光致变色天然白色方钠石宝石学和光谱学特征探究

方钠石为似长石类矿物，常作为无机光致变色材料广泛应用于建筑、照明和放射量测定等领域。高品质方钠石宝石以其迷人的光致变色效应为大众熟知，并得到消费者认可，价格也不断攀升。因天然宝石级方钠石的宝石学特征和光致变色机理研究较薄弱，为鉴定提供理论依据并探究其变色机制，特选取具光致变色效应的天然白色方钠石分别进行UVA(365nm)和UVC(254 nm)紫外光致变色实验，并结合傅里叶变换红外光谱(FTIR)、紫外-可见光吸收光谱(UV-Vis)和电子顺磁共振波谱(EPR)，探讨其光致变色机理。紫外光辐照实验显示，天然白色方钠石具有橙粉色-橙红色荧光，受UVA紫外光辐照5分钟变为紫红色，可维持1~2 min，使用白光（700~400 nm）照射可以快速褪至初始白色；受UVC紫外光辐照，紫色调浓度持续加深且产生持久稳定的光致变色；初始白色样品紫外荧光强度明显强于光致变色后紫红色样品。FTIR吸收光谱指示，5 250 cm-1处强吸收峰归属于H₂O的弯曲振动与伸缩振动的组合吸收，证实存在结晶水；4 698和4 555 cm-1两处弱吸收峰与金属阳离子(M)和O—H相互吸引而成的面外弯曲振动γ(M—OH)有关（M=Mg2+, Na+, K+, Al3+）；1 002 cm-1处强吸收峰与标准方钠石吸收特征相比，向高波数偏移约20 cm-1，指示硅氧四面体结构中AlⅣ含量减少，结合EPR波谱结果认为上述现象可能由Mn2+和Ti3+替换AlⅣ—O四面体结构所致，并分别形成以3 511 G(g=2.002)处为中心的多条超精细谱线及3 573 G(g=1.967)处单一谱线。UV-Vis和EPR谱指示，白色方钠石短波紫外产生持续光致变色产生的主要原因是，因S2-₂替代Cl-导致体系存在部分Cl缺失（空位V_Cl）来平衡电价，S2-₂受紫外线激发分解：S2-₂→S-₂+e-，e-进入导带而被V_Cl捕获形成色心，位于色心的电子转变为激发态而产生539 nm宽吸收带，伴有向紫外区的拖尾增强吸收，导致样品呈紫红色，EPR 3 480 G(g=2.02)处单一谱峰可作为判别依据。

Gemological and Spectroscopic Characteristics of Natural White Sodalite With Tenebrescence

Sodalite is a member of the feldspathoids, which is widely applied in the field of construction, illumination, and dosimetry. High-quality sodalite is known mainly as its tenebrescence under ultraviolet (UV) irradiation. This attracting optical phenomenon has been recognized by consumers, leading the price of sodalite soaring. For the lack of systematic research on gemstone sodalite, its gemological characteristics and mechanism of tenebrescence are investigated in this study. Photochromic white sodalites were selected in the research of tenebrescence under UV irradiation, respectively exposed to UVA (365 nm) and UVC (254 nm). The mechanism of tenebrescence was investigated by FTIR, UV-Vis, and EPR. The UV irradiation research showed: (1) natural white sodalite has an orangish-pink to orangish-red fluorescence, which might change into purplish-red for 1～2 min when exposed to UVA in 5 min, however, it could be removed by visible-light (700～400 nm) quickly; (2) the purplish-red color got continuous enhanced and became steady even exposed to visible-light; (3) the fluorescence of natural samples was stronger than their photochromic state. FTIR spectra of natural samples showed: (1) 5 250 cm-1 strong absorption attributed to stretching and bending vibration of H₂O, demonstrating the existence of hydrate; (2) 4 698 and 4 555 cm-1 weak absorption were induced by the out-of-plane bending vibration (γ) between metallic cations (M) and hydroxyl radicals (O—H); (3) 1 002 cm-1 strong absorption moved 20 cm-1 towards high wave-number direction compared to the standard spectrum for substitution of Mn2+ and Ti3+ in AlⅣ—O, which could be demonstrated by the EPR signals of several hyperfine lines around 3 511G (g=2.002) and a single line at 3 573 G (g=1.967). UV-Vis and EPR spectra indicated that, the main cause of tenebrescence in white sodalite was associated with Cl vacancy generated by the substitution between S2-₂ and Cl-. For the balance of charge valence, S2-₂ was excited by UV irradiation and generated free electron(S2-₂→S-₂+e-). The free electron then jumped into subduction band and traped by V_Cl, which formed color center and led to wide absorption band around 539 nm along with strengthen absorption towards ultraviolet region and finally generate purplish-red. By the way, single EPR line at 3 480 G(g=2.02) might be the proof to capability of tenebrescence.