CRYSTAL-GROWTH STUDIES OF NATURAL-GAS CLATHRATE HYDRATES USING A PRESSURIZED OPTICAL-CELL

The crystal-growth behavior of structure I (sI), structure II (sII), a nd structure H (sH) clathrate hydrates has been studied using a specia lly designed, pressurized optical cell. Single crystals of each hydrat e type, methane sI, methane-propane (95%-5%) sII, and methane-methylcy clopentane sH, were grown in equilibrium with aqueous liquid + vapor /- liquid hydrocarbon. Each structure type exhibits characteristic cry stal morphology, which suggests that crystal habit in natural settings , such as sea-floor outcrops, may allow visual identification of hydra te types. In addition, the relative growth rates for different Miller planes for each crystal type were determined. The relative growth-rate schemes and resulting crystal morphology of each structure can be rel ated to the unit-cell density distribution of the small cages in each structure. Four-phase P-T equilibrium data for methane-methylcyclopent ane sH data were also measured using optical methods. Evaluation of th ese and previously published phase-equilibrium data for all three know n hydrate structures strongly suggests that hydrate assemblages of coe xisting sII and sH should be common in natural settings.