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 similar to 2 min when exposed to UVA in 5 min, however, it could be removed by visible-light (700 similar to 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 weak absorption were induced by the out-of-plane bending vibration (gamma) 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 Al-IV-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 Si and Cl . For the balance of charge valence, S-2(2) was excited by UV irradiation and generated free electron(S-2(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.