Transformations in methane hydrates

Detailed study of pure methane hydrate in a diamond cell with in situ optic al, Raman, and x-ray microprobe techniques reveals two previously unknown s tructures, structure II and structure H, at high pressures. The structure I l methane hydrate at 250 MPa has a cubic unit cell of a = 17.158(2) Angstro m and volume V = 5051.3(13) Angstrom (3); structure H at 600 MPa has a hexa gonal unit cell of a = 11.980(2) A, c = 9.992(3) Angstrom, and V = 1241.9(5 ) Angstrom (3). The compositions of these two investigated phases are still not known. With the effects of pressure and the presence of other gases in the structure, the structure Ii phase is likely to dominate over the known structure I methane hydrate within deep hydrate-bearing sediments underlyi ng continental margins.