Clathrate hydrate
From Wikipedia, the free encyclopedia
Clathrate hydrates (or alternatively gas clathrates, gas hydrates, clathrates, hydrates etc) are a class of solids in which gas molecules occupy "cages" made up of hydrogen-bonded water molecules. These "cages" are unstable when empty, collapsing into conventional ice crystal structure, but they are stabilised by the inclusion of the apolar gas molecules within them. Most low molecular weight gases (including O2, N2, CO2, CH4, H2S, Ar, Kr, and Xe), as well as some higher hydrocarbons and freons will form hydrate under certain pressure-temperature conditions.
Clathrates are believed to occur in large quantities on some outer planets, moons and trans-Neptunian objects, binding gas at fairly high temperatures. Clathrates have also been discovered in large quantity on Earth, i.e. in giant natural methane clathrate deposits on the deep ocean floor (e.g. in the northern headwall flank of the Storegga Slide, which is a part of the Norwegian continental shelf) and in permafrost regions (e.g. the Mallik gas hydrate field in the Mackenzie Delta of northwestern Canadian Arctic). Hydrocarbon clathrates are a problem for the petroleum industry, since their formation inside gas pipelines frequently leads to plug formation in the latter. Deep sea deposition of carbon dioxide clathrate to remove this greenhouse gas from the atmosphere has also been proposed.
Kieffer et al. (2006) suggest that the geyser activity in the south polar region of Saturn's moon Enceladus originates from clathrate hydrates, where carbon dioxide, methane, and nitrogen are released when exposed to the vacuum of space by the "Tiger Stripe" fractures found in that area.[1]
[edit] Structure
Gas hydrates usually form two crystallographic cubic structures – structure (Type) I and structure (Type) II[2] of space groups 
and 
respectively. Rather seldom, a third hexagonal structure of space group P6 / mmm maybe observed (Type H).[3]
The unit cell of Type I consists of 46 water molecules, forming two types of cages – small and large. The small cages in the unit cell are two against six large ones. The small cage has the shape of pentagonal dodecahedron (512) and the large one that of tetrakaidecahedron (51262). Typical guests forming Type I hydrates are CO2 and CH4.
The unit cell of Type II consists of 136 water molecules, forming also two types of cages – small and large. In this case the small cages in the unit cell are sixteen against eight large ones. The small cage has again the shape of pentagonal dodecahedron (512) but the large one this time is hexakaidecahedron (51264). Type II hydrates are formed by gases like O2 and N2.
The unit cell of Type H consists of 34 water molecules, forming three types of cages – two small of different type and one huge. In this case, the unit cell consists of three small cages of type 512, twelve small ones of type 435663 and one huge of type 51268. The formation of Type H requires the cooperation of two guest gases (large and small) to be stable. It is the large cavity that allows structure H hydrates to fit in large molecules (e.g. butane, hydrocarbons), given the presence of other smaller help gases to fill and support the remaining cavities. Structure H hydrates were suggested to exist in the Gulf of Mexico. There thermogenically-produced supplies of heavy hydrocarbons are common.
[edit] See also
[edit] References
- ^ Kieffer, Susan W., Xinli Lu, Craig M. Bethke, John R. Spencer, Stephen Marshak, Alexandra Navrotsky (2006). "A Clathrate Reservoir Hypothesis for Enceladus' South Polar Plume". Science 314 (5806): 1764-1766. DOI:10.1126/science.1133519.
- ^ von Stackelberg & Müller, 1954
- ^ Sloan E. D., Jr. (1998) Clathrate hydrates of natural gases. Second edition, Marcel Dekker Inc.:New York.
