1. National Gemstone Testing Center，Shanghai Laboratory，Shanghai 200122，China 2. School of Ocean and Earth Sciences，Tongji University，Shanghai 200092，China 3. Laboratory of Gem and Technological Materials，Tongji University，Shanghai 200092，China
Abstract：Dendrite shaping as micro fractures is commonly found in surface of nephrite gravel and it reveals its weathering process. However, study on the petrological and mineralogical characteristics of dendrite in nephrite gravel is so far limited. Therefore, Xingjiang nephrite gravel was selected as researching sample, while electron microprobe(EPMA), Raman spectroscopy (Raman)and scanning electron microscope(SEM)were used to study the sample in terms of the chemical composition, Raman spectra and microstructure characteristics. The results are shown as follows, EPMA analysis indicates that MnO and BaO are the major chemical compositions for dendrite. The contents of MnO and BaO are 49.045%~54.012% and 9.012%~10.961% respectively. Raman spectra analysis shows that matrix of nephrite gravel mainly consists of tremolite, while dendrite mainly consists of supergene manganese minerals and organic matter. Raman peaks relating to Mn—O stretching vibration mode of MnO6 octahedra in supergene manganese minerals appear at nearly 475, 498, 510, 575 and 617 cm-1. In addition, intensity and sharpness of the Raman peaks mentioned above being different indicates that supergene manganese minerals are with different crystallinity in each test location. Raman peak relating to C—C stretching vibration mode in organic matter appears at 1 590 cm-1; while Raman peak appearing at 1 370 cm-1 is related to structure defect and disordered arrangement. SEM result reveals that the dendrite thin film covers on and contacts abruptly with the tremolite fibers which are arranged in pilotaxitic texture. Chemical composition, Raman spectra and microstructure characteristics comprehensively indicate that the formation of dendrite in nephrite gravel is related to multi-stage deposition of manganese compounds and organic matter in the river.
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