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What Is a Clathrate?

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  • Written By: Phil Riddel
  • Edited By: Jessica Seminara
  • Last Modified Date: 24 November 2016
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A clathrate is a type of hydrate, or water compound, in which molecules of another substance are trapped within a cage-like structure composed of water molecules. The trapped molecule is usually a gas at normal pressure and temperature. Clathrates are ice-like solids that generally form at high pressures and low temperatures. Among the best known and most studied is a methane hydrate that occurs naturally in large deposits under the seabed in many parts of the world. These may be a potential source of energy, but there is also concern that a sudden release of large amounts of clathrate methane, perhaps prompted by global warming, could be disastrous.

The basic unit of most clathrates is a dodecahedron composed of 20 water molecules arranged to form 12 pentagonal faces, with a hollow center that can be occupied by a “guest molecule.” The structure is essentially held together by hydrogen bonds between the water molecules, but stabilized by the guest molecules. Since the dodecahedra cannot be packed together to fill all the available space, other polyhedral shapes also occur, so that a lattice is formed. Due to this variation in the cage shapes, and the fact that not all of the cages are necessarily occupied, clathrates cannot be given precise chemical formulae. Clathrate guest molecules can be hydrocarbon gases, such as methane or ethane, oxygen, nitrogen and carbon dioxide.

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Methane hydrate is the clathrate compound that has generated the most interest. This compound occurs in large amounts in various locations around the edges of all the continents and in the permafrost regions of Siberia and Alaska. It has been estimated that these deposits constitute the largest reserve of hydrocarbons on the planet, far exceeding known reserves of coal, oil and natural gas. They are thought to have formed from methane produced by microbial activity in the anaerobic conditions in sediments just below the surface of the seafloor or on land where temperatures are sufficiently low. Even in tropical regions, seafloor temperatures are low enough for clathrate formation, where the pressure allows them to solidify at a few degrees above the freezing point.

Given the vast quantities of methane stored in these deposits, they have been considered as a potential source of natural gas. There may, however, be serious technical difficulties involved in its extraction that render it uneconomical. The Soviet Union made a number of unsuccessful attempts to extract gas from Siberian permafrost clathrate deposits during the 1960s and 1970s. There is also concern that the methods used to release the trapped gas may destabilize the deposits, which could potentially lead to subsidence and landslides.

Although clathrate deposits may represent a huge untapped energy resource, they could also pose a serious hazard. They are not stable outside the temperature and pressure conditions where they occur and there is concern that global warming may render them unstable. This poses a twofold threat.

Firstly, the melting of clathrate ice mixed with sediments at continental edges could result in massive landslides and consequent tsunamis. There is evidence from the relatively recent geological past that this may have happened off the coast of Norway. Secondly, methane is a potent “greenhouse” gas that traps heat in the atmosphere to an even greater extent than carbon dioxide. The sudden release of huge amounts of this gas could accelerate global warming, which might in turn cause further destabilization. Again, there is geological evidence that this may have happened in the past through natural processes and there is, as of 2011, particular concern about methane hydrate in permafrost deposits.

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