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The impact of transition metal cations and absorbed water on color transformation of turquoise

Citation

Wang, Xueding; Guo, Ying (2021), The impact of transition metal cations and absorbed water on color transformation of turquoise, Dryad, Dataset, https://doi.org/10.5061/dryad.qjq2bvqd5

Abstract

Thirty-five gem-quality turquoise samples with continuous hue were investigated using energy dispersive X-ray fluorescence spectroscopy (EDXRF), ultraviolet-visible spectroscopy (UV-Vis), infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). The chemical and spectra analyses indicate that Fe3+ primarily controls green hue of turquoise, where the absorption band performs bathochromic shift from 426 nm to 428 nm with increasing V content in the solid-solution series turquoise-chalcosiderite. Zn2+ suppress blue and V3+ and Cr3+ enhance the absorption on blue, green and red light, the combination of which results in vivid greenish yellow of faustite. The substitution of Al by trivalent cations (primarily Fe3+ and V3+) can enhance polarity of the phosphate group (PO4)3-, resulting in the absorption strength of Cu-rich turquoise lower than that of chalcosiderite and faustite. The ROH (R781/R833) derived from absorption double peaks at 781 cm-1 and 833 cm-1 can distinguish faustite from turquoise and chalcosiderite with value > 1, while the rare faustite with high V content only shows single absorption peak at 798 cm-1. Increasing absorbed water can decrease lightness of all color turquoise and enhance chroma for blue turquoise based on the CIE 1976 L*a*b* color system, which causes obvious color difference for loose turquoise, primarily darkening lightness.

Methods

The dataset was collected in the Lab of Gemological Research in school of Gemmology, China University of Geosciences, Beijing. It has been processed by box chart analysis, regression analysis, clustering analysis, discriminant analysis, and correlation analysis.