The results of a microscopy, SEM-EDS, XRD, FTIR, and Raman spectra study of the nacres of the Pinctada martensii pearls from Zhanjiang city, China shows that they can be classified as the high-quality, medium-quality, and inferior-quality pearls. Aragonite, the main inorganic mineralogy in the nacres, was crystallized and grown up in the compartments formed by the silk and radial organic sheets originating from organic matters secreted by the mantle of mollusks. The crystalline orientations of aragonite tablets were changed from the (002), (012) and (102) crystalline plane nets in the early to the (002) crystalline plane net only in the later. The formation processes of the microstructure of the nacres could be divided into three stages. In the early stage, the precursor particles of aragonite nucleated and grew up fast; then, porous aragonite aggregates consisting of the fine aragonite crystals were formed. In the middle stage, the aragonite crystals directionally grew up to form the aragonite tablets and microlayers. The surface of the aragonite tablets and microlayers are rough and few porous, and the edges of the crystals were serrated. In the last stage, the aragonite tablets in the aragonite microlayer mixed perfectly together to form high-quality aragonite layer whose surface was smooth and perfect. 1. Introduction The nacres of pearl, products of the biomineralization of the mollusks, are natural nano-organic-inorganic composite materials. Their special physical and chemical properties have excited more and more attentions of material scientists, mineralogists, and biologists. For instance, the nacres of pearl are widely used in the production of the pearl wool and pearl fiber because their break resistance performance can reach about 3000 times of that of the pure inorganic materials [1]. The special physical and chemical properties of the nacres of pearl depend on their unique microstructure whose formation process are typical self-assembly process of nanomaterials. Additionally, the degree of order of aragonite crystals in the nacres highly impacts the quality of the pearl. Currently, based on the above reasons, the research of the microstructure of the nacres of pearl has become an international hotspot and frontier subject. Up to now, to explain the formation mechanism of the microstructure of the nacres of pearl, the compartment theory [2], the bridge theory [3, 4], and the template theory [5, 6] have been established. Although the established modes are effective on the studies of the configuration and formation process of microstructural
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