Crystal structures of gas hydrates

All gas hydrates have repetitive crystal units, and a unit cell consists of several cavities sharing faces, edges, and/or vertices, with guest molecules trapped in those 

Cavities Small Large Small Large Small Medium Large 

Structure I comprises two 5 cavities and six cavities, formed by 46 water molecules. It is clear from Fig. 10.3a that one 5 cavity is located at each of the eight vertices. Considering the surrounding unit cells, each of the vertices only contains 1/8 of a 5 cavity. By adding the 5 cavity at the center of the strucmre, the total number of 5 cavities becomes two per tmit cell. For the 5 6 cavities, each of the six faces contains two halves of cavities and the total number becomes six. Similar calculation can be done on other structures. 

Structure II (see Fig. 10.3b) is a face-centered cubic lattice, consisting of 16 5 cavities and 8 cavities. Each unit cell of sH contains 136 water molecules. This structure can be regarded as completely defined by 5 cavities (which share the pentagonal faces), whereas the residual voids form the cavities (which share all the hexagonal faces). The two kinds of cavities are connected by sharing pentagonal faces. 

---