应用扫描电镜-红外与反射光谱研究海水灰色Akoya珍珠特异的矿化微结构特征

近年来海水灰色Akoya珍珠是珍珠类饰品的新宠,前期研究主要聚焦在对其海水属性、各结构单元的元素赋存特征、辐照处理及其鉴别方法等方面。本文借助紫外可见(UV-Vis)反射光谱仪、显微红外光谱仪、扫描电镜等技术,对具有白色内核的一类灰色珍珠的宝石学与其精细结构特征进行研究。结果表明:①在珍珠的珠核与珍珠层之间,基本存在厚约几十微米的褐色有机质过渡层。在珠核至珍珠表面的径向上,靠近褐色过渡层的珍珠层区域中存在无特定形态区域,该区域物相组成所对应的红外光谱中可见分别归属球文石(约1444cm^-1、887cm^-1)与方解石(约1410cm^-1、872cm^-1、708cm^-1)的特征吸收。同时,在接近珍珠表面处的珍珠层中,文石板片形貌较不规则;而在珍珠径向上并位于珍珠层的中间区域中,文石则呈规则的板片形貌,且自内核至珍珠表层方向上,文石板片的厚度呈现渐薄特征。②整个珍珠表面的反射光谱与外层单一的珍珠层的光谱特征一致,上述褐色过渡层对整个珍珠的UV-Vis反射光谱无直接影响,因此该褐色过渡层是否对珍珠呈现灰色产生影响有待进一步商榷。本研究工作对灰色Akoya珍珠的呈色机理探究及形成属性的鉴别具有较重要的指导意义,同时可进一步丰富人们对具有0.3~0.6mm薄层珍珠层的珍珠品类精细结构及矿化特征的认知。

Study on the Unique Mineral Microstructure of Seawater Cultured Gray Akoya Pearl by SEM, FTIR and Reflection Spectroscopy

BACKGROUND: Seawater cultured gray Akoya pearls have become popular as jewelry in the recent years. In the early stage, some research focused mainly on investigating the cultured environment of seawater or freshwater pearls, element occurrence characteristics of each structural unit, irradiation treatment and the identification method of irradiated pearls.
OBJECTIVES: To further study the gemological characteristics and fine microstructure of a type of gray pearl with a white nucleus.
METHODS: Ultraviolet-visible reflection spectrum, micro-infrared spectrum and scanning electron microscope methods were used.
RESULTS: A brown transition layer of organic matter between the nacre and nucleus was discovered, which measures several microns in thickness. A layer with no fixed morphology composed of calcite and vaterite in the nacre near the brown transition layer was also discovered. Quasi plates of aragonite exist in the nacre near the surface of the pearl. The morphology of these aragonite tablets in the middle area of the nacre was more regular, the thickness of individual aragonite plate gradually decreased in the direction from the nucleus to the surface of the pearls. The reflectance spectrum of the entire pearl surface was consistent with the spectral characteristics of the outer single nacre. The brown transition layer had no direct effect on the UV-Vis reflectance spectrum of the entire pearl. Therefore, whether or not the brown transition layer affected the gray appearance of the pearl needs further discussion.
CONCLUSIONS: The research work has important guiding significance for the coloring mechanism of gray Akoya pearls and the identification of the formation attributes. It can also aid in the recognition of the fine structure and mineralization characteristics of pearls with a thin layer of nacre of 0.3mm to 0.6mm.