The formation of meso- and macroporous gas hydrates

We present results of experimental studies on the formation of gas hydrates (clathrates) at conditions of geophysical interest. Clathrate hydrates for med by a reaction of gas at ice Ih surfaces are always found to be mesoporo us to macroporous with pores sizes between 100 to 400 nm and pore volumes o f approximately 25-40% for CH4, Ar and Nz hydrate, and smaller pores of 20 to 100 nm with a porosity of approximately 10-20% for CO2 hydrate. The thre e-dimensional sponge-like microstructure occurs in single crystalline grain s of typically a few mu m size and was observed by field-emission scanning electron microscopy. It forms over a wide range of p-T conditions below the ice Ih melting. The porous microstructure is stable for at least several m onths, even close to the clathrate decomposition, and is proposed to be for med by local differences in the energy balance between hydrate formation an d ice decompositon. The results presented are considered of potential major importance for the understanding of the behaviour of natural gas hydrates found e.g. in polar ice sheets and permafrost regions, and also in some cel estial bodies.