硅硼镁铝石的宝石学特征研究

Gemmological Characteristic of Grandidierite

硅硼镁铝石是一种极其稀少的宝石,近年来只在马达加斯加发现了宝石级样品。为研究其宝石学特征并为其鉴定提供依据,对样品进行了常规宝石学测试,以及X射线荧光光谱、红外光谱、拉曼光谱等现代大型仪器的测试。结果表明,硅硼镁铝石颜色为蓝绿色到绿蓝色,抛光面呈玻璃光泽,折射率在1.578^(-1).619之间,有较大可至0.041的双折射率,且具有明显的蓝绿/深绿/无色三色性。通过对X射线荧光光谱的分析,硅硼镁铝石的颜色可能与其含有的铁元素有关,样品中Fe/Mg+Fe值在0.017～0.037之间,且比值越高,样品的铁含量越高,颜色也越浓郁。样品的红外光谱主要分布于中红外指纹区1300～1500cm^(-1)、800～1100cm^(-1)和400～800cm^(-1)三个区间,其中1300～1500cm^(-1)的一系列吸收峰与硼氧三角形BO_3的伸缩振动有关;800～1100cm^(-1)具有较强的吸收峰,这与硅硼镁铝石中的硅氧骨干有关。400～800cm^(-1)的吸收可能与(Al,Mg,Fe)-O和硅氧骨架的变形有关。在拉曼光谱中硅硼镁铝石的硼氧三角形BO_3、硅氧四面体SiO_4、扭曲的AlO_5、MgO_5和FeO_5的三角双锥以及八面体AlO_6的吸收峰均有体现。

Grandidierite,considered one of the rarest gems,has been discovered in Madagascar.8 grandidierite samples from Madagascar are studied by gemmological identification moethods and advanced instruments,including energy dispersive X-ray fluorscence spectrometer,Fourier transform infrared spectrometer,and Raman spectrometer.The results show that the colour is from strong bluish green to greenish blue,and the polished facet of grandidierite is in glassy lustre.The refractive index is 1.578~1.619,and birefringence is 0.041.It's pleochroism show bluish green,deep green and colorless.Through the analysis of X-ray fluorescence spectra,we preliminarily concluded that the colour of grandidierite may be related to its iron content,and the higher of the iron content,the more intense of the color.It is also characterized by infrared spectra and Raman spectra.The infrared spectra of the sam-ple are mainly distributed in the middle infrared fingerprint area of 1300^(-1) 500 cm^(-1),800^(-1) 100 cm^(-1) and 400-800 cm^(-1).A series of absorption peaks in 1 300^(-1) 500 cm^(-1) are related to the stretching vibration of the BO_3 triangles;800^(-1) 100 cm^(-1) have strong absorption peaks,which is related to the SiO_4 tetrahedra in grandidierite.The absorption of 400~800 cm^(-1) may be related to the deformation of (Al,Mg,Fe)-O and the SiO_4 tetrahedra.In the Raman spectroscopy test,absorption peaks related to the BO_3 triangles;SiO_4 tetrahedra;distorted AlO_5,MgO_5,and FeO_5 trigonal bipyramids;and AlO_6 octahedra are showed.