Water diffusion and interlayer structure in Mg-smectite hydrates was investigated by molecular dynamics and Monte Carlo computer simulation. Smectites with only tetrahedral layer charge (Mg-beidellite) were examined at 300 and 400 K, each with two layers of adsorbed water. Previous quasielastic neutron-scattering experiments have indicated that nonsolvating water molecules in Mg-smectites exhibit faster diffusional motion within a "cage" formed by the solvated Mg(H2O)(6)(2+) counterions than they do via inter-cage motion. Our 500-1175 ps MD simulations suggested that the dimension of this "cage" region is approximately 5.5 Angstrom, but two-phase diffusional behavior was observed only for water in Mg-beidellite at 300 K. The counterions in Mg-beidellite formed outer-sphere surface complexes with the clay mineral siloxane surface, but nonsolvating water: molecules showed a significant tendency to occupy ditrigonal cavities in the surface. This behavior could be responsible for the small equilibrium layer spacing in Mg-beidellite, as well as a lower water self-diffusion coefficient.