Structure I is cubic (diploidal). Pentagonal cages of water molecules enclosing a methane molecule occupy the corners of the unit cell, and a rotated cage occupies the center (left above). Other water molecules link the dodecahedra, forming hexagonal rings (right, above).
The interstices between the dodecahedra are occupied by cages with hexagonal ends and 12 pentagonal faces. Some of the pentagonal faces are shared with dodecahedra, others with 14-faced cages. The hexagonal rings joining the dodecahedra are also ends of the 14-faced cages. These cages join end to end to form columns, which share faces with intervening perpendicular columns. Two sets of columns are shown above.
In the diagram above, we see the third set of columns perpendicular to the other two sets.
In the diagram above, we see the 14-faced cages of a unit cell. These fit between the dodecahedral cages.
The diagram above shows the dodecahedra at the corners of a unit cell together with the intervening 14-faced cages. The rotated dodecahedron at the center of the unit cell is concealed, but its counterpart in the next cell above is shown.
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