电子辐照紫晶的宝石学特征

Gemmological Characteristic of Electron Irradiated Amethyst

对东非紫晶、玻利维亚紫黄晶和合成紫黄晶样品进行电子辐照处理,对比观察电子辐照前后紫晶样品的紫外-可见光谱、中红外吸收光谱,分析辐照处理前后紫晶的宝石学性质变化情况及致色机理。实验表明,电子辐照处理能够有效加深特定区域的紫晶颜色,具体表现为:随着辐照剂量增加,可见光区540 nm吸收峰增强。电子辐照处理导致东非紫晶的紫外光区221 nm吸收峰向228 nm移动;所有紫晶在中3 614 cm^-1处的吸收峰向3 620~3 623 cm^-1移动,这些光谱特征具有宝石学鉴定意义。对热处理褪色紫黄晶的辐照处理可令紫色恢复并浓集在布儒斯特纹处,进一步证实紫晶中Fe元素的分布具有方向性。Fe元素占位对紫晶致色意义重大,中红外光谱范围内探测到的结构水的吸收峰与颜色之间缺乏相关性。

Amethyst from East Africa and ametrine from Bolivia and laboratory were all irradiated under electron beam and then measured by micro UV-Vis spectrometer and MIR spectrometer.The absorption spectra have been studied to analyze the change after the treatment and to explain the colour mechanism in some degree.Treatment experiments showed that the electron irradiation can intensify the violet colour effectively only in specific colour zones of amethyst with the phenomenon that the absorption band centered at 540 nm gets higher.The radiation induced the peak shift from 221 nm to 228 nm in ultraviolet region of amethysts from East Africa,and the peak shift from 3 614 cm^-1 to 3 620-3 623 cm^-1 in middle infrared region of all the amethysts,which may have significance on identification of the treated amethyst.The irradiation treatment on heated ametrine made the completely faded sample’s violet reverse,and the violet concentrated along Brewster fringes.This experiment confirmed further that the Fe in amethyst has strongly directional selectivity that supports the viewpoint where the occupations of Fe do matter on coloration.Also,we found detectable structural water bands in MIR region have no obvious correlation with colour.