Graphitization of hydrogen-rich organic matter under high temperature and pressure

Abstract The graphitization of deep carbon, particularly hydrogen-rich organic matter (HROM), under extreme conditions is crucial for understanding the evolution of carbonaceous materials within geological settings. Despite the significance, the detailed mechanisms of HROM transforming into crystalline graphite under high-pressure remain poorly understood. This study addresses this gap by conducting high-temperature and high-pressure (HTHP) experiments on liptinite, a type of HROM extracted from coal. We subjected liptinite to pressures ranging from 500MPa to 3GPa and temperatures up to 1073K to simulate geological conditions. Using techniques such as optical microscopy, Raman spectroscopy, X-ray diffraction, and high resolution transmission electron microscopy, we characterized the structural and valence state changes of liptinite. Key findings include the transformation of liptinite into a three-dimensional graphene foam and nanocrystalline graphite at pressures above 1.5GPa. We observed an increase in sp2 carbon content and a decrease in sp3 carbon, indicating a reduction in defects and an enhancement in aromatic carbon networks. The study also revealed that HROM contributes significantly to the horizontal growth but less to the vertical stacking of carbon layers during graphitization, which has implications for the differential graphitization processes of sedimentary organic matter. This research provides unique insights into the graphitization process of HROM and its implications for the geological stability and preservation of organic carbon in the lithosphere. The findings contribute significantly to the understanding of carbon evolution and the formation of graphite deposits.