Study of the structural evolution of the 10 Å unstable hydrate of kaolinite during dehydration by XRD and SAXS

Abstract

This work deals with understanding the structural evolution of the dehydration of the 10 Å unstable hydrate of kaolinite. The method used to characterize this hydrate is based on a comparison between the experimental and the calculated X-ray diffraction profiles. The study was achieved in two steps: (i) the quantitative interpretation of 00lreflections enabled the determination of the number of intercalated water molecules, their positions and the stacking mode of the clay layers along the normal to the (a,b) plane; and (ii) the study of thehklreflections withhand/ork≠ 0 enabled the characterization of the structural evolution in the (a,b) plane of the hydrated kaolinite during dehydration. The hydrate is made up of two demixed phases. The first phase is homogenous and corresponds to a 10 Å hydrated kaolinite, characterized by two H2O molecules per Si2Al2O5(OH)4situated atZ= 7.1 Å from the surface oxygen. Two adjacent layers are translated with respect to each other, withT1= −0.155a+ 0.13b+ 10n. The abundance of this phase decreases during dehydration. The second phase is made up of 10 Å hydrated layers, 8.4 Å hydrated layers and 7.2 Å dehydrated kaolinite layers, randomly interstratified. The abundance of this second phase increases during dehydration. The corresponding interlayer shifts are respectivelyT21= −0.155a+ 0.13b+ 10nfor the 10 Å hydrated layer,T22= −0.355a+ 0.35b+ 8.4nfor the 8.4 Å hydrate andT23= −0.36a− 0.024b+ 7.2nfor the natural kaolinite. In addition to these interlayer shifts, some translation defects are introduced, such as −b/3, which exists in the initial kaolinite. The interpretation of the small-angle X-ray scattering (SAXS) patterns showed that the particle thickness remained the same before and after the hydration treatments, whereas X-ray diffraction (XRD) results indicated that the hydration of kaolinite caused a decrease of the mean number of layers \bar m per crystallite from 40 to 20 layers. This decrease is related to the presence of H2O molecules situated within the micropores in the kaolinite particles that leave their interlayer space after heating at 573 K. The resulting dehydrated compound is characterized by the same basal distance and mean number of layers \bar m per crystallite as for the natural kaolinite, while the proportion of the defects, such as the −b/3 translation, increases in the completely dehydrated compound (45%) compared with the natural kaolinite (10%).