Cages and Three Crystal Structure

To date gas hydrates have been found to occur in three different crystal structures (Sloan 1998). Structures I and II both crystallize within a cubic system, whereas the third stmcture (also denominated H) crystallizes within a hexagonal system, analogous to water-ice (Fig. 14.1 Table 14.1). The stmcture of gas hydrate can be seen as a packing of polyhedral cages. Five types of hydrate cages are known, from 

Stmcture I is most frequently observed. Its unit cell consists of 8 cages 2 small (5 ) and 6 large cavities (5 6 ). Inside each cavity resides a maximum of 1 guest molecule, such that 8 guests molecules are associated with 46 water molecules in stmcture I (2[5 ] 6[5 6 ] 46HjO). A unit cell of stmcture II consists of 24 cages, 

When all hydrate cages are filled, the three crystal types have similar concentrations of 85 mol% water arrd 15 mol% guest molecirles. Stmcture I hydrate with CH and has minimiun (stoichiometric) hydration mtmbers of 5.75 and 7.67, respectively. Only large cavities in the Stmcture II hydrate are occupied with C3HJJ (and i-C Hj ), and such hydrates have a hydration mtmber of 17 (e.g. Sloan 1998). However, hydration 

Cavity Small Large Small Large Small Medium Large 

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