By employing DFT approaches we studied the adsorption of a single Ag atom and its cation on different sites of a pure alpha-quartz (001) surface and on an alpha-quartz (001) surface containing an Al defect. The energetically different adsorption sites of Ag and Ag+ were revealed and the profiles of diffusion were calculated. Diffusion of an Ag atom through the pure alpha-quartz surface is predicted to have a much lower barrier than an Ag cation. However in case of diffusion through the alpha-quartz surface with an Al defect the barriers are almost the same for both cation and neutral Ag. (C) 2013 Elsevier B. V. All rights reserved.