The phase transition of methane hydrate in water-methane-ammonia system was investigated under pressures up to 20 GPa using synchrotron X-ray powder diffraction (XRD) combined with diamond anvil cells. The XRD experiments revealed that the sI cage structure (MH-I) of methane hydrate transforms into an sH cage structure (MH-II) at approximately 1 GPa, further transforms into a filled-ice Ih structure (MH-III) at approximately 2 GPa, and remains in this structure under pressures up to at least 20 GPa. Ammonia was observed as ammonia hemihydrate phase-II above 3.8 GPa. It is therefore considered that methane hydrate can coexist with aqueous ammonia below 3.8 GPa and coexist with ammonia hemihydrate phase-II above 3.8 GPa. The transition pressures of methane hydrate in the investigated system were consistent with those in water-methane system. These results indicate that, although ammonia is thought to inhibit methane hydrate formation, methane hydrate can be stable in water-methane-ammonia system up to at least 20 GPa and at room temperature. The pressure range in this study covered the pressure conditions inside icy moons, indicating that methane hydrate has a potential to be the main constituent of them.
|Journal||Journal of Physics: Conference Series|
|Publication status||Published - 2020 Aug 17|
|Event||27th AIRAPT International Conference on High Pressure Science and Technology - Rio de Janeiro, Brazil|
Duration: 2019 Aug 4 → 2019 Aug 9
ASJC Scopus subject areas
- Physics and Astronomy(all)