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

DOI: 10.1007/s00410-019-1559-5 DOI Help

Authors: Paolo Nimis (Università degli Studi di Padova) , Ross J. Angel (University of Pavia) , Matteo Alvaro (University of Pavia) , Fabrizio Nestola (Università degli Studi di Padova) , Jeff W. Harris (University of Glasgow) , Nicola Casati (Swiss Light Source) , Federica Marone (Swiss Light Source)
Co-authored by industrial partner: No

Type: Journal Paper
Journal: Contributions To Mineralogy And Petrology , VOL 174

State: Published (Approved)
Published: April 2019
Diamond Proposal Number(s): 7616

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.

Keywords: Diamond; Magnesiochromite; Inclusion; Crystallographic orientation; Syngenesis; Protogenesis

Subject Areas: Earth Science

Beamlines: I15-Extreme Conditions

Other Synchrotrons: SLS