Methane hydrate behavior under high pressure

Phases changes in a water-methane system were investigated in a pressure range from 0.2 to 5.5 GPa using a diamond anvil cell. In-situ X-ray diffractometry and optical microscopy revealed methane hydrate behavior from growth to decomposition into high-pressure ice and solid methane at room temperature. Methane hydrate crystallized at 0.2-0.3 GPa from liquid, and it was compressed continuously until 2.3 GPa, maintaining structure I. Below 0.7 GPa the cage occupancy was unchanged. At 1.5 GPa methane hydrate partly decomposed to ice IV and fluid methane. The remaining methane hydrate kept structure I, but the cage occupancy was changed; i.e., small cages became vacant. At 2.1 GPa, coexisting ice VI transformed to ice VII and fluid methane solidified to phase I, while methane hydrate remained. At this pressure, structure I of methane hydrate was still maintained, and an additional change of cage occupancy occurred. The change in the cage occupancy is consistent with the change in compressibility observed on the compression curve. At 2.3 GPa, all of the methane hydrate decomposed into ice VII and phase I of solid methane.