| In this paper,the gem mineralogical characteristics of the Cenxi"old jade"were systematically studied using conventional gemological instruments,polarizing microscope,X-ray powder diffractometer,scanning electron microscope,electron microprobe,Fourier infrared spectrometer and other large-scale testing instruments.Meanwhile,the name of the jade was determined,and the genesis of the jade was preliminarily discussed.The raw Cenxi"old jade"is rounded in appearance.According to color characteristics,two series namely dark color and light color can be divided from the Cenxi"old jade".The dark one is mainly black and gray-blue,randomly with white and yellow veins.The rough surface of the black sample is dull,whereas the gray-blue sample is silky luster.The jade is opaque to translucent with dense and partially visible radial,fibrous,and occasionally flashes of mineral cleavage surface.The light one is dominated by off-white,yellow-brown,black and dark green randomly distributed clumps,with yellow veined or banded surface and glass luster.The yellow-brown sample is flake and fibrous with columnar flashes on the surface.The relative density,refractive index,and Mohs hardness of the Cenxi"old jade"are 3.04 to 3.18,1.64 to 1.66,6.5 to 7,respectively.No fluorescence can be observed under long and short wave ultraviolet light.The Cenxi"old jade"has a delicate structure,mainly composed of micro-fiber interweaving structure and micro-fiber crystallization structure.Bundle-like,column-like crystalline structure and micro-sheet structure can also be observed.The major minerals of the jade are fibrous and columnar sillimanite,while the minor minerals include muscovite,biotite,chlorite,and apatite.The content and distribution characteristics of the minor minerals in different samples are slightly different.A small amount of monazite and zircon also present in the jade.According to conventional gemological parameters and mineral composition of jade,and the naming rules of China's latest"Name of Jewelry and Jade"?GB/T16552-2017?,the research sample should be named sillimanite jade.The infrared spectrum of sillimanite jade shows typical spectral characteristics of sillimanite.The main absorption peaks are approximately 1180cm-1,960cm-1,899cm-1,820cm-1,748cm-1,688cm-1,628cm-1,560cm-1,510cm-1,482cm-1,and 445cm-1.When heated to 950?,the weight loss of the sample is only 1.42%.This is mainly attributed to the adsorbed water in jade and the structural water released by water-bearing minerals such as muscovite,biotite and chlorite in jade,indicating that the thermal stability of sillimanite jade is very good.According to the structural characteristics and mineral compositions of the jade and geological tectonic movement in the Cenxi area,it is preliminarily concluded that the Cenxi sillimanite jade was transformed from a regional metamorphism of original rocks that rich in aluminum and high iron and a comprehensive alteration of later ore-forming fluids. |
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