Diamond-anvil cell observations of a new methane hydrate phase in the 100-MPa pressure range

A new high-pressure phase of methane hydrate has been identified based on i ts high optical relief, distinct pressure-temperature phase relations, and Raman spectra. In-situ optical observations were made in a hydrothermal dia mond-anvil cell at temperatures between -40 degrees and 60 degreesC and at pressures up to 900 MPa, Two new invariant points were located at -8.7 degr eesC and 99 MPa for the assemblage consisting of the new phase, structure I methane hydrate, ice Ih, and water, and at 35.3 degreesC and 137 MPa for t he new phase-structure I methane hydrate-water-methane vapor. Existence of the new phase is critical for understanding the phase relations among the h ydrates at low to moderate pressures, and may also have important implicati ons for understanding the hydrogen bonding in H2O and the behavior of water in the planetary bodies, such as Europa, of the outer solar system.