Gas hydrates grown at gas-ice interfaces were examined by electron microscopy and found to have a sub-micrometer porous structure. In situ observations of the formation of porous CH4- and CO2- hydrates from deuterated ice Ih powders were made at different pressures and temperatures, using time-resolved neutron diffraction data from the high-flux D20 diffractometer (ILL, Grenoble) as well as in-house gas consumption measurements. The CO2 experiments conducted at low temperatures are particularly important for settling the open question of the existence of CO2 hydrates on Mars. We found that at similar excess fugacities, the reaction of CO2 was distinctly faster than that of CH4. A phenomenological model for the kinetics of the gas hydrate formation from powders of spherical ice particles is developed with emphasis on ice-grain fracturing and sample-consolidation effects due to the outward growth of gas hydrate. It describes (1) the initial stage of fast crack-filling and hydrate film spreading over the ice surface and the two subsequent stages which are limited by (2) the clathration reaction at the ice-hydrate interface and/or by (3) the diffusive gas and water transport through the hydrate shells surrounding the shrinking ice cores. In the case of CO2-hydrate, the activation energies of the ice-surface coating in stage 1 are estimated to be 5.5 kJ/mol at low temperatures and 31.5 kJ/mol above 220 K, indicating that water molecule mobility at the ice surface plays a considerable role in the clathration reaction. Comparable activation energies of 42.3 and 54.6 kJ/mol are observed in the high temperature range for the reaction- and diffusion-limited stages 2 and 3, respectively.