Three types of clathrate structures have been identified: Structure I, Structure II, and Structure H (Von Stackelberg and Muller, 1951; Davidson et al., 1984; Ripmeester et al., 1987). Each of these clathrate structures contains small and large cavities between cages of water molecules. The type of clathrate structure formed from water and gas depends on the size ("van der Waals diameter") of the gas molecules (Davidson et al., 1984; Handa and Tse, 1986; Ripmeester and Ratcliffe, 1990).
Many pure gases that have average molecular van der Waals diameters between 410 and 580 pm (e.g., CH4, CH3Cl, SO2, and Xe) form Structure I clathrates with two cavities of average diameter of 782 pm and six cavities of average diameter of 866 pm per unit cell (Davidson, 1973; Davidson et al., 1984; Handa and Tse, 1986). In the case of impure gas, trace gases that can fit into Structure I cavities will be incorporated into the clathrate lattice during gas hydrate formation (e.g., Davidson, 1973). This incorporation of trace gases involves fractionation such that the composition of gas in hydrate, gas dissolved in water, and the original gas may be significantly different. The degree of fractionation between any two gases during Structure I clathrate formation should depend largely on temperature and the difference between average van der Waals diameters for molecules of the two gases (Barrer and Stuart, 1957; Davidson, 1973).
Several authors (Nikitin, 1937; Barrer and Stuart, 1957; Davidson, 1973) have demonstrated experimentally or theoretically that formation of a Structure I clathrate in the presence of noble gases will cause large fractionation between the noble gases. Such fractionation is exemplified in Table 1, which lists experimental and "theoretical" Kr/Ar, Xe/Kr and Xe/Ar enrichment factors during formation of CH3Cl hydrate (from Davidson, 1973, p. 231). Note that enrichment factors indicate preferential enrichment of the heavier noble gas into Structure I gas hydrate (Xe > Kr > Ar) and that fractionation increases with decreasing temperature. Nikitin (1937) reported that small quantities of He and Ne, along with relatively large quantities of Ar and Rn (Rn > Ar >> Ne > He), were incorporated into the clathrate lattice during SO2 hydrate formation. Barrer and Stuart (1957) and Davidson (1973) omit discussion of He or Ne (as well as Rn) fractionation (presumably because diameters of He and Ne molecules are too small to fit into Structure I cavities so that their fractionation values are very small to zero). To our knowledge, no experimental or theoretical data have been presented for noble gas fractionation during CH4 hydrate formation.