The short-range order of a set of Lu-saturated smectites, modified by hydrothermal treatment at 400 °C, which give rise to a new crystalline phase (Lu2Si2O7), has been analyzed. In particular, the modifications undergone by the tetrahedral Si atoms of the lattice and the possible diffusions of O-T layer ions to other structural positions were investigated. Single-pulse, magic-angle-spinning nuclear magnetic resonance (MAS-NMR) measurements from the 1H, 7Li, 27Al, and 29Si nuclei, as well as two-dimensional 1H-27Al cross-polarization MAS-NMR experiments, have been made on both the initial and the hydrothermally treated samples. The results show that (1) the formation of the Lu2Si2O7 takes place from Si atoms surrounded by Al tetrahedra in the samples that contain tetrahedral Al; (2) there is a diffusion of octahedral cations to the interlayer positions during the hydrothermal treatments, compatible with the X-ray diffraction data reported for this reaction; and (3) the octahedral Al migrates to the vacant tetrahedral positions that appear during the formation of the new Lu2Si2O7 phase. A clear relationship between each sample composition and its reactivity is defined.