津巴布韦金刚石中石墨包裹体及金刚石异常双折射特征分析

Analysis of Abnormal Birefringence and Graphite Inclusions in Zimbabwean Diamonds

津巴布韦马朗(Marange)金刚石矿以产出混合习性(八面体与近立方体)金刚石为特征,其石墨包裹体仅存在于近立方体区。石墨包裹体的形态、分布及金刚石的异常双折射与应变特征,能反映其从开始结晶到被搬运至地表过程中经历的地质作用。因此,对津巴布韦混合习性金刚石及石墨包裹体的研究不仅能提供与其他产地金刚石有对比意义的数据,且由于其本身生长习性的特殊性,故两生长区的精细分析对帮助理解不同习性金刚石在地质过程中的行为差异具有重要价值。本文采用DiamondView;、偏光显微镜、扫描电镜、能谱仪、红外光谱仪和拉曼光谱仪对津巴布韦混合习性金刚石及石墨包裹体进行系统观察分析。结果显示,石墨包裹体存在于近立方体区的定向椭圆裂隙中,属于由金刚石石墨化而成的同生-次生包裹体。近立方体区与H有关的红外吸收更强,而八面体区与N有关的吸收更强,这种对不同杂质元素的差异富集,导致金刚石呈现主要与裂隙及不同生长区有关的异常双折射。金刚石八面体区LO=TO带拉曼位移为1332.05~1332.20cm^(-1),半高宽为4.21~4.37cm^(-1),对应应力约0.06~0.27GPa;近立方体区LO=TO带拉曼位移为1331.93~1332.47cm^(-1),半高宽为3.67~4.08cm^(-1),对应应力约0.01~0.64GPa。总体来看近立方体区的残余应力更大,应变更强。本研究确定了津巴布韦混合习性金刚石中石墨包裹体的定向性,发现了证明其同生-次生性的新证据,并揭示了两生长区金刚石的应变特征差异,对了解津巴布韦金刚石的形成环境和不同金刚石的物理化学性质差异具有重要意义。

BACKGROUND: The 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.OBJECTIVES: To 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.METHODS: The growth structure and growth sectors of Zimbabwean diamonds were observed by DiamondView;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.RESULTS: Graphite 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 1332.05-1332.20 cm^(-1), the FWHM was 4.21-4.37 cm^(-1), which corresponded to stress of 0.06-0.27 GPa. The Raman shift of LO=TO band in cuboid sectors was 1331.93-1332.47 cm^(-1), the FWHM was 3.67-4.08 cm^(-1), which corresponded to stress of 0.01-0.64 GPa. In general, the residual stress and strain were greater in cuboid sectors.CONCLUSIONS: The 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.