Study on the Spectral Characteristics and the Color-Change Effect of Spinel
WANG Zi-min1, MAO Xiao-tian1, YIN Zuo-wei1*, CHEN Chang2, CHENG Tian-jia1
1. Gemmological Institute, China University of Geosicences（Wuhan），Wuhan 430074，China
2. Yunnan Land and Resources Vocational College，Kunming 652501，China
Abstract：Spinel is a precious gem known for its vivid color and fame in historical record. Colour changing is a common optic phenomenon in gemology normally seen in alexandrite, sapphire, spinel, garnet, etc. The colour changing in gemstones could mostly be attributed to chromium ions and vanadium ions, but there are few reports on color-change spinel, and there is a lack of research on color-causing elements and discoloration mechanism. The subjects of this study include an color-change spinel (blue under D65 illumination, violet-blue under A illumination) and two blue spinels without discoloration effect (there is no significant change in tone under the two light sources). The spectral details and chemical composition of spinel samples could be obtained via laser-ablation ion-coupled-plasma mass spectrometer (LA-ICP-MS), ultraviolet-visible-light spectrometer, Raman spectrometer and photoluminescence spectrometer. Compositional analyses of LA-ICP-MS show that all three samples are magnesia-alumina spinel, the main chemical components are MgO and Al2O3 and contain trace elements such as Fe, V, Cr, and Zn etc. The concentration of Fe ion in the color-change spinel is relatively higher, while there is an only minute amount of concentration of Co ion, and the content of V ions in the color-change spinel is similar to that in the blue spinel. UV-Vis spectrum of the color-change sample reveal several absorption peaks: 387, 461, 478, 527, 559, 590, 627, 668 nm.The spin causes the absorption peaks at 559, 590 and 627 nm allowed transition 4A2→4T1(4P) of the d electron spin of Co ion and split by spin-orbit coupling. In addition, the Fe2+d-d spin forbidden transition5E(D)→3T1(H) in tetrahedral coordination also produces an absorption peak at 559 nm. Specifically, the absorption band around 559 nm, which resulted mutually from Fe ion and Co ion, is the principal reason of color-change effect in spinel. A comparison of Raman spectra shows no difference between the color-change spinel and the other two blue spinels, where the Raman displacement of 311, 405, 663 and 765 cm-1 correspond to F2g(1), Eg, F2g(3) and A1g vibration, respectively. Photoluminescence analysis of the color-change sample shows that the energy level 4T1(P) of Co2+ ion in Td symmetric position subdivides into three secondary levels. 686, 650 and 645 nm luminescence peaks are generated as electrons drop from excited states of these three 4T1(P) sub-levels to ground state of 4A2(F).The content of Co ion in color-change spinel is very low, and the content of Fe ion is high. Due to the fluorescence quenching of the Fe ion, the sample has no red luminescence.
王梓民，毛晓天，尹作为，陈 畅，成天嘉. 变色尖晶石的光谱学特征及变色机理研究[J]. 光谱学与光谱分析, 2022, 42(11): 3541-3545.
WANG Zi-min, MAO Xiao-tian, YIN Zuo-wei, CHEN Chang, CHENG Tian-jia. Study on the Spectral Characteristics and the Color-Change Effect of Spinel. SPECTROSCOPY AND SPECTRAL ANALYSIS, 2022, 42(11): 3541-3545.
[1] Marfunin A S. Spectroscopy, Luminescence and Radiation Centers in Minerals. Springer Science & Business Media, 2012.
[2] LI Li-ping, YE Dong(李立平, 业 冬). Journal of Gems and Gemmology(宝石和宝石学杂志), 2003, 5(4): 17.
[3] Schmetzer K, Gubelin E. New Yearbook for Mineralogy Treatises, 1980, 9(1): 428.
[4] Chauvire B, Rondeau B, et al. Gems & Gemology, 2015, 51(1): 2.
[5] D’Ippolito V, Andreozzi G B, et al. Physics and Chemistry of Minerals, 2015, 42(6): 431.
[6] Andreozzi G B, D’Ippolito V, et al. Physics and Chemistry of Minerals, 2019, 46(4): 343.
[7] Taran M N, Koch-Muller M, Langer K. Physics and Chemistry of Minerals, 2005, 32(3): 175.
[8] Kuleshov N V, Mikhailov V P, et al. Journal of Luminescence, 1993, 55(5-6): 265.
[9] Sardar D K, Gruber J B, et al. Journal of Applied Physics, 2002, 91(8): 4846.
[10] Bosi F, Halenius U, et al. American Mineralogist, 2012, 97(11-12): 1834.
[11] Yumashev K V, Denisov I A, et al. Applied Physics B, 2000, 70(2): 179.
[12] Nataf L, Rodríguez F, Valiente R. Physical Review B, 2012, 86(12): 125123.
[13] Anghel S, Boulon G, et al. Journal of Luminescence, 2011, 131(12): 2483.
[14] D’Ippolito V, Andreozzi G B, et al. Journal of Raman Spectroscopy, 2015, 46(12): 1255.
[15] O’horo M P, Frisillo A L, et al. Journal of Physics and Chemistry of Solids, 1973, 34(1): 23.
[16] Gaft M, Reisfeld R, Panczer G. Modern Luminescence Spectroscopy of Minerals and Materials. Second Edition. Switzerland: Springer International Publishing Switzerland, 2015.
