Gemmological and Spectral Characteristic of Zultanite: Colour-Change Diaspore
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Abstract
Zultanite, an exotic gemstone, is only found inbir Mountains, Menderes Massif, Turkey as well as colour-change diaspore. In this paper, the authors studied the gemmological and spectral characteristics of Zultanite samples by means of conventional gemmological testing methods, Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), and ultraviolet-visible spectrometry (UV-Vis). The results indicated that the infrared absorption bands at 2 998、2 928 cm-1 are associated with —OH stretching vibrations. The infrared absorption band at 1 075 cm-1 is ascribed to —OH bending vibration, and 970 cm-1 is ascribed to —OH deformation vibration. Another two infrared absorption bands at 2 119、1 986 cm-1 are assigned to —OD stretching vibration. Raman bands at 1 192、1 048 cm-1 are associated with —OH bending vibrations, and 792、667、555 cm-1 are ascribed to —OH deformation vibration. The bands at 500、449、398、332、289、259 cm-1 are assigned to Al—O vibration. The most intense band at 449 cm-1 is ascribed to Al—O—Al bending vibration. The results of LA-ICP-MS test showed that the main impurity elements of Zulanite samples are Fe、Si、Ti with trace elements Ga and Cr. UV-Vis absorption spectroscopy analysis showed that four strong absorption bands at 371、385、397、448 nm are induced by d—d electronic transition of Fe3+ and charge transfer of Fe3+—Fe3+. The broad and weak band at 575 nm is induced by the charge transfer of Fe2+—Ti4+ as well as trace Cr3+, which the authors propose to be the main mechanism of colour change effect in Zultanite.
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