Dehydroxylation induced structural transformations in montmorillonite: an isothermal FTIR study

Abstract

Fourier transform infrared (FTIR) measurements on thermally shocked (290<T<1100 K, t=1 and 24 h) and thermally soaked (T=803 K, 0<t<25 h) Ca-montmorillonite from Texas show that the dehydroxylation temperature for this mineral is much lower than previously reported in the literature and depends upon the thermal soak time used. The effect of thermal shock temperature on vibrational frequencies, originating from the silicate sheet, and the intensity of Al-OH-Al stretching mode suggest that Camontmorillonite dehydroxylates at 903 <T<923 K and 773<T<823 K for 1 h and 24 h thermally shocked samples, respectively. Our similar measurements on the Mn and Fe exchanged-montmorillomte from Texas demonstrate that the dehydroxylation temperature range is also affected by the type of cations present in the interlayer. The thermal soak (T=803 K, 0<t<25 h) measurement on Ca-montmorillonite indicates that the dehydroxylation of this montmorillonite proceeds via the development of intermediate structural phases. As noted by previous authors, the dehydroxylation of Mg²⁺ and Fe³⁺ octahedral sites does not affect the overall structure. However, if ∼75 percent of the hydroxyls attached to Al³⁺ are lost, then the lattice manifests an intermediate structural phase. In this intermediate phase, the structure of both octahedral and silicate layers is affected. On a further loss of hydroxyls (∼90% of Al³⁺ hydroxyls), the final montmorillonite dehydroxylate phase develops. The vibrational analysis of an isothermally treated sample suggests that the final phase is induced due to the rearrangement of the silicate oxygens, which leave the coordination around Al to be 5. The dehydroxylation results of montmorillonite (Texas) have been discussed in terms of known mechanisms, and it appears that the dehydroxylation starts at the surface and proceeds via proton delocalization at trans hydroxyl positions, followed by the protons' migration across the vacant cation sites with the formation of H₂O molecules below the hexagonal holes.