Geology topics

R.J. Hemley

Publications and source records attributed to R.J. Hemley.

3 recordsLinked to original sources

USGS2011

Structural stability of methane hydrate at high pressures

The structural stability of methane hydrate under pressure at room temperature was examined by both in-situ single-crystal and powder X-ray diffraction techniques on samples with structure types I, II, and H in diamond-anvil cells. The diffraction data for types II (sII) and H (sH) were refined to the known structures with space groups Fd 3 m and P 6 3 / mmc , respectively. Upon compression, sI methane hydrate transforms to the sII phase at 120 MPa, and then to the sH phase at 600 MPa. The sII methane hydrate was found to coexist locally with sI phase up to 500 MPa and with sH phase up to 600 MPa. The pure sH structure was found to be stable between 600 and 900 MPa. Methane hydrate decomposes at pressures above 3 GPa to form methane with the orientationally disordered Fm 3 m structure and ice VII ( Pn 3 m ). The results highlight the role of guest (CH 4 )-host (H 2 O) interactions in the stabilization of the hydrate structures under pressure.

Geoscience Frontiers↗

USGS2001

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 its high optical relief, distinct pressure-temperature phase relations, and Raman spectra. In-situ optical observations were made in a hydrothermal diamond-anvil cell at temperatures between -40?? and 60 ??C and at pressures up to 900 MPa. Two new invariant points were located at -8.7 ??C and 99 MPa for the assemblage consisting of the new phase, structure I methane hydrate, ice Ih, and water, and at 35.3 ??C and 137 MPa for the new phase-structure I methane hydrate-water-methane vapor. Existence of the new phase is critical for understanding the phase relations among the hydrates at low to moderate pressures, and may also have important implications for understanding the hydrogen bonding in H2O and the behavior of water in the planetary bodies, such as Europa, of the outer solar system.

Journal of Physical Chemistry A↗

USGS2000

Transformations in methane hydrates

Detailed study of pure methane hydrate in a diamond cell with in situ optical, Raman, and x-ray microprobe techniques reveals two previously unknown structures, structure II and structure H, at high pressures. The structure II methane hydrate at 250 MPa has a cubic unit cell of a = 17.158(2) A?? and volume V = 5051.3(13) A??3; structure H at 600 MPa has a hexagonal unit cell of a = 11.980(2) A??, c = 9.992(3) A??, and V = 1241.9(5) A??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 underlying continental margins.

Proceedings of the National Academy of Sciences of↗