Crystallographic relationships between diamond and its olivine inclusions

The syngenicity of inclusions in diamonds is generally assumed on the basis of morphological criteria. It has been suggested that an epitactic relation between an inclusion and its host would represent a more robust proof of syngenicity. Yet the crystallographic relations between inclusions and their hosts are rarely determined and a systematic survey of these relations for the different mineral species is lacking. Modern in-situ single-crystal X-ray analysis can provide a high-quality, non-destructive, three-dimensional examination of the geometrical relations between the crystal lattices of the inclusion and of the diamond host. We will present preliminary results of an on-going single-crystal X-ray diffraction study of olivine inclusions in diamond. We have investigated four diamonds from the Udachnaya kimberlite (Yakutia). Three of the diamonds include a single olivine and the fourth diamond includes three olivines. All olivines show a negative-crystal shape. For the first three diamonds, the included olivines show the following orientations (within ±12°): a) the (111) plane of the diamond is parallel to the (-1-33) plane of the olivine; b) the (-111) plane of the diamond is parallel to the (010) plane of the olivine; c) the (1-11) plane of the diamond is parallel to the (150) plane of the olivine. In the fourth diamond, all three included olivines showed a similar orientation, with the (001) plane of the diamond being parallel to the (010) planes of the olivines. Our preliminary data indicate a wide range of possible orientations for olivine in diamond. Our database is being extended to improve the statistical significance of our results for both single and multiple inclusions of olivine in diamond.