Gemology,Mineralogy And Trace Element Study On Green Nephrite From Qinghai Province

There are varieties of nephrite occurring in Qinghai province,China,including the green nephrite.However,the present research on nephrite from Qinghai mainly focuses on the white nephrite.Green nephrite from Qinghai is hardly studied.Samples of green nephrite from Qinghai and its wall rock were tested by standard gemological methods,Fourier transform infrared spectroscopy?FTIR?,laser Raman spectroscopy,electron microprobe analysis?EMPA?,back scattered electron imaging?BSE?,X-ray fluorescence?XRF?,inductively coupled plasma mass spectrometry?ICP-MS?,stable isotope mass spectrometry,to study its gemological and petrographic characteristics,bulk-rock chemical composition,and O-H isotope composition.Furthermore,it was compared with green nephrite from Pakistan,Canada,New Zealand,and Russia.Based on the geological background and previous data,the genesis and formation processes of green nephrite from Qinghai were discussed.Green nephrite from Qinghai shows a greasy to waxy luster,with a typical felted texture,the refractive index ranges from 1.60 to 1.61,Mohs hardness is about 6.5,which is consistent with the theoretical value of nephrite.The result of FTIR and Raman spectroscopy of the samples indicates that they are nephrite.In the infrared spectrum,some peaks are missing in the range of1150⁸50 cm^-1,and peaks in the range of 800⁶00 cm^-1 have drifted to low frequency.In the Raman spectrum,the peak of 930cm^-1 is missing.They are both related to the increase of Fe²⁺content resulting from the replacement of Mg²⁺in green nephrite.The result of EMPA suggests that the green nephrite from Qinghai is mainly composed of tremolite,with minor actinolite,diopside,zircon,chromite,chlorite,zoisite,and rutile.Among these minerals,diopside was replaced by tremolite in the Al-rich environment to form nephrite,chromite provides Cr for tremolite,and the presence of zircon provides a possibility for the dating of green nephrite from Qinghai.The result of EDXRF of nephrite from Qinghai suggests that the depth of green color is positively correlated with the content of Cr and Fe.The bulk-rock chemical composition measurement and O-H isotope mass spectrometry experiment were carried out on green nephrite from Pakistan,Canada,New Zealand,Qinghai and Russia.According to the content of Fe₂O₃ and FeO,the ratio of Fe/?Fe+Mg??higher than 0.06?,and the concentration of Cr and Ni?much more higher than those in dolomite-related nephrite?,the samples belong to serpentinite-related nephrite.The bulk-rock REE patterns exhibit a lower?REE concentration than those in dolomite-related nephrite.The O-H isotope values of samples indicate the ore-forming fluids are mainly metamorphic water,differing from the dolomite-related nephrite,which is mainly magmatic water,meteoric water and CO₂ released from dolomite.There is no significant difference in bulk-rock chemical composition and O-H isotope values among green nephrite from different deposits.