Clathrate

Clathrates (from the Latin clatratus, meaning "with bars") are crystalline solids which look like ice, and which occur when water molecules form a cage-like structure around smaller 'guest molecules'. The most common guest molecules are methane, ethane, propane, isobutane, normal butane, nitrogen, carbon dioxide and hydrogen sulfide.

Clathrates, also called gas hydrates, were discovered in 1810 by Sir Humphrey Davy. In the 1930s clathrates were a problem, clogging gas pipelines under cold conditions. In clathrates, water crystallizes into ice in a cubic system, rather than in the normal hexagonal structure. In the most common hydrate structures, (Structure I), the cages are arranged in body-centered packing; the unit cell contains 46 molecules of water and up to eight of methane. If all cages were occupied by methane, 1 m3 of solid hydrate could contain 170.7 m3 of methane gas at standard temperature and pressure. In nature, 1 m3 of hydrate contains up to 164 m3 of methane.

Recently clathrates have received attention as a possible energy source. They may play a role in large undersea slumps which could result in tsunamis, and in climate variability.