Silicon is the slowest diffusing species in olivine. As a consequence, silicon mobility controls the processes requiring the motion of all the constitutive atoms of olivine. Among these processes, the most important is probably creep since the ductile deformation of olivine plays a central role in upper mantle convection and thus Earth dynamics. Only a few experimental studies investigated the diffusion of silicon in olivine, likely because its sluggishness makes it hard to measure as it requires analytical techniques with down to nanoscale resolutions. We report new diffusion experiments of silicon in olivine, and a new analysis that includes results from previous studies. The volume diffusion of silicon in mantle olivine is likely not controlled by intrinsic defects (e.g. point defects created by thermal agitation) but by extrinsic defects (e.g. atomic impurities). This implies that, in addition to temperature, silicon diffusion and thus mantle viscosity may vary significantly as a function of the chemical environment of the various mantle settings. Noticeably, silicon diffusivity is enhanced by up to three orders of magnitude in presence of hydrogen, and to a lesser extent in presence of iron. Hydrogen is likely to play a key role as its distribution in the mantle is highly variable, increasing silicon diffusivity and thus decreasing mantle viscosity in hydrogen-rich areas and increasing mantle viscosity in hydrogen-poor areas. Furthermore, the diffusivity of silicon in olivine grain boundaries is about six orders of magnitude faster than in volume. Although confined to a few nanometer-thick intergranular medium, grain boundary diffusion could control silicon transport in mantle rocks as long as grain size remains below millimeters, and will be favored in cold settings such as the lithosphere or the shallow asthenosphere.