Study on the Microstructure of the Pinctada martensii Pearls and Its Significance

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.