坦桑尼亚Umba出产颜色丰富的刚玉, 该研究对象是一颗来自Umba的具有特殊变色效应的蓝宝石, D65光源(色温6 500 K)下呈现淡黄色, A光源(色温2 856 K)下呈现淡紫红色。 为了研究这颗变色蓝宝石紫外-可见光光谱中的谱峰归属与变色成因, 该研究使用电荷补偿理论来分析此样品紫外-可见光光谱中的谱峰归属。 采用紫外-可见分光光度计(UV-Vis)和激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)对这颗变色蓝宝石进行了测试。 结果发现, 变色蓝宝石紫外-可见光光谱中存在位于377, 388和450 nm处的3个吸收峰和1个以560 nm为中心的宽缓吸收带。 样品的颜色主要受450 nm处吸收峰和以560 nm为中心的吸收带影响, 其中以560 nm为中心的吸收带造成了这颗蓝宝石的变色效应。 根据激光剥蚀电感耦合等离子体质谱仪的测试结果, 样品中主要杂质元素有Fe, Ti, Cr, V和Mg等。 样品紫外-可见光光谱中377, 388和450 nm处的吸收峰是由Fe³⁺导致。 蓝宝石中的Cr³⁺, V³⁺, Fe²⁺-Ti⁴⁺对都可以在560 nm附近产生吸收, 结合电荷补偿理论分析, 刚玉中的Mg²⁺会优先和Ti⁴⁺进行电荷补偿, 样品中Mg含量要稍微高于Ti, 推测样品中几乎所有Ti⁴⁺会与Mg²⁺进行电荷补偿, 因此样品中几乎不会存在Fe²⁺-Ti⁴⁺对。 Fe²⁺-Ti⁴⁺对电荷转移产生的吸收特征具有很强的偏振性, 尤其是在580 nm以后的吸收特征会随着偏振方向的改变而有很明显的变化。 偏振紫外-可见光光谱测试发现以560 nm为中心的吸收带没有明显的偏振性, 进一步验证了样品中几乎没有Fe²⁺-Ti⁴⁺对, 因此以560 nm为中心的吸收带主要是由于Cr³⁺和V³⁺造成的。 样品的颜色主要是由Fe³⁺, Cr³⁺和V³⁺引起的, 而变色效应主要是由Cr³⁺和V³⁺导致。 结合电荷补偿机制与偏振-紫外可见光光谱来解释这颗变色蓝宝石的紫外-可见光光谱中以560 nm为中心的吸收带的归属, 为研究刚玉紫外-可见光光谱中较为常见的位于560 nm左右吸收带的归属提供了一种新的研究思路。

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.