166-11 The heat is on: Transforming cloud inclusions in asteriated diamonds

Session: Mineralogical Characterization of Economic Resources: From Critical Minerals to Gemstones

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Asteriated diamonds are geological curiosities that generally show visible, often symmetric gray clouds; these aesthetically pleasing gems demonstrate interesting diamond growth mechanisms and have been long prized by collectors. This study analyzed the effects of low-pressure, high-temperature annealing on the infrared absorption, Raman, and photoluminescence (PL) spectroscopy, as well as the inclusion characteristics, of similar diamond plates with cubo-octahedral clouds. The samples exhibited strong inclusion-related zoning, comprising micron-sized particles identified as graphite, which grew noticeably larger with annealing at temperatures ranging from 300°C to 1700°C. Specifically, the graphite inclusions, detected by Raman spectroscopy, had an initial grain size of approximately 1 micron, which increased to 3 microns after annealing at 1200°C and further expanded to 14 microns after annealing at 1700°C. Moreover, the hexagonal morphology of these grains became discernible with optical microscopy, and their orientation within the {111} plane was determined. To further investigate the effects of annealing on defects and graphite inclusions, infrared absorption and PL spatial maps were collected after each temperature step. Additionally, photomicrographs and Raman spectra were collected to study the evolution of graphite inclusions. The results show that the graphitic inclusions grew significantly larger as the stressed diamond surrounding them converted to graphite. Furthermore, many nitrogen-related optical centers, including NV^- and H3 centers, were no longer detected after high-temperature annealing within the cuboid regions, suggesting their potential transformation to hydrogen-bearing complexes such as NVH and N₂VH. Notably, the presence of methane was detected in the unannealed cubo-octohedral diamonds, but was no longer observed in Raman spectra after annealing at 1200°C. The disappearance of methane, combined with changes in inclusion morphology, could provide a potential method for detecting heat treatment in mixed-habit diamond samples.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7898

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