硬水铝石是珠宝市场上流行的彩色宝石, 因其具有独特的变色效应（日光下呈现棕黄绿色, 白炽灯下呈现紫红色）而被广大消费者欢迎。 研究硬水铝石的变色成因对于宝石的切割加工、 优化处理以及价值评估有着重要意义。 目前硬水铝石的变色成因研究较少, 考虑到硬水铝石的化学成分和晶体结构与刚玉类似, 刚玉的颜色成因理论研究较为成熟, 因此, 为研究硬水铝石的变色成因, 本文选取与硬水铝石变色效应非常相近的变色刚玉, 从二者的微量元素、 紫外-可见光光谱与晶体结构等方面进行对比研究。 运用激光剥蚀电感耦合等离子体质谱仪测试了样品的微量元素, 根据测试结果, 硬水铝石样品中致色元素主要有Fe、 Cr、 V、 Ti, 刚玉样品中致色元素主要有Mg、 Ti、 Fe、 V、 Cr, 二者的微量元素种类相似, 但含量有差异。 运用紫外-可见光光谱与偏振紫外-可见光光谱表征了变色硬水铝石与变色刚玉在可见光区的吸收特征, 发现硬水铝石中存在位于387、 398、 438和448 nm处的吸收峰以及中心位于572 nm左右的宽缓吸收带, 正是这一宽缓吸收带导致了变色效应, 与之对应的, 变色刚玉中存在位于377、 388和450 nm处的吸收峰以及中心位于560 nm左右的宽缓吸收带。 二者在可见光区的吸收特征非常类似。 有差异的是, 变色刚玉的560 nm吸收带不存在明显的偏振性, 而硬水铝石的572 nm吸收带存在偏振性, 晶体中电荷转移导致的吸收特征往往具有偏振性。 通过二者晶体结构的对比分析与刚玉中的电荷补偿理论分析, 推测硬水铝石中398 nm吸收峰由Fe3+导致, 387、 438和448 nm吸收峰由Fe3+-Fe3+离子对导致, 572 nm处的宽缓吸收由Cr、 V、 Fe2+-Ti4+离子对产生。 硬水铝石的变色效应是在Cr、 V、 Fe2+-Ti4+离子对的综合作用下产生的。 本研究通过类比变色效应相近的刚玉, 研究了硬水铝石的变色成因, 为研究宝石中的类似问题提供了新思路。

Diaspore is a popular color gemstone in the jewelry market. It is popular with consumers because of its unique Alexandrite effect (brownish green in the sunlight and purplish red in the incandescent lamp). The study on the Alexandrite effect origin of diaspore is of great significance to the cutting, the treatment and the value evaluation of gemstones. There are few studies on the Alexandrite effect origin of diaspore. Considering that the chemical composition and crystal structure of diaspore are similar to corundum, the theory on color origin of corundum is relatively mature. Therefore, to research the Alexandrite effect of diaspore, the corundum, which is very similar to the Alexandrite effect of diaspore, is selected for innovative comparative study from trace elements, UV-Vis spectrum and crystal structure. The trace elements in the samples were measured by laser ablation inductively coupled plasma mass spectrometer. According to the test results, the main chromogenic elements in diaspore are Fe, Cr, V, Ti and the main chromogenic elements in corundum are Mg, Ti, Fe, V, Cr. The types of trace elements in the two samples are similar, but the contents are different. Ultraviolet-visible and polarized ultraviolet-visible spectra were used to characterize the absorption characteristics of the samples in the visible light region. It was found that there were absorption peaks at 387, 398, 438 and 448 nm and absorption bands with centers at about 572 nm in the spectrum of diaspore. It was this absorption band that caused the Alexandrite effect. Correspondingly, there were absorption peaks at 377, 388 and 450 nm and an absorption band with the center at about 560 nm in the spectrum of diaspore. Their absorption characteristics in the visible light region are very similar. The difference is that the absorption band at 560 nm of corundum does not have obvious polarization, while the absorption band at 572 nm of diaspore has polarization. The absorption characteristics caused by charge transfer in crystals often have polarization. Through the comparative analysis of their crystal structures and the charge compensation theory in corundum, we speculate that the absorption peak at 398 nm in diaspore is caused by the d—d electron transition of Fe3+, the absorption peaks at 387, 438, 448 nm are caused by the Fe3+-Fe3+ pair, and the absorption band at 572 nm is caused by the Cr, V, Fe2+-Ti4+ pair. The alexandrite effect in diaspore is caused by Cr, V, Fe2+-Ti4+ pair. This study innovatively studied the Alexandrite effect origin of diaspore by comparing it with corundum, providing a new idea for studying similar problems in gemstones.