Gemmological and Spectroscopic Characteristics and Fluorescence Mechanism of Orange-Yellow Sodalite from Afghanistan

Orange-yellow sodalite is a newly emerged gem variety in the market in recent years, and relevant researches on its gemmological characteristics, cause of colour and fluorescence mechanism remain scarce in China. In this study, 3 orange-yellow sodalite samples with distinct fluorescence were tested for their gemmological characteristics, chemical composition, spectroscopic features, cause of colour and fluorescence mechanism by conventional gemmological testing methods, scanning electron microscopy, laser ablation inductively coupled plasma mass spectrometry, Raman spectroscopy, Fourier transform infrared spectroscopy, ultraviolet-visible spectrophotometry and fluorescence spectroscopy. The results showed that the 3 orange-yellow sodalite samples have a refractive index of 1.48 and a relative density ranging from 2.29 to 2.30. Under long-wave ultraviolet (LWUV) light, it exhibits orange-red to orange fluorescence. Under short-wave ultraviolet (SWUV) light, the orange sodalite samples show blue-white fluorescence, while the yellow sodalite sample displays pale green fluorescence. The colour of the orange-yellow sodalite is unstable: perceptible fading occurs after one year indoor storage, accompanied by a significant reduction in fluorescence intensity under SWUV excitation. Chemical composition analysis revealed that orange-yellow sodalite samples are primarily composed of major elements including Na, Al, Si and Cl, with trace elements such as Li, B, S, K, Cu and Ga. The UV-Vis absorption spectra showed a strong absorption band in the ramge of 310-340 nm in the ultraviolet region, and a broad and gentle absorption band covering the range of 400-600 nm in the blue region. It is inferred that the absorption band in the visible region may originate from the electron transitions of polysulfide colour centers with different valence states and chain lengths, among which the absorption peaks of neutral S₄ and S₅^2- are most consistent with the bands observed in this study. The blue-white fluorescence under SWUV light is jointly contributed by Cu, Ga and S elements, whereas the orange fluorescence under LWUV light is mainly induced by S₂^- ions. This study provides an important experimental evidence for elucidating the cause of colour and fluorescence mechanism of orange-yellow sodalite.