Research on Spectral Characteristics and Coloration of Natural Cobalt Spinel
TAO Long-feng1, 2, SHI Miao2, XU Li-juan2, HAN Xiu-li1*, LIU Zhuo-jun2
1. College of Mining Engineering, North China University of Science and Technology, Tangshan 063210, China
2. School of Gemmology and Materials Science, Hebei GEO University, Shijiazhuang 050031, China
Abstract：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 4T1g(4F)→4T1g(4P) of Co2+ makes the orange-yellow region (550~630 nm) produce absorption bands, while the transition 3T1g→3T1g(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.
陶隆凤，史 淼，徐丽娟，韩秀丽，刘倬君. 天然钴尖晶石的谱学特征及颜色成因[J]. 光谱学与光谱分析, 2022, 42(07): 2130-2134.
TAO Long-feng, SHI Miao, XU Li-juan, HAN Xiu-li, LIU Zhuo-jun. Research on Spectral Characteristics and Coloration of Natural Cobalt Spinel. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2022, 42(07): 2130-2134.
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