Ｘ線回折によるガスハイドレートの高圧相変化と地球惑星科学的意義

• Main Authors: 平井, 寿子, 山本, 佳孝, 川村, 太郎
• Format: Article in Journal/Newspaper
• Language: Japanese
• Published: 北海道大学低温科学研究所
• Subjects: 431.67; Methane hydrate
• Online Access: http://hdl.handle.net/2115/8333

ガスハイドレートは，従来，内包するゲストサイズにより構造が選択されると考えられていたが，近 年ダイヤモンドアンビルを用いた高圧Ｘ線回折実験によって新たな相の存在や多様な相変化が明らか となった．本稿では水素からキセノンまでのガスハイドレートに対し，ゲストサイズと圧力とをパラ メータとする相変化を概観する．また，メタンハイドレートの特異的な高圧安定性を基に，海王星や タイタンなどの氷惑星・衛星における存在の可能性を検討する． Recent high-pressure studies on gas hydrates, including methane hydrate, have accumulated to the extent that we now have a better understanding of the phase changes in gas hydrates. A feature of phase changes in terms of pressure and guest size was summarised earlier in this paper. Gas hydrates with a guest size from argon to methane eventually take a common filled ice Ih structure, although they have different initial and intermediate structures. In the latter part, retention by the filled ice Ih structure of methane hydrate up to 42 GPa is described. In-situ x-ray diffractometry and optical observation revealed the existence of the filled ice structure with as volume change of 40% and large anisotropic compressibility. Symmetrization of the hydrogen bonds in water molecules forming the fundamental structure was suggested to occur. The meanings for geological and planetary sciences of the phase changes and the retention of a very high pressure of methane hydrate are also discussed. 3章 クラスレートハイドレート研究のフロンティア