Defects in a mixed-habit Yakutian diamond: Studies by optical and cathodoluminescence microscopy, infrared absorption, Raman scattering and photoluminescence spectroscopy

Widespread occurrences in the crystallisation history of natural diamonds are epochs of mixed-habit growth in which normal {1 1 1}-faceted growth is accompanied by non-faceted growth on curved surfaces of mean orientation ∼{1 0 0}, termed ‘cuboid’. This paper analyses mixed-habit-related phenomena in a near-central, (1 1 0)-polished slice of an octahedron from the Mir pipe, previously studied principally by SIMS probes analysing N impurity content and C and N isotope composition. In the present work, newly studied features include dislocation content, fine structure in cathodoluminescence (CL) patterns, refined IR absorption data, Raman and photoluminescence (PL) microspectroscopy and microscopy of internal non-diamond bodies. Topographic imaging and spectroscopic techniques traced the specimen's morphological evolution from a cubo-octahedral core containing complex relative development of {1 1 1} and cuboid sectors, both populated by graphite crystallites, diameters up to ∼5 μm, lying on all diamond host {1 1 1}. Coherently overgrowing the core was a zone of widely but smoothly varying relative development of {1 1 1} and cuboid sectors, both on birefringence evidence dislocation-free, emitting strongly from cuboid sectors the PL spectra associated with Ni–N-vacancy complexes. An enclosing octahedral shell of solely {1 1 1} lamellae terminated mixed-habit growth. High-resolution FTIR absorption measurements of I(B′), the integrated absorption due to {1 0 0}-platelet defects, showed from its absence or weakness that total or substantial platelet degradation had taken place in the mixed-habit zones, indicating that these had undergone conditions close to the diamond–graphite phase boundary in their history.