Mineralogical Characteristics and Color Genesis of Vesuvianite Jade from Hanzhong, Shaanxi Province, China

A new type of vesuvianite jade has recently been discovered in Hanzhong City, Shaanxi Province, China. However, a systematic investigation into its mineralogical characteristics and the origin of its color is currently lacking. In this study, the gemological, mineralogical, and spectroscopic properties of the Hanzhong vesuvianite jade were comprehensively analyzed using a suite of modern analytical techniques, including standard gemological testing, polarizing microscopy, X-ray powder diffraction, Fourier-transform infrared spectroscopy, laser Raman spectroscopy, UV-visible absorption spectroscopy, and X-ray fluorescence spectroscopy. The origin of the jade’s color was also preliminarily investigated. The results indicate that the samples are primarily composed of vesuvianite, with associated minerals including minor amounts of grossular, chlorite, and diopside, and trace amounts of calcite, epidote, chromite, and titanite. The pale green patches consist mainly of chlorite and grossular, the dark green bands are predominantly chlorite, and the dark brown patches are composed of abundant, disseminated microcrystalline chromite intermixed with uvarovite (calcium chromium garnet). The major chemical components of the vesuvianite jade matrix are SiO2, Al2O3, and CaO. Specifically, SiO2 ranges from 37.01 to 38.54 wt.%, Al2O3 from 18.48 to 22.84 wt.%, and CaO from 37.16 to 40.04 wt.%. Minor amounts include MgO (0.76–4.39 wt.%) and FeOT (total iron expressed as FeO, 0.56–2.09 wt.%). The yellowish-green color of the matrix originates from a combination of ligand-to-metal charge transfer of Fe3+, crystal field transitions of Fe3+, and intervalence charge transfer between Fe2+ and Fe3+ in vesuvianite. The emerald-green color of the patches results from the synergistic effect of Fe and Cr; Fe provides a yellowish-green background color, upon which the crystal field transitions of Cr3+ (indicated by a doublet at 686/696 nm) impose strong absorption in the red region, resulting in a more vivid green hue.