A Study Of Gemological Mineralogy Of Green Nephrite From Qiekuotai Deposit In Manasi County,Xinjiang

Manasi county in Xinjiang province has one of the most important green nephrite mines in China.The mining history can be traced back to the period of Emperor Qianlong of Qing dynasty.The nephrite mine was found again in 1973 and later put into production.The green nephrite deposit in Manasi is the ultrabasic type of metasomatic replacement deposit.It is located in the ophiolitic melange belt of early Palaeozoic in North Tianshan,Xinjiang.The nephrite is formed by the contact replacement between the ultrabasic rock and the basic country rock.It shows an occurence of wedge-shape or lenticular structure,where the ultrabasic rock branches intruded into the country rock.The ultrabasic rock is strongly serpentinized.And some sepentinite has been altered into talc and magnesite.The nephrite in Qiekuotai has different tone of green color,such as pale green,vivid green,or yellowish green.Some nephrite has brown color like caramel.The green nephrite always has opaque “black spot” inclusions,or “green spots” inclusions which the color is lighter than green nephrite.The average reflective index of nephrite is 1.61.The average of relative density of nephrite is 2.96.The nephrite has a vitreous luster,opaque to translucent transparency.It doesn't show an obvious absorption spectrum observed by a spectroscope.And it is inert under the ultraviolet light.The major mineral of the nephrite in Qiekuotai is tremolite.The textures of tremolite are felt-like blastic texture,fiber blastic texture,harbor-like blastic texture and coarse grained blastic texture.The average ratio of Mg/(Mg+Fe2+)is 0.934.The minor minerals are chromite,chlorite and uvarovite.Chromite is the main composition of "black spots",while uvarovite is the main composition of "green spots".The analyse of the result of EMPA shows the average formation temperature and pressure of chromite are 1336.54? and 3.188 GPa.The average formation temperature of chlorite is 141.5?.The formation temperature of uvarovite is 650-350?.Compare with the nephrite from other mines,the nephrite in Qiekuotai has less Cr content,with an average of 83.78 ppm.Also the nephrite in Qiekuotai has less total amount of REE,which average is 2.165.The nephrite samples are light REE enrichment type,most with positive Eu anomalies.The nephrite samples are lack in Hf?Zr?Ta content,but enrich in Sr content.The nephrite samples have higher ratio of Rb/Ba and Sr/Ba.The Fe2+?Fe3+ inter-valence charge transfer transition in tremolite,causes a strong visible light absorbance from 550 nm to 800 nm,and gives the nephrite a green color.The different visible light absorbance from 270 nm to 500 nm gives nephrite a tone from pale green to yellowish green,due to Fe2+?O metal-to-ligand charge transfer intensity.