Hydrothermal reactivity of Lu-saturated smectites: Part I. A long-range order study

Changes produced after hydrothermal treatments at 400 OC and at different p ressures in a set of Lu-saturated smectites have been compared with those o ccurring in a mixture of 2SiO(2):1Lu(2)O(3). The effect of the mineralogica l composition of the smectite on the reactivity has been analyzed. The grow th of a new crystalline phase, Lu2Si2O7 (detected by means of X-ray diffrac tion), at a different pressure for each smectite, has allowed us to establi sh a reactivity order among the samples. All the samples were much more rea ctive than the mixture of oxides. The reactivity of the smectites was modul ated by their different mineralogical compositions. First, smectites having Al in the tetrahedral sheet were the most reactive. This finding is interp reted as due to a strong electrostatic interaction created by the Al that a ctivates neighboring Si for the reaction. Second, a higher reactivity was o bserved for the smectites with total occupancy of the octahedral sheet, due to the simultaneous disruption of the tetrahedral-octahedral shared oxygen layer and the diffusion of octahedral cations into the interlayer space. T hird, the layer charge of the smectite does not induce any variation in the reactivity for the set of samples analyzed.