Gas hydrates (clathrates) are non-stoichiometric, crystalline, inclusion compounds. These compounds can enclathrate light gases such as methane and ethane. Hydrates are stable only at high pressures and low temperatures and have been found to occur throughout natural environments such as ocean margins and permafrost regions.
Hydrates form three common structures: sI, sII, and sH.
Methane and ethane both form sI as pure components. The sI unit cell consists of a 6 large cages (51262), 2 small cages (512), formed from 46 H2O molecules.
Certain mixtures of methane + ethane can form sII. The sII unit cell consists of a 8 large cages (51264), 16 small cages (52), formed from 136 H2O molecules.
During a December 2003 dive to 1022 m in Monterey Bay, we created a sample of hydrate from a methane-ethane (80-20 mol%) gas mixtures. We analyzed the sample in situ with DORISS. The methane-ethane mixture forms an sII hydrate as seen in the spectrum below.
In July 2004 we visited the Hydrate Ridge site off the coast of Oregon. As seen in the pictures below, we were able to extract hydrate crystals from the seafloor and analyze them with DORISS. The hydrates were sI hydrates composed primarily of methane.