Structural Transformation of 2:1 Dioctahedral Layer Silicates during Dehydroxylation-Rehydroxylation Reactions

Abstract

Abstract--The structural transformation of dioctahedral 2:1 layer silicates (illite, montmorillonite, glauconite, and celadonite) during a dehydoxylation-rehydroxylation process has been studied by X-ray diffraction, thermal analysis, and infrared spectroscopy. The layers of the samples differ in the distribution of the octahedral cations over the cis-and trans-sites as determined by the analysis of the positions and intensities of the 11l, 02l reflections, and that of the relative displacements of adjacent layers along the a axis (c cos ~3/a), as wei1 as by dehydroxylation-temperature values. One illite, glauconite, and celadonite consist of trans-vacant (tv) layers; Wyoming montmorillonite is composed of cis-vacant (cv) layers, whereas in the other illite sample ta, and cv layers are interstratified. The results obtained show that the rehydroxylated Al-rich minerals (montmorillonite, illites) consist of tv layers whatever the distribution of octahedral cations over cis-and trans-sites in the original structure. The reason for this is that in the dehydroxylated state, both tv and cv layers are transformed into the same layer structure where the former trans-sites are vacant.The dehydroxylation of glauconite and celadonite is accompanied by a migration of the octahedral cations from former cis-octahedra to empty trans-sites. The structural transformation of these minerals during rehydroxylation depends probably on their cation composition. The rehydroxylation of celadonite preserves the octahedral-cation distribution formed after dehydroxylation. Therefore, most 2:1 layers of celadonite that rehydroxylate (--75%) have cis-vacant octahedra and, only in a minor part of the layers, a reverse cation migration from former trans-sites to empty octahedra occurred. In contrast, for a glauconite sample with a high content in WA1 and VJA1 the rebydroxylation is accompanied by the reverse cation migration and most of the 2:1 layers are transformed into tv layers.

Dehydroxylation mechanisms in Al3+/Fe3+ dioctahedral phyllosilicates by quantum mechanical methods with cluster models

“…Thus, the FeFe pairs are the most favoured for dehydroxylation and the AlAl pair the less favoured one. This is consistent with the reactivity order postulated previously (Heller-Kallai and Rozenson, 1980;Bray and Redfern, 2000;Muller et al, 2000) based on infra-red spectroscopy studies of several phyllosilicates at high temperature. The structures of the critical points along the reaction coordinate are similar each other (Fig.…”

“…Infrared spectroscopic studies of the dehydroxylation reaction for various phyllosilicates determined an estimate for the thermal stability of the M-OH bond and the rate of H 2 O release (Heller-Kallai and Rozenson, 1980;Emmerich et al, 1999;Bray and Redfern, 2000;Muller et al, 2000). The frequency of the vibration modes of the OH groups showed notable differences with cation substitution.…”

Novel 2:1 structure of phyllosilicates formed by annealing Fe3+, Mg-rich dioctahedral mica

“…3c). As reported in the previous works (Heller-Kallai and Rozenson 1980;Muller et al 2000c) celadonite was dehydroxylated and ferrous iron present in the natural state was oxidized by the present annealing process. Hence the composition of the annealed specimen is expressed approximately K(Fe 3+ ,Mg) 2 Si 4 O 11 .…”

Effects of Chemical Structure on the Stability of Smectites in Short-Term Alteration Experiments

“…The initial composition of the octahedral sheet discriminates these two bentonite types (Figure 9) (Figure 9). Muller et al (2000) and Wolters (2005) vacancies of clay minerals alone is, therefore, insufficient to explain the alteration potential of clays/ bentonites.…”