Abstract:Tourmaline is a borosilicate with complex crystal structure and chemical composition. The most common species of tourmaline on jewelry markets are elbaite and some dravite. There are numerous textbooks and articles in related fields focusing on elbaite rather than dravite. In this study, six light-yellow to brownish yellow tourmaline gemstones from Mozambique were examined. Standardgemological testing was followed by LA-ICP-MS, infrared spectroscopy, ultraviolet-visible spectroscopy, fluorescence spectroscopy and Raman spectroscopy testing. Standard gemological testing revealed that the physical and optical properties of the samples are basically in agreement with those of general tourmaline, but all samples have medium to strong green fluorescence under short-wave (254 nm) UV light, while general tourmaline is inert. In addition, the samples contain a lot of dark or grey granular mineral inclusions, but no tubular inclusions, gas-liquid two-phase inclusions, which are common in tourmaline. LA-ICP-MS testing shows that the samples belong to dravite and their average crystal structure formula is (Ca0.15Na0.85)1.00(Mg2.89Fe0.02Al0.09)3.00Al6(Si6O18)(B0.95□0.05O3)3(OH)4. The samples with fewer inclusions were selected for infrared spectroscopy testing. The vibration peaks of hydroxyl group and Si—O are found in the 2 000~6 000 cm-1 region, indicating that the samples contain water. There is a broad absorption peak between 400~500 nm in the Ultraviolet-visible spectra. The peak position is about 445 nm, which may be related to the charge transfer of Fe2+-Ti4+ and exchange-coupled Fe2+-Fe3+. The samples emit medium-strong green fluorescence under 254 nm UV light. There are 534 nm strong peak and 475 nm shoulder peak in fluorescence spectra. The fluorescence may be related to the Ti and Fe in the samples. Laser Raman spectra of the tourmaline samples are performed. The result is in accordance with the Raman spectra of dravite (species of tourmaline). The innovation of this paper is mainly embodied in the following two aspects: (1) the samples turn out to be dravite, whose spectral characteristics have not been studied in detail. (2) These dravites have a unique fluorescence under short-wave UV light, which has not been documented, and the reason for the origin of the fluorescence has been deduced by the author.
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