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Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us?

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Abstract

We have studied by X-ray diffractometry the crystallographic orientation relationships (CORs) between magnesiochromite (mchr) inclusions and their diamond hosts in gem-quality stones from the mines Udachnaya (Siberian Russia), Damtshaa (Botswana) and Panda (Canada); in total 36 inclusions in 23 diamonds. In nearly half of the cases (n = 17), [111]mchr is parallel within error to [111]diamond, but the angular misorientation for other crystallographic directions is generally significant. This relationship can be described as a case of rotational statistical COR, in which inclusion and host share a single axis (1 df). The remaining mchr–diamond pairs (n = 19) have a random COR (2 df). The presence of a rotational statistical COR indicates that the inclusions have physically interacted with the diamond before their final incorporation. Of all possible physical processes that may have influenced mchr orientation, those driven by surface interactions are not considered likely because of the presence of fluid films around the inclusions. Mechanical interaction between euhedral crystals in a fluid-rich environment is therefore proposed as the most likely mechanism to produce the observed rotational COR. In this scenario, neither a rotational nor a random COR can provide information on the relative timing of growth of mchr and diamond. Some multiple, iso-oriented inclusions within single diamonds, however, indicate that mchr was partially dissolved during diamond growth, suggesting a protogenetic origin of these inclusions.

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Acknowledgements

This research was supported by ERC Starting Grant INDIMEDEA to FN (agreement n. 307322). MA was supported by ERC Horizon 2020 research and innovation programme (agreement n. 714936 for the project TRUE DEPTHS). JWH thanks the Diamond Trading Company (a member of the DeBeers Group of Companies) for the donation of the diamonds used in this study. We acknowledge the Paul Scherrer Institut, Villigen, Switzerland, and the Diamond Light Source, UK, for provision of synchrotron radiation beamtime at the TOMCAT beamline of the SLS (experiment e15427) and at the I15 beamline of the DLS (experiment EE7616), respectively. We are grateful to M. Bruno for reviewing an early version of the manuscript and useful discussion, and to O. Navon and an anonymous reviewer for their formal reviews, which helped us improve the paper.

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Correspondence to Paolo Nimis.

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Communicated by Othmar Müntener.

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Nimis, P., Angel, R.J., Alvaro, M. et al. Crystallographic orientations of magnesiochromite inclusions in diamonds: what do they tell us?. Contrib Mineral Petrol 174, 29 (2019). https://doi.org/10.1007/s00410-019-1559-5

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  • DOI: https://doi.org/10.1007/s00410-019-1559-5

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