马达加斯加黄色方柱石的谱学特征研究

利用常规宝石学仪器、电子探针、傅里叶变换红外光谱仪、激光拉曼光谱仪、紫外-可见分光光度计和三维荧光光谱仪等，对马达加斯加黄色方柱石的宝石学性质及谱学特征进行了研究分析。马达加斯加方柱石的宝石学特征与方柱石理论值基本一致；方柱石样品颜色均匀，具有玻璃光泽，原石晶型较为完好且表面普遍可见纵纹及褐红色杂质，部分样品表面可见晕彩效应，样品内部可见多种包裹体，如黑云母、无色晶体包裹体等。红外光谱分析结果表明，样品在指纹区均显示1 039，1 105和1 196 cm-1处 Si（Al）—O伸缩振动吸收峰；752 cm-1处Si—Si（Al）伸缩振动吸收峰；551，687和624 cm-1处O—Si (Al)—O 弯曲振动吸收峰；459 cm-1处Si—O—Si的弯曲振动与Na（Ca）—O伸缩振动的耦合吸收峰；416 cm-1处Si—O—Si弯曲振动吸收峰。红外光谱官能团区的诊断性鉴定依据为：3 530和3 592 cm-1处O—H振动引起的吸收峰；2 499，2 629和2 964 cm-1处CO2-₃振动产生的吸收峰。拉曼光谱分析结果表明，桥氧弯曲振动产生459和538 cm-1两处吸收峰；Al—O振动导致775 cm-1吸收峰；硅氧四面体Q₄结构单元振动产生1 114 cm-1吸收峰。紫外-可见光吸收光谱可知，马达加斯加方柱石为过渡金属元素致色，铁离子的存在导致了379和420 nm两处吸收峰，且420 nm吸收峰的强弱影响着方柱石的颜色深浅。致色原因为占据了晶体结构中四面体位置的Fe2+与Fe3+之间电荷转移，从而产生黄色。三维荧光光谱分析显示，方柱石具有较为一致的发光行为，均可见一强一弱两个荧光峰，多集中在302 nm(λ_ex)/343 nm(λ_em)附近。电子探针成分分析结果表明样品属于方柱石族系列中的针柱石，Ma值范围为66%～69%, 平均Ma值为68.1%，且随着Ma值的增高，双折射率随着变小。谱学测试作为无损测试技术，适用于鉴定宝石品种。对鉴定马达加斯加方柱石具有重要的意义，同时为产地溯源、区分优化处理品种提供数据支持。

Study on the Spectral Characteristics of Scapolite From Madagascar

The gemological and spectroscopic properties of the Madagascar scapolite is studied based on analyses of fifteen samples using EMPA, FTIR, Raman spectroscopy, UV-Vis Spectroscopy, fluorescence spectroscopy and some gemological instruments. The gemological characteristics of Madagascar scapolite are consistent with the theoretical values of scapolite; The samples are uniform in color and have a glassy luster. The raw stone crystal is relatively intact. Longitudinal stripes and maroon impurities are commonly seen on the surface of them. Iridescence can be seen on the surface of some samples, and a variety of inclusions can be seen inside the samples, such as biotite and colorless crystal inclusions. The infrared spectrum analysis shows that absorption peaks of 1 039, 1 105, 1 196 cm-1 in the fingerprint area are attributed to the Si(Al)—O group. 752 cm-1 peak is due to Si—Si(Al) stretching, 551, 687, 624 cm-1 peaks are due to O—Si (Al)—O bending vibration. Bending vibration of Si—O—Si associated with Na(Ca) —O stretching jointly results in a 459 cm-1 peak. 416 cm-1 is due to the bending vibration of Si—O—Si. Absorption peaks related to functional group area are mainly due to different vibrational modes and frequencies of CO2-₃ (2 499, 2 629, 2 964 cm-1) and O—H(3 530 and 3 592 cm-1), which are diagnostic for the identification of scapolite. The Raman Spectroscopic analysis indicates that the bending vibration of bridge oxygen produces 459 and 538 cm-1 peaks; Al—O vibration leads to a 775 cm-1 peak. The vibration of SiO₄ tetrahedron unit generates 1 114 cm-1. UV-Vis spectrum shows 379 and 420 nm, which are caused by electron transfer between Fe2+ and Fe3+ in tetrahedron position. The yellow color of Madagascar scapolite is due to transition metal elements. The intensity of the 420 nm peak directly affects the color depth of scapolite. Analysis of 3D luminescence shows a relatively uniform luminescence phenomenon, which shows two fluorescence peaks, one strong and one weak, mostly centered at 302 nm(λ_ex)/343 nm(λ_em). The EMPA analysis result indicates that the sample belongs to Dipyre in the scapolite series. The Ma value is around 66%~69%, and the average value is 68.1%, and with the increase of the Ma value, the refractive index decreases. As a nondestructive testing technique, spectrum testing is suitable for identifying gem varieties. It is of great significance for the identification of Madagascar scapolite. It provides data support for the traceability of origin and the differentiation of scapolite varieties.