main attentions are paid to the microstructure of nacre according to the study of X-ray diffraction > scanning electron microscopy -, transmission electron microscopy and micro-Raman spectra, and attentions also to the organic matrix especially the carotenoids in nacre according to in situ study of resonance Raman spectra. Finally, the effects of carotenoids in nacres or pearls on their commercial value, and the formation mechanism of nacres are discussed preliminarily in terms of the previous works and the results done in this paper.Main conclusions of this work are drawn as follows:1) In nacre of Hyriopsis cumingii ( Lea) shell, the aragonite crystallites exist two kinds of orientation, one, which is generally accepted, is the c axis of aragoniteperpendicular to nacreous layers, the second, which is proposed in this paper, is the [012] axis of aragonite perpendicular to nacreous layers. And also, these two crystal orientations varying with the different positions in the same nacre of shells clearly shows the structurally inhomogeneities of nacre. These results offer new insight into the study of the nacre formation.2) The carotenoids successfully probed in nacre by in situ resonance Raman spectra, are considered one of the most important factors responsible for the coloration of nacre and pearl. This suggestion not only provides new scientific evidence for the nacre used as pharmaceutical and hygienical products, but also is of significance to the bleaching of pearl so as to increase the quality and the commercial value of pearls.3) In shell of Hyriopsis cumingii (Lea) , the prismatic layers and the nacreous layers are all composed of aragonite minerals, and aragonite crystallites in prismatic layers show obviously preferential orientation of [012] axis perpendicular to the shell layers, so it is considered that the orientation of aragonite crystallites in prismatic layers perhaps play an important role in deciding the orientation of aragonite crystallites in nacre and acts as one of template to resulting the formation of aragonite with [012] orientation perpendicular to nacreous layers in nacre.