Investigation of the mineralogical composition and origin analysis of black jadeite

Analyzing black jadeite variety is helpful not only to distinguish black jadeite from the common omphacite jadeite, but also to learn the origin of jadeite by studying these special specimens. The basic gemological properties, mineral composition, structural characteristics, spectroscopic features, and color-causing mechanisms of black jadeite were studied through testing methods such as Polarizing microscope, X-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy. Recent studies have confirmed the distinct crystalloblastic textures of black jadeite-namely columnar, granular, and fibrous-alongside its characteristic fracture structures. This research further identifies the presence of secondary minerals, primarily opaque black impurities consisting of a graphite and disordered graphite mixture, as revealed by Raman spectroscopy and X-ray diffraction analyses. These analyses have elucidated that the variable distribution of graphite mixtures contributes to differing degrees of light absorption, thereby forming various shades of black, a critical factor in the gemstone's coloration. This study suggests that these carbonaceous materials are a byproduct of the carbonization of organic matter within metamorphic fluids, subsequently forming dispersed graphite structures within the mineral matrix and its fractures, indicative of multi-stage metamorphic processes. Additionally, the identification of graphite supports the theory that jadeite's protolith originated from the subduction processes of the Neotethys Ocean, accompanied by metasomatism and high-pressure metamorphism of olivinite.