Hexakaidecahedron

In structure I, the unit cell contains 136 water molecules which are arranged so that they form 16 pentagonal dodecahedra (small cavities) and 8 hexakaidecahedra (large cavities).

Other possible cage structures which the water molecules may adopt are the tetra- and the hexakaidecahedron.

Moreover, it is impossible to separate gas molecules completely because the structure of propane hydrate consists of hexakaidecahedrons and dodecahedrons.