Natural Gas Hydrates: Overview
Steven Dutch, Professor Emeritus, Natural and Applied Sciences, Universityof Wisconsin - Green BayNatural gas hydrates are a curious kind of chemical compound called a clathrate.Clathrates consist of two dissimilar molecules mechanically intermingled but nottruly chemically bonded. Instead one molecule forms a framework that traps theother molecule. Natural gas hydrates can be considered modified ice structuresenclosing methane and other hydrocarbons, but they can melt at temperatures wellabove normal ice.
At 30 atmospheres pressure, methane hydrate begins to be stable attemperatures above 0 C and at 100 atmospheres it is stable at 15 C. Thisbehavior has two important practical implications. First, it's a nuisance to thegas company. They have to dehydrate natural gas thoroughly to prevent methanehydrates from forming in high pressure gas lines. Second, methane hydrates willbe stable on the sea floor at depths below a few hundred meters and will besolid within sea floor sediments. Masses of methane hydrate "yellowice" have been photographed on the sea floor. Chunks occasionally breakloose and float to the surface, where they are unstable and effervesce as theydecompose.
The stability of methane hydrates on the sea floor has a whole raft ofimplications. First, they may constitute a huge energy resource. Second, naturaldisturbances (and man-made ones, if we exploit gas hydrates and aren't careful)might suddenly destabilize sea floor methane hydrates, triggering submarinelandslides and huge releases of methane. Finally, methane is a ferociouslyeffective greenhouse gas, and large methane releases may explain sudden episodesof climatic warming in the geologic past. The methane would oxidize fairlyquickly in the atmosphere, but could cause enough warming that other mechanisms(for example, release of carbon dioxide from carbonate rocks and decayingbiomass) could keep the temperatures elevated.
There are three types of methane hydrate structure. They all includepentagonal dodecahedra of water molecules enclosing methane. This geometryarises from the happy accident that the bond angle in water is fairly close tothe 108 degree angle of a pentagon. Generally, the dodecahedra are slightlydistorted so that three dodecahedra can share an edge. This requires a dihedral(inter-face) angle of 120 degrees, whereas the dihedral angle of a truedodecahedron is 116.5 degrees. Between the dodecahedra are other cages of watermolecules with different shapes. In practice, not all cages are occupied byhydrocarbons, but occupancy rates of over 90 per cent occur.
 | In all the diagrams below and on the linked pages, each vertex is occupied byan oxygen atom and the midpoint of each edge is a hydrogen atom. This atom isattached to one oxygen as part of a water molecule and hydrogen bonded to theother. In the diagram at left one cage is shown with oxygen atoms in blue and hydrogen in red. A methane molecule is shown inside one of the cage skeletons. |
Structure I
Structure I is cubic. A dodecahedral cage is centered at the corners of theunit cell and a rotated dodecahedron is in the center of the cell. Thedodecahedra are linked by 14-faced cages that consist of hexagonal ends and 12pentagons. In the diagram below dodecahedra are in magenta. The central rotateddodecahedron is hidden but its counterpart in the next unit cell above is shownat top.
Structure II
Structure II is also cubic. 16-faced cages consisting of 12 pentagons and 4hexagons are arranged tetrahedrally (left side of diagram below). Theinterstices are filled by dodecahedral cages (right, below).
H Structure
The H (for hexagonal) structure consists of three cages: dodecahedra, cageswith 4, 5 and 6-sided faces, and "barrels" consisting of 12 pentagonaland 8 hexagonal faces. The "barrels" can hold large hydrocarbonmolecules. They are surrounded by a hexagonal net of the 4-5-6 cages, and layersof these cages alternate with hexagonal nets of dodecahedra.
References
Henriet, J.-P., Mienert, J., 1998; Gas hydrates : relevance to world marginstability and climate change, London : The Geological Society, GeologicalSociety special publication no. 137, 338 p.
Kleinberg, Robert; Brewer, Peter, 2001; Probing gas hydrate deposits. American Scientist. vol. 89; no. 3, Pages 244-251.
Holder, Gerald-D (editor); Bishnoi, P. R. (editor), 2000; Gas hydrates;challenges for the future. Annals of the New York Academy of Sciences. 912; NewYork Academy of Sciences. New York, NY, United States. Pages: 1039.
Paull, Charles K. (editor); Dillon, William P. (editor), 2000; Natural gashydrates; occurrence, distribution, and detection. Geophysical Monograph 124,American Geophysical Union. Washington, D.C., United States. Pages: 315.
Haq, Bilal U., 1998; Gas hydrates; greenhouse nightmare? Energy panacea orpipe dream? GSA Today. vol. 8; 11, Pages 1-6. Geological Society of America(GSA). Boulder, CO, United States
Smelik, Eugene A.; King, H. E. Jr., 1997; Crystal-growth studies of naturalgas clathrate hydrates using a pressurized optical cell. American Mineralogist.vol. 82; 1-2, Pages 88-98. Mineralogical Society of America. Washington, DC,United States.
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1 August 2003, Last Update