The formation of lutetium disilicate from lutetium-saturated montmorillonite induced by hydrothermal treatments in mild conditions has been studied. Changes in the silicate lattice and in the species located in the interlayer space have been examined by using XRD, MAS-NMR, and Lu L(III) EXAFS techniques, as well as soft-energy XES of Al and Si, the main silicate constituents. Results obtained showed that the Lu2Si2O7 phase was formed to a considerable extent at 300 degrees C and that the reaction started at a temperature as low as 200 degrees C. These results imply that the previously reported higher temperature, 400 degrees C, is not a thermodynamic barrier for the solid-state reaction and offer an effective mechanism to immobilize these ions. Capabilities of soft XAFS techniques in the study of complex systems such as aluminosilicate minerals are shown.