Analysis of Abnormal Birefringence and Graphite Inclusions in Zimbabwean Diamonds

SUN Chengyang; LU Taijin; SONG Zhonghua; HE Mingyue; DENG Yi

doi:10.15898/j.cnki.11-2131/td.202111050165

SUN Chengyang, LU Taijin, SONG Zhonghua, HE Mingyue, DENG Yi. Analysis of Abnormal Birefringence and Graphite Inclusions in Zimbabwean Diamonds[J]. Rock and Mineral Analysis, 2022, 41(2): 199-210. DOI: 10.15898/j.cnki.11-2131/td.202111050165

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Analysis of Abnormal Birefringence and Graphite Inclusions in Zimbabwean Diamonds

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BACKGROUNDThe Marange diamond deposit in Zimbabwe is characterized by producing mixed-habit (octahedral and cuboid) diamonds. Graphite inclusions in these diamonds only exist in cuboid sectors. The morphological and distributional characteristics of graphite inclusions and the abnormal birefringence and strain characteristics of diamonds can reflect the geological process experienced by diamonds from the beginning of crystallization to being transported to the Earth's surface. Therefore, the study of diamonds and graphite inclusions in Zimbabwe can provide comparative data for diamonds from other deposits. Besides, due to the peculiarity of growth habits, detailed analysis would be of great value to help understand the behavioral differences of diamonds with different growth habits in geological processes. OBJECTIVESTo determine if graphite inclusions in Zimbabwean diamonds are syngenetic or epigenetic, and to reveal the relationship between graphite inclusions and the infrared absorption spectrum, Raman scattering spectrum as well as birefringence and strain characteristics of diamonds.METHODSThe growth structure and growth sectors of Zimbabwean diamonds were observed by DiamondView^TM image system. The morphological and distributional characteristics of graphite inclusions and abnormal birefringence in diamonds were analyzed by scanning electron microscopy (SEM) and polarized light microscopy. Analysis of distribution and relative concentration of impurity elements in different growth sectors was conducted by infrared spectroscopy. Strain characteristics of diamonds in different growth sectors were analyzed by Raman spectroscopy and projection diagram of corresponding results.RESULTSGraphite inclusions in cuboid sectors of Zimbabwean diamonds were syngenetic-epigenetic inclusions located in directional elliptical cracks. According to infrared spectra of different growth sectors, cuboid sectors showed stronger infrared absorption related to elemental hydrogen, while octahedral sectors showed stronger absorption related to elemental nitrogen. This enrichment of different impurity elements leading to abnormal birefringence was mainly related to cracks and different growth sectors in diamond. The Raman shift of LO=TO band in octahedral sectors was 1332.05-1332.20cm^-1, the FWHM was 4.21-4.37cm^-1, which corresponded to stress of 0.06-0.27GPa. The Raman shift of LO=TO band in cuboid sectors was 1331.93-1332.47cm^-1, the FWHM was 3.67-4.08cm^-1, which corresponded to stress of 0.01-0.64GPa. In general, the residual stress and strain were greater in cuboid sectors.CONCLUSIONSThe determination of the orientation of graphite inclusions in mixed-habit diamonds in Zimbabwe, provides new evidence to prove their syngenetic-epigenetic nature, and reveal the difference in the strain characteristics of diamonds in the two growth regions. This research is helpful for understanding the formation environment of diamonds in Zimbabwe and of different diamonds. The differences in physicochemical properties are of great significance.

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Analysis of Abnormal Birefringence and Graphite Inclusions in Zimbabwean Diamonds

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