Physico-thermal conditions for the occurrence of natural graphite: Geological estimation and experimental reproduction

The physico-thermal conditions for the occurrence of carbon materials, particularly highly crystalline graphite, in natural ores are briefly summarized by classifying them into four classes, metamorphic, fluid-deposited, Acapulco-Lodran, and ureilite. The temperature-pressure conditions for experimental graphite syntheses are reviewed by focusing on the effects of pressure, coexisting minerals, coexisting iron and hydrothermal pressurization. Under pressure, graphitization occurred at a temperature above 1,600 °C and a pressure of 0.3 GPa, and proceeded by a heterogeneous process involving the abrupt conversion of the T-component to the G-component due to the acceleration of graphitization at points of stress concentration. The coexistence of calcium carbonate (limestone in nature and CaCO3 in a reagent grade) accelerates graphitization at a temperature above 1,000 °C under 0.3 GPa pressure by a cooperative catalytic action of Ca ions and CO molecules. Under hydrothermal conditions, various carbon materials, such as diamond, graphite, carbon nanotubes, etc., were obtained, but no highly crystalline graphite has been obtained by fluid-deposition in nature. In close contact with iron ore in the Acapulco-Lodran meteorite, highly crystalline graphite occurred, which may related to experimental results on the formation of graphite by precipitation from carbon-saturated molten iron melt atmospheric pressure.