The design of new clay-based materials, such as acid catalysts and barrier components for nuclear waste repositories, demands further structural knowledge of the interaction of lanthanides with the lattice of clays upon heating. The phase Lu2Si2O7 (shown) has been generated upon hydrothermal treatment of lutetium-saturated montmorillonite, at temperatures extremely lower than those up to now reported. Study of the local environment of Lu(III) by EXAFS is consistent with quantitative formation of the disilicate.