Study on the Mineralogical and Optical Characteristics of Blue Opal from Lishui Investigated by XRF-SEM-XRD-FTIR

Opal deposits and their corresponding mineralogical characteristics are rarely reported in China as far as can be ascertained. Recently, a type of blue quasi-opal deposit was discovered in Jinyun, Lishui, Zhejiang Province, which has attracted the attention of gem researchers. While the related investigation about this blue quasi-opal has not been reported elsewhere, in this paper, the mineralogical and optical characteristics of blue quasi-opal collected from Jinyun, Zhejiang Province are carried out primarily by X-ray Fluorescence Spectrometry (XRF), Field Emission-Scanning Electron Microscopy (FE-SEM), X-ray Powder Diffraction (XRD) and Fourier Transformation Infrared Spectroscopy (FTIR). The results show that the main compositions of blue quasi-opal mineral are silica (95%) and a small amount of Fe₂O₃ and Al₂O₃. Meanwhile, an iridescent effect is found on the surface of both unheated and heated samples, which resembles those of natural opals. The XRD spectra with sharp diffraction peak are consistent with those of cristobalite, which is obviously different from some microcrystalline (such as Opal-CT, Opal-C) and amorphous opals (Opal-A). In addition, its infrared spectrum shows a typical frequency shift compared to natural opals. As a consequence, these blue ores are preliminary classified as quasi-opals and named as a variant of volcanic-type opals since the geological structure of Jinyun area has the characteristics of Mesozoic volcanic rocks. Based on FE-SEM investigation, the iridescent effect is attributed to imbricate structure on ore surface. Further research is needed to confirm their exact category.