The Study on UV-Vis Spectrum of a Special Color-Changed Sapphire
CHEN Chao-yang1, HUANG Wei-zhi1, SHAO Tian1, SHEN Che1, LI Zhi-bin2, Andy Hsitien Shen1*
1. Gemmological Institute，China University of Geoscience (Wuhan)，Wuhan 430074，China
2. School of Civil and Resource Engineering, University of Science and Technology Beijing, Beijing 100083, China
Abstract：Many colorful corundum crystals were found in Umba, Tanzania. The sample of this study is a sapphire with special color-changed effect from Umba. It shows slight yellow under D65 light source (colour temperature is 6 500 K) and slight purplish red under A light source (colour temperature is 2 856 K). In order to study the UV-Vis spectroscopy and the origin of the color-changed effect of this sapphire, the charge compensation theory was innovatively used to analyze the assignment of absorption characteristic in UV-Vis spectrum. UV-Vis spectrophotometer and laser ablation inductively coupled plasma mass spectrometer (LA-ICP-MS) were used to test the sample. The result showed that there are three absorption peaks at 377, 388 and 450 nm and a absorption band centered at 560 nm in UV-Vis spectrum of the sample. The color of the sample is mainly affected by the peak at 450 nm and the band centered at 560 nm, which leads to the color-changed effect. According to the results of LA-ICP-MS, the trace elements in this sample are Fe, Ti, Cr, V, Mg, etc. The absorption peaks at 377, 388 and 450 nm in the UV-Vis spectrum of samples are caused by Fe3+. The absorption features near 560 nm may be caused by Cr3+, V3+, Fe2+-Ti4+ pairs in sapphire. Combining with charge compensation theory, Mg2+ will preferentially compensate with Ti4+ in corundum. The content of Mg in sample is slightly higher than that of Ti. So almost all Ti4+ will compensate with Mg2+ and almost no Fe2+-Ti4+ pair exists in the sample. The absorption characteristic of charge transfers between Fe2+ and Ti4+ has strong polarization. The absorption characteristic caused by Fe2+-Ti4+ pairs, especially after 580 nm, will change obviously with the change of polarization direction. Polarized UV-Vis Spectrum of this sapphire shows that there is no obvious polarization feature on the absorption band centered at 560 nm, which could further confirm that there is almost no Fe2+-Ti4+ pair in the sample. Thus the absorption band centered at 560 nm is mainly caused by Cr3+ and V3+. The color of sample is mainly caused by Fe3+, Cr3+, V3+ and the color-changed effect is mainly caused by Cr3+ and V3+. This research innovatively combines charge compensation mechanism and polarized UV-Vis spectroscopy to explain the assignment of the absorption band centered at 560 nm in the UV-Vis spectrum of this color-changed sapphire. It provides a new method to study the assignment of the absorption band around 560 nm which is a common absorption characteristic in UV-Vis spectrum of corundum.
Key words：Sapphire; Color-changed effect; UV-Vis spectrum; Assignments of spectral characteristics
陈超洋，黄伟志，邵 天，沈 澈，李志彬，沈锡田. 特殊变色蓝宝石的紫外-可见光光谱研究[J]. 光谱学与光谱分析, 2019, 39(08): 2470-2473.
CHEN Chao-yang, HUANG Wei-zhi, SHAO Tian, SHEN Che, LI Zhi-bin, Andy Hsitien Shen. The Study on UV-Vis Spectrum of a Special Color-Changed Sapphire. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2019, 39(08): 2470-2473.
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