Type II structure is cubic. In addition to pentagonal dodecahedra, hydrocarbons are also enclosed by polyhedra with 4 hexagonal and 12 pentagonal faces (of course somewhat distorted. The polyhedron has tetrahedral symmetry and is shown below in a number of orientations.
Clusters of these solids link tetrahedrally by sharing faces.
The voids between three 16-faced solids are filled by triads of dodecahedra (of course, to join this way, the dodecahedra must also be slightly distorted, since the dihedral angle of a true pentagonal dodecahedron is only 116.5 degrees, not 120 degrees)
The result is that tetrahedral clusters of 16-faced polyhedra (below, left) are surrounded by cages of dodecahedra (below right).
Below are views of a cluster of 16-faced polyhedra and a cluster with enclosing dodecahedra, viewed along a 4-fold inversion axis.
Henriet, J.-P., Mienert, J., 1998; Gas hydrates : relevance to world margin stability and climate change, London : The Geological Society, Geological Society 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; New York Academy of Sciences. New York, NY, United States. Pages: 1039.
Paull, Charles K. (editor); Dillon, William P. (editor), 2000; Natural gas hydrates; 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 or pipe 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 natural gas 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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Structure I
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