天然钴尖晶石的谱学特征及颜色成因

尖晶石(Mg, Fe, Zn, Mn)(Al, Cr, Fe)₂O₄]是一种典型的镁铝氧化物，常用于宝石、陶瓷及微晶玻璃材料中。近年来，一种颜色为矢车菊蓝色的天然尖晶石晶体出现在市场中，备受收藏者和设计师们的喜爱，其价格也不断攀高。天然钴尖晶石常呈矢车菊蓝色，透明，长波紫外光下显示弱-中等的绿色荧光，短波呈惰性，变色钴尖晶石在日光下常呈矢车菊蓝色而在白炽灯下呈紫红色。采用电子探针、电感耦合等离子体质谱、傅里叶变换红外光谱、显微聚焦激光拉曼光谱、紫外可见吸收光谱和阴极发光等测试技术获得了天然钴尖晶石和变色钴尖晶石的谱学特征，并探讨了其颜色成因和变色机理。结果表明：天然钴尖晶和变色钴尖晶石属于镁尖晶石，其主要化学成分为MgO和Al₂O₃，平均含量分别为25.77%和71.37%；此外，过渡族元素Zn，Fe，Co和V含量较高，其平均含量分别为1 337.85，831.53，99.52和58.26 μg·g-1。天然钴尖晶石与变色钴尖晶石的红外光谱和拉曼光谱与普通尖晶石的特征峰基本一致，其中红外光谱在517，589和704 cm-1处的主位均发生红移，其红移范围在5~33 cm-1之内；拉曼特征峰集中在300~800 cm-1范围之内。结合化学成分分析、紫外可见吸收光谱和阴极发光测试结果，认为天然钴尖晶石的颜色是因晶格中所含Co2+，Fe3+和V3+中的电子跃迁共同作用所致。由于Co2+发生自旋禁阻跃迁4T_1g(4F)→4T_1g(4P)使得可见光橙黄区（550~630 nm）内产生吸收带，而V3+的外层电子跃迁（3T_1g→3T_1g(3P)）和Cr3+的外层电子跃迁（4A₂→E₂）使得可见光蓝紫区（400~490 nm）内产生吸收线，可见光中红光和蓝光均匀透过，从而使其产生变色效应。该研究基本确定了天然钴尖晶石的谱学特征和成色机理，以及变色钴尖晶石的变色效应，为其科学鉴别提供了可靠的理论依据，有助于天然钴蓝色尖晶石与普通蓝色尖晶石、合成钴蓝色尖晶石的区分，具有重要的理论研究和商贸学应用价值。

Research on Spectral Characteristics and Coloration of Natural Cobalt Spinel

Spinel(Mg,Fe,Zn,Mn)(Al,Cr,Fe)₂O₄] is a magnesium aluminum oxide mineral used for gemstones and glass-ceramics. In recent years, a natural spinel has been found as a gemstone with the color of cornflower blue.It has been loved by collectors and designers and has an increasing price. The cobalt spinels are often cornflower blue and transparent. It shows weak to medium green fluorescence under long-wave ultraviolet and no fluorescence at short-wave. The cobalt spinel with alexandrite effect displayed cornflower blue color in sunlight and purplish-red color in incandescent light. Three natural cobalt spinel samples with spectral characteristics and coloration were investigated with EPMA, FTIR, Raman spectrometer, UV-VIS-NIR spectrometer, and cathodoluminescence spectrometer, and these testing results were compared with those of ordinary spinel without alexandrite effect. The results show that the alexandrite effect and cobalt spinel belong to magnesium spinel. The cobalt spinel was composed of MgO and Al2O3, with an average content of 71.37% and 25.77%, respectively; The contents of transition metals such as Zn, Fe, Co and V were relatively high, and with an average content of 1 333.85, 831.53, 99.52 and 58.26 μg·g-1, respectively. It was found that their infrared spectra and Raman spectra are the same as those of ordinary spinel: the infrared spectrum at 517, 589, and 704 cm-1 are red-shifted, and the red shift range is 5~33 cm-1, and the Raman peaks are concentrated at 300~800 cm-1. Compared the UV-Vis-NIR-spectra with the results of chemical analysis and cathodoluminescence test, it is suggested that the color of natural cobalt spinel is due to the combined action of electronic transitions in Co2+, Fe3+ and V3+ contained in the lattice. The spin-forbidden transition 4T_1g(4F)→4T_1g(4P) of Co2+ makes the orange-yellow region (550~630 nm) produce absorption bands, while the transition 3T_1g→3T_1g(3P) of V3+ and the transition 4A₂→E₂ of Cr3+ makes the blue-purple region (400~490 nm) absorption lines are generated to uniformly transmit the red and blue light so that it produces alexandrite effect. Discolored cobalt spinels are often cornflower blue in sunlight and purplish-red in incandescent lamps. This research confirmed the spectral characteristics and coloration mechanism of natural cobalt spinels and the alexandrite effect of cobalt spinels. This study provided a basis for their scientific identification of natural cobalt blue spinels and is beneficial for readers to distinguish natural cobalt blue spinels from ordinary blue spinels and synthetic cobalt blue spinels. It has important theoretical research and commercial application value.