Mineralogical Characteristics Of The Black Quartzite Jade In Linwu District,Hunan Province

A sort of dark gray-black quartzite jade commercially named "Tongtian Black Jade" has recently been discovered in Mount Tongtian in Linwu City of Hunan Province,the finished product of which as exquisite structure.In this paper,the studies on quartzite jade conducted by the predecessors are taken into consideration to analyze the samples by conventional gemological test,and put under observation with the microscope and thin-section lens,as well as to analyze by means of X-ray powder diffracttion,election probe,X-ray fluorescence spectrometer,infrared spectrum,ultraviolet and visible spectrophotometer and ruman spectroscopy,to research systematically the gemological,mineralogical and spectral characteristics of the sample and to discuss further its metallogenic mechanism and color genesis.The gemological characteristics of the black quartzite jade in this area are similar to those of the other quartzite jades,but with relatively low hardness.The Mohs hardness of most of the samples falls in the range between 4.5 to 5.5.Under the microscope,most of the samples exhibit the cryptocrystalline-fine-grained structure,and some of them can be observed with white dotlike and needle-like minerals,while under the polarizing microscope,many texture of crytalloblastic /palimpsest are observable.Based on the analysis of election probe and X-ray powder diffraction,the main mineral of the sample is quartz,and the secondary minerals are mica,feldspar and clay minerals,with a small amount of andalusite,almandine,graphite,and the iron mineral.The crystallinity of quartz in the samples is relatively high,which is higher than 9(cell parameters are close to the standard values).which is regional metamorphite of the typical parametamorphic rock formation.According to the sample's main structure,configuration features and chemical,mineral composition,it is regarded as typical low-to-moderate temperature hydrothermal regional metamorphism of metamorphic rock series,which is classified as greenschist facies,with mineralization temperature between 400? to 450?.The virgin rock is made of sedimentary rock abundant in aluminum and quartz,feldspar.The tectonic environment that it forms belongs to continental borderland.The results of infrared spectroscopy show that the sample contains some part of absorbed water and a small amount of consititutional water,and the absorption of Si-O bond is splits obvious,which further indicates that the crystallization degree of quartz,which is the main mineral of the sample,is on the higher side.When analyzed with Raman spectroscopy,weak absorption peaks of coesite were demonstrated at some locations,indicating that the crystallinity of the sample is not as high as that of the single crystal quartz.The analysis of ultraviolet-visible spectrophotometer showed that the absorption wavelength of the sample falls in the range between 450?500nm,where the color saturation is on the relatively higher side.With the results of the analyses mentioned earlier,it can be basically concluded that the sample is colored by the impurity of carbon.