尖晶石作为一种珍贵的宝石材料, 因其瑰丽的颜色外观和悠久的历史而广受称赞。 变色效应作为宝石学中一种常见的光学现象, 在变石, 蓝宝石, 尖晶石, 石榴石等宝石中都可以出现。 通常将宝石的变色效应归结为Cr离子和V离子所致, 但是目前有关变色尖晶石的报道较少, 缺乏致色元素和变色机理的研究。 本次研究对象是一颗具有变色效应的尖晶石(在D65光源下呈蓝色, 在A光源下呈蓝紫色), 和两颗不具有变色效应的蓝色尖晶石(两种光源下色调无明显变化)。 运用电感耦合等离子体质谱仪(LA-ICP-MS)、 紫外可见光谱仪、 拉曼光谱仪、 光致发光光谱仪获取样品的成分和光谱信息。 LA-ICP-MS化学成分测试结果表明, 三颗尖晶石均为镁铝尖晶石, 主要化学成分为MgO和Al₂O₃, 并含有Fe, V, Cr, Co和Zn等微量元素, 在变色尖晶石中含有较多的Fe离子和微量的Co离子, 不含有Cr离子, 并且变色尖晶石与无变色效应的蓝色尖晶石中V离子含量相近。 变色尖晶石紫外可见吸收光谱具有位于387, 461, 478, 527, 559, 590, 627和668 nm处的吸收峰, 其中387, 461, 478和668 nm吸收峰与Fe离子有关。 559, 590和627 nm处的吸收峰是由Co离子d轨道电子自旋允许跃迁⁴A₂→⁴T₁(⁴P)并经自旋-轨道耦合作用分裂所致。 此外, 四面体配位中的Fe²⁺d—d自旋禁阻跃迁⁵E(D)→³T₁(H)同样在559 nm处产生吸收峰。 由Co离子和Fe离子共同作用, 在559 nm附近产生的吸收宽带是尖晶石产生变色效应的主要原因。 拉曼光谱测试结果显示变色尖晶石与其他两颗蓝色尖晶石无差异, 可见311, 405, 663和765 cm^-1四个特征拉曼位移峰, 依次对应F_2g(1), E_g, F_2g(3)和A_1g振动。 光致发光光谱(PL)测试发现变色尖晶石中处于T_d对称位置的Co²⁺的⁴T₁(P)能级会分裂成为三个子能级, 电子由三个⁴T₁(P)激发态的子能级回落到⁴A₂(F)基态而产生位于686, 650和645 nm处的发光峰。 变色尖晶石中Co离子含量很低, 并且Fe离子含量较高, 受到Fe离子荧光猝灭作用, 样品无红色发光现象。

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 F_2g(1), E_g, F_2g(3) and A_1g vibration, respectively. Photoluminescence analysis of the color-change sample shows that the energy level ⁴T₁(P) of Co²⁺ ion in T_d 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.