变色石榴石是众多石榴石族矿物中的一种特殊品种, 通常为含微量Cr和V的镁铝—锰铝榴石。 国内少有人研究, 国外主要研究变色石榴石的紫外可见吸收光谱与颜色计算, 但未见具红色荧光变色石榴石的相关报道, 缺乏荧光光谱研究。 本次研究对象为一颗产自坦桑尼亚Umba地区的变色石榴石, 具明显的变色效应, 且在长波紫外荧光灯下发出红色荧光。 通过化学成分测试与光谱测试来详细探讨其变色效应及荧光机制。 采用激光剥蚀电感耦合等离子体质谱仪(LA-ICP-MS)确定该变色石榴石为镁铝榴石与锰铝榴石的类质同象替代中间产物, 含有Cr, V和Fe等微量元素, 其平均端元组分为Prp_46.28Sps_38.40Grs_13.57Alm_2.33Uvt_0.35。 紫外-可见吸收光谱中蓝紫区409, 422, 430和486 nm处的强吸收尖峰为Mn²⁺的自旋禁阻跃迁造成, 459和503 nm的弱吸收峰则与Fe²⁺有关, 黄区以571 nm为中心的宽缓吸收带由Cr³⁺和V³⁺的自旋允许跃迁共同造成。 Cr和V是产生变色效应的主要原因。 蓝紫区和橙黄区的强吸收, 导致两个“透射窗”绿光区与红区的透过率相当, 从而使石榴石产生变色效应: 日光灯下为黄绿色, 白炽灯下呈紫红色。 3D荧光光谱中690 nm处的发射峰为Cr³⁺的多重禁阻跃迁² E_g→⁴ A_2g所致, 两侧的678和704.5 nm的荧光峰也为Cr³⁺所致。 可见光中用紫光(400~440 nm)和黄光(550~600 nm)最能激发出样品的红色荧光, 这与Cr³⁺的两个自旋允许跃迁(⁴ A_2g→⁴ T_1g和⁴ A_2g→⁴ T_2g)对应的吸收宽带有关。 当光源的能量正好能允许这两个吸收跃迁发生时, 电子吸收能量从基态跃迁至这两个激发态, 然后再从最低激发态² E_g回到基态发射荧光, 此时的荧光产额最强。 相比于前人研究中的变色石榴石, 本文样品中的Fe²⁺含量较低(<0.3 Wt%), Fe²⁺具有荧光猝灭的作用, 因而推测Cr高Fe低是本文变色石榴石能够被激发出红色荧光的主要原因。 目前国内还没有关于天然变色石榴石的荧光研究, 这为日后进一步研究石榴石的荧光提供了谱学依据与理论基础。

Color-change garnet is a special variety in the garnet group, usually belongs to pyrope-spessartine solid-solution series and contains a trace of Cr and Fe. Few people have studied it in China, and no color-change garnets with obvious red fluorescence under ultraviolet radiation have been reported at home and abroad. A color-change garnet with red fluorescence from the Umba region of Tanzania was studied by chemical and spectral tests. Laser ablation inductively coupled plasma mass spectrometry was used to study sample’s chemical properties. The result shows that this color-change garnet belongs to the pyrope-spessartine solid-solution series and contains a certain amount of grossular end-member, with some trace elements of Cr, V, Fe. The average composition of it is Prp_46.28Sps_38.40Grs_13.57Alm_2.33Uvt_0.35. In ultraviolet-visible absorption spectrum, sharp absorption peaks at 409, 422, 430 and 486 nm are assigned to the spin-forbidden transition of Mn²⁺, and additional peaks at 459, 503 nm is caused by Fe²⁺, the broad absorption band peaked at 571 nm in the yellow region which is responsible for the color-change effect is attributed to the spin-allowed transition of Cr³⁺ and V³⁺ together. Two absorption maxima in violet-blue and yellow-orange regions and comparably equal transmittance of green and red light cause the color change effect, garnet appears greenish-yellow in day light and becomes purplish-pink under incandescent light. The emission peak at 690 nm in the fluorescence spectrum is ascribed to the² E_g→⁴ A_2g multiplicity-transition of Cr³⁺. And red fluorescence is most excited by violet light (400~440 nm) and yellow light (500~560 nm), that correspond to absorption bands caused by two spin-allowed transitions of Cr³⁺,⁴ A_2g→⁴ T_1g and⁴ A_2g→⁴ T_2g respectively. Compared with other color-changing garnets in previous studies, this sample contains lower Fe²⁺. It is speculated that high Cr and low Fe makes the color-change garnet can be excited to red fluorescence under ultraviolet radiation. The detailed spectra study of color-change garnet with red fluorescence and discussionon of its color-change effect and fluorescence mechanism provide spectral data and theoretical basis for further research on special garnets’ fluorescence in the future.