Mineralogical Characteristics And Color-causing Mechanism Of Qin-jade

The gemological and mineralogical characteristics of the quartz of Qin-jade in Luonan County,Shaanxi Province were studied by means of optical microscopes,X-ray diffractometer,electron probe,infrared spectrometer,thermogravimetric analyzer,UV-Visible-near infrared spectrophotometer and other equipment.The formation of color and deposit genesis of Qin-jade were discussed.The color of Qin-jade is rich and varied,there are mainly red,purple,green and light yellow-green,translucent-opaque,with glassy luster.Its refractive index ranges from 1.53 to 1.55,and relative density from 2.65 to 2.69,Mohs hardness 6.5-7 and the UV fluorescence is inert.This kind of quartz jade normally in the form of granular crystal structure and a special layered ring structure,and main mineral allomorphic granular quartz with a particle sizes of 5-20?m,which accounts for over 90%.Auxiliary minerals of the jade consist of hematite,goethite,chlorite,mica,halloysite,with small amorphous iron oxide and iron compounds.The chemical composition has a small amount of impurity elements such as K,Mg,Al,Fe and Ti besides Si and O.The color of Qin-jade is closely related to the content and presence minor minerals,iron compounds and impurity elements.Red-brown iron compounds such as hematite,goethite and amorphous hydrated iron oxide of 1-10?m size were disseminated randomly among quartz particles,with w(FeO)content varying from 84.39% to 7.08% and containing about 10% water,which is the main cause of red color genesis of the jade.Different shades of green are caused by the uniform distribution of pieces chlorite(10?m)and fine scale sericite(2-10?m)in jade,while mica,halloysite and other clay minerals lead to the appearance of pale yellow.Purple color in jade is resulted from the replacement of silicon in quartz crystal by 0.15% to 0.23% titanium and iron.The distribution characteristics of red-brown iron compounds and chlorite in different regions makes it forming "red and green" or "red and white" color stripe.The siliceous jade ore body of the Jixian system in the Middle Proterozoic of Luonan County,Shaanxi Province is produced in a layered and layer-like manner.The siliceous rock in w Al/w(Al+Fe+Mn)ratio is 0.45% to 0.58%,and the content of aluminum is as high as 6.50%-17.59%,the average value of ?REE is 5.11?g/g,and the ?Eu,?Ce and(La/Ce)N values are 0.86,1.02 and 0.85.It is speculated that the siliceous rocks in the area are the products of sedimentary or diagenetic stages,and the formation of the massive and lenticular jade is related to the changes in the redox environment during the later metamorphism.