  Experimental results on the formation of molecular hydrogen on amorphous
silicate surfaces are presented for the first time and analyzed using a rate
equation model. The energy barriers for the relevant diffusion and desorption
processes are obtained. They turn out to be significantly higher than those
obtained earlier for polycrystalline silicates, demonstrating the importance of
grain morphology. Using these barriers we evaluate the efficiency of molecular
hydrogen formation on amorphous silicate grains under interstellar conditions.
It is found that unlike polycrystalline silicates, amorphous silicate grains
are efficient catalysts of H$_{2}$ formation within a temperature range which
is relevant to diffuse interstellar clouds. The results also indicate that the
hydrogen molecules are thermalized with the surface and desorb with low kinetic
energy. Thus, they are unlikely to occupy highly excited states.
