Gemmological and Mineralogical Characteristics of Vesuvianite Jade from Hanzhong, Shaanxi Province

In recent years, the vesuvianite jade with good quality has been discovered in Hanzhong, Shaanxi Province, and its preliminary researches have been carried out on gemmological characteristics. However, there is still a lack of systematic research on the mineralogical characteristics of the vesuvianite jade and its relationship with the quality. In this study, the gemmological, mineralogical and spectral characteristics of vesuvianite jade from Hanzhong, Shaanxi Province, were systematically analyzed using conventional gemmological tests, polarizing microscope, electron probe, infrared spectrometer, ultraviolet-visible spectrometer, etc. The colour genesis of vesuvianite jade was initially explored. The vesuvianite jade samples are mainly green, with uneven colour distribution, and different shades of light green, yellowish green and emerald green can be seen locally; they are dense and massive, with micro-metamorphic structures, and unequal grain-like metamorphic, columnar and fibrous structures can be seen locally; the main mineral is vesuvianite and the secondary minerals include garnet, diopside, chlorite, epidote and calcite. The main chemical compositions of vesuvianite jade is: w (SiO₂ ) = 37.10 %-38.29 %, w (Al₂O₃) = 17.49 %-19.67 %, w (CaO) = 38.18 %-39.28 %, with a small amount of FeO and MgO. The infrared spectrum of sample is basically consistent with that of vesuvianite. The colour of sample mainly depends on the colour of the vesuvianite jade crystals and distribution. The chemical compositions and UV-Visible spectral characteristics of different colours samples indicate that the electron transition of Fe²⁺ and the charge transfer between Fe²⁺-Fe³⁺ in the vesuvianite crystals lead to the green colour. The yellow color is mainly caused by the charge transfer between O^2- → Fe²⁺ in the vesuvianite crystals, and the electron transition of Fe³⁺ plays a supporting role.