Six olivine inclusions in diamond are remnants of a syngenetic monocrystal

Abstract

We report the discovery of a diamond containing six thin, flat olivine inclusions. Cathodoluminescence (CL) imaging and X-ray tomography revealed inclusion alignment within and parallel to a single diamond growth zone. A novel application of Fourier Transform Infrared spectroscopy to diamond inclusions established that they shared a common crystallographic orientation. Whilst the olivine (100) and diamond (111) planes were parallel, the olivine b- and c-axes were not parallel to any of the diamond's major crystallographic axes. The shared morphology, spatial distribution and crystallographic orientations of the inclusions can only be explained by a single crystal origin, which has previously been used as proof of protogenesis. However, unusual CL dead zones, thought to be related to the partial dissolution and replacement of the monocrystal, are exclusively observed in the plane of the inclusions, implying that the original crystal was never equant when interacting with the diamond. As we are not aware of any mechanism resulting in olivine independently adopting such a high aspect ratio (over 60:1) in the mantle, we favour a model wherein the monocrystal grew as a thin layer on a pre-existing diamond surface before being partially dissolved and rapidly encased. We therefore conclude that the olivine inclusions are remnants of a syngenetic monocrystal, and that crystallographic orientation alignment between inclusions does not always imply a protogenetic relationship.