We have used equilibrium molecular dynamics and grand canonical Monte Carlo simulations to compute the transport diffusivities of CH4 and CF4 adsorbed in a fully atomistic model of silicalite over a wide range of temperatures and pore loadings. Our results provide a direct test of the accuracy of the Darken approximation, an expression frequently used to relate transport and self-diffusivities in zeolites. The Darken approximation is accurate for CH4 in silicalite at 298 K, but is less accurate at higher temperatures. In contrast, CF4 deviates strongly from the Darken approximation at all temperatures we have examined. This observation suggests that substantial deviations from the Darken approximation may be a common feature of molecular diffusion in zeolites.