Inclusion and point defect characteristics of Marange graphite-bearing diamonds after high temperature annealing

Abstract

This study gives an analysis of the effect of low-pressure, high-temperature annealing on the infrared, Raman, and photoluminescence (PL) features, as well as the inclusion characteristics, of cubo-octahedral diamond plates from the Marange deposits in Zimbabwe. The samples showed strong inclusion-related zoning which consists of micron-sized particles identified as graphite and these grew noticeably larger with annealing at temperatures of 300°C to 1700°C. Within the natural diamonds, the graphite inclusions (detected by Raman spectroscopy) had a grain size of approximately 1μm, which increased to 3μm after 1200°C and 14μm after 1700°C annealing and their hexagonal morphology was discernible. From the geometry of these grains, we determined that they were oriented within the {111} family of planes. The infrared absorption and PL spatial maps were collected after every temperature step to study the effects of annealing on the defects, and photomicrographs and Raman spectra were collected to study the graphite inclusions. The graphitic inclusions grew much larger as the stressed diamond surrounding them converted to graphite. Many nitrogen-related optical centers, including NV− and H3 are no longer detected after high temperature annealing within the cuboid regions as these may have been transformed to hydrogen-bearing complexes such as NVH and N2VH. The presence of CH4 is detected in the unannealed Marange diamonds, but was no longer observed in Raman spectra after 1200°C annealing. This CH4 disappearance along with changes in inclusion morphology could provide a method to detect heat treatment if these mixed-habit samples are sourced to create treated black gem diamond.