Hydrothermal syntheses of graphite from bituminous coal at 0.5-5 kbar water vapor pressure and 300-600°C

Abstract

Hydrothermal experiments were carried out to synthesize graphite from bituminous coal at water vapor pressure of 0.5-5 kbar using 1% Li2CO3 or 2% Ni metal as a catalyst. Oxygen fugacities were controlled by hematite+magnetite (HM), nickel+nickel oxide (NNO) or quartz+ fayalite +magnetite (QFM) buffers during six runs at 1 kbar. Graphite crystallized from bituminous coal at about 375°C, 0.5 kbar; 405°C, 1 kbar; 390°C, 2 kbar and 320°C, 5 kbar under uncontrolled fo2 conditions. The transition boudary from bituminous coal to graphite has a positive inclination below 1 kbar and a negative inclination over 1 kbar under uncontrolled fo2 conditions. The negative inclination of the transition boundary was probably due to the experiments that the oxygen fugacity was incompletely buffered by the escape of hydrogen gas from the system through the Ag-Pd capsule-wall. Under the foe controlled conditions by NNO buffers, graphite crystallized between 400°C and 420°C at 1 kbar. When fo2 were controlled by NNO or QFM, the crystallinity of graphite increased abruptly to reach a fully-ordered state at 420°C. The experimental results were compared with graphitization of carbonaceous material in black schists from some metamorphic districts, and also with oxygen isotope geothermometry. In natural systems, graphitization depends on metamorphic temperature and oxygen fugacity, but not on metamorphic pressure to a great extent.