Mineralogical Characteristic of Black Quartzose Jade from Linwu, Hunan Province

In recent years, quartzose jade has gradually entered the market and been accepted by the public. A new kind of gray-black or black quartzose jade named "Tongtianmoyu" has been found in Linwu, Hunan Province. Mineral components and chemical compositions were analyzed by conventional gemmological instruments, polarizing microscope, X-ray powder diffraction(XRD) spectrometer, and electron probe micro-analyzer(EPMA). Spectroscopy characteristics were analyzed by Fourier transform infrared(FTIR) spectrometer, Raman spectrometer, and ultraviolet-visible (UV-Vis) spectrometer. The results turn out that the main structure of the quartzose jade is cryptocrystalline-fine grain structure. Further, crystalloblastic structure and palimpsest structure were observed under the polarizing microscope. In addition to organic carbon, it contains inorganic minerals, of which quartz accounts for about 50%, feldspar and muscovite account for about 30%, clay minerals account for about 10%, and a small amount of andalusite, almandite, pyrite and other Fe-minerals. Combining the results of structure and composition analyses, it is inferred that the quartzose jade was formed by siliceous shale under an epimetamorphism process. The characteristic absorption peaks of quartz can be seen in the FTIR and Raman spectra, and the Si-O bond absorption and division indicate the high crystallization degree of quartz. Characteristic absorption of Fe at visible wavelength of 450-500 nm was measured by UV-Vis spectrometer. Based on the results of filling degree of organic carbon, Raman spectrum and UV chromatogram, it can be inferred that the colour of the samples is mainly caused by carbon impurities.