[S-MP47_1AM2] 鉱物の物理化学

[SMP47-10] メタンハイドレートの高圧相への転移のメカニズム

*門林 宏和¹、平井 寿子¹、平尾 直久²、大石 泰生²、大竹 道香³、山本 佳孝³、小島 洋平¹ (1.愛媛大学地球深部ダイナミクス研究センター、2.高輝度光科学研究センター、3.産業技術総合研究所)

キーワード：メタンハイドレート, 相転移メカニズム, 高圧, X線回折, ラマン分光

Methane hydrate (MH), called as “burning ice”, is expected to be a fruitful natural resource, at the same time, methane is rather effective greenhouse gas than carbon dioxide. It is also thought to be a major constituent of icy bodies in and outside the solar system. MH is composed of hydrogen-bonded host water molecules forming cages or frameworks that include guest methane molecules. Three phases of MH have been known so far. The low-pressure phase, sI, is stable below 0.8 GPa and it transforms into a hexagonal structure, sH, at 0.8 GPa and further transforms to a filled ice Ih (FIIhS) at 1.8 GPa at room temperature. At these phase transitions, release of water content is accompanied. As described above, the existence of phase transitions and the structures of transformed high-pressure phases have been clarified by the previous studies. However, transition mechanisms from the lower-pressure phase to individual high-pressure phases have been unresolved issue. In this study, high-pressure experiments were performed to investigate mechanisms of the phase transitions of MH at high pressures. Clamp type and lever-spring type diamond anvil cells were used in this study. The pressure and temperature conditions were from 0.2 to 3.0 GPa and 300 K, respectively. Pressure measurements were made via a ruby fluorescence method. The initial samples of MH were prepared by ice-gas interface method. The samples were characterized via time-resolved X-ray diffractometry using synchrotron radiation at BL-10XU, SPring-8, and time-resolved Raman spectroscopy at GRC, Ehime University. As for sI-sH phase transition, Raman spectroscopy revealed that 5¹² cages of sI survived during the transition and that the 5¹² cages remained as same 5¹² cages of sH structure. And, 5¹²6² cages of sI changed to 4³5⁶6³ and 5¹²6⁸ cages of sH. The results suggested that the sI-sH transition may follow a martensitic-like mechanism because of being maintaining 5¹² cages unchanged in sH structure. On the other hand, at sH-FIIhS transitions, Raman spectroscopy detected abrupt collapse of all constituent cages in sH and release of fluid or solid methane molecules. And then, the framework of FIIhS was gradually reconstructed, absorbing the released methane molecules into the FIIhS structure. The results indicated that the sH-FIIhS transition follows reconstructive mechanism. The explanations may be reasonable, because the former transition is from a cage to another cage structure, and the latter one is from a cage structure to different framework of a filled ice structure.