马达加斯加硅硼镁铝石的宝石学特征
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摘要
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品质。为研究其宝石学特征并为鉴定提供依据,对8块马达加斯加硅硼镁铝石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、拉曼光谱等大型仪器的测试分析。结果表明,硅硼镁铝石样品的颜色为蓝绿色到绿蓝色,抛光面呈玻璃光泽,折射率为1.578～1.619,双折射率约为0.041,且具有明显的蓝绿色/深绿色/无色三色性。硅硼镁铝石的颜色可能与其含有的Fe元素有关,且Fe质量分数越高,颜色越浓郁。硅硼镁铝石样品具有特征的红外光谱以及拉曼光谱,这与其结构中的硼氧三角形(BO_3)、硅氧四面体(SiO_4)、扭曲的三角双锥(AlO_5、MgO_5和FeO_5)以及八面体(AlO_6)有关:红外光谱中,最为特征的硼氧三角形(BO_3)的振动位于1 300～1 500 cm~(-1),而位于800～1 100 cm~(-1)处的吸收峰与硅氧骨干有关;拉曼光谱中特征的硼氧三角形(BO_3)振动反映在800～1 100 cm~(-1)区间。红外光谱以及拉曼光谱测试可作为其鉴定依据。
Grandidierite, one of the rarest gems, has been discovered recently in Madagascar. 8 pieces of grandidierite samples from Madagascar were collected and studied by gemmological testing methods and advanced instrumentation including energy dispersive X-ray fluorescence spectrometer, Fourier transform infrared spectrometer, and Raman spectrometer. The results showed the colour of the grandidierite is from strong bluish green to greenish blue, and its polished facet is glassy lustre. The refractive index is 1.578-1.619, and birefringence is 0.041. It's pleochroism of bluish green, deep green and colourless. Through the analysis of X-ray fluorescence spectra, it can be preliminarily concluded that the colour of grandidierite may be related to its Fe element, and the higher the Fe content, the more intense the colour. It is also characterized by infrared spectra and Raman spectra, which are related to the BO_3 triangles, SiO_4 tetrahedra, distorted AlO_5, MgO_5, and FeO_5 trigonal bipyramids, and AlO_6 octahedra. In infrared spectra, the absorption peak of vibration of BO_3 triangles is in 1 300-1 500 cm~(-1) region while the absorption in 800-1 100 cm~(-1) region is related to the silica skeleton. The vibration of the BO_3 triangles in the Raman spectrum is reflected in the range of 800-1 100 cm~(-1). Infrared spectroscopy and Raman spectroscopy can be used as methods for grandidierite identification.
Grandidierite, one of the rarest gems, has been discovered recently in Madagascar. 8 pieces of grandidierite samples from Madagascar were collected and studied by gemmological testing methods and advanced instrumentation including energy dispersive X-ray fluorescence spectrometer, Fourier transform infrared spectrometer, and Raman spectrometer. The results showed the colour of the grandidierite is from strong bluish green to greenish blue, and its polished facet is glassy lustre. The refractive index is 1.578-1.619, and birefringence is 0.041. It's pleochroism of bluish green, deep green and colourless. Through the analysis of X-ray fluorescence spectra, it can be preliminarily concluded that the colour of grandidierite may be related to its Fe element, and the higher the Fe content, the more intense the colour. It is also characterized by infrared spectra and Raman spectra, which are related to the BO_3 triangles, SiO_4 tetrahedra, distorted AlO_5, MgO_5, and FeO_5 trigonal bipyramids, and AlO_6 octahedra. In infrared spectra, the absorption peak of vibration of BO_3 triangles is in 1 300-1 500 cm~(-1) region while the absorption in 800-1 100 cm~(-1) region is related to the silica skeleton. The vibration of the BO_3 triangles in the Raman spectrum is reflected in the range of 800-1 100 cm~(-1). Infrared spectroscopy and Raman spectroscopy can be used as methods for grandidierite identification.
引文
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[2] Bruyère D,Delor C,Raoul J,et al.A new deposit of gem-quality grandidierite in Madagascar[J].Gems & Gemology,2016,52(3):266-275.
[3] 杨岳清,王勇,吕庆田,等.福建南平花岗伟晶岩中的电气石研究[J].岩石矿物学杂志,2010,29(3):235-242.
[4] 袁婷.同一方向碧玺的红外光谱谱学特征[J].超硬材料工程,2008,20(6):57-60.
[5] 法默 V C.矿物的红外光谱[M].应育浦,等译.北京:科学出版社,1982:289-303.
[6] Stephenson D A,Moore P B.The crystal structure of grandidierite,(Mg,Fe)Al3SiBO9[J].Acta Crystcallographica.Section B,Structural Science,2010,24(11):1 518-1 522.
[7] Zhao C,Liao L,Xia Z,et al.Temperature-dependent Raman and infrared spectroscopy study on iron-magnesium tourmalines with different Fe content[J].Vibrational Spectroscopy,2012,62(5):28-34.
[8] Hoang L H,Hien N T M,Xiang B C,et al.Raman spectroscopic study of various types of tourmalines[J].Journal of Raman Spectroscopy,2011,42(6):1 442-1 446.