Ordered Interstratification of Dehydrated and Hydrated Na-Smectite

Abstract

Abstract--The 001 spacing of Na-smectite was found to vary from 9.6 ,~ at 0% relative humidity (RH) to 12.4 ~ at 60-65% RH. The 9.6-,~ spacing corresponds to dehydrated Na-smectite, and the 12.4-,~ corresponds to Na-smectite with one water layer. A regular series of intermediate values resulted from ordered interstratification of the 9.6-and 12.4-~ units. Ordered interstratification was confirmed by the presence of a 001 spacing of 9.6 + 12.4 A = 22 A. This peak appeared under experimental conditions at about 35% RH. It appeared for calculated simulations of ordered stacking of 50/50 mixtures (_+ 10%) of 9.6-and 12.4-,~ units. The 004 peak of this 22-,~ spacing interacted with the 002 of the 9.6-A spacing of ordered mixtures of more than 50% 9.6-/k units and with the 002 of the 12.4-~ spacing of ordered mixtures of more than 50% 12.4-~ units. The result of this interaction was a complex peak, the position of which was a function of the ratio of 9.6-and 12.4-,~ units. This complex peak was noted for experimental and for calculated conditions. Calculated tracings assuming ordered stacking matched the experimental tracings closely, whereas those assuming random stacking did not.Ordering was apparently due to the interaction of the positive charge of the interlayer cation repelling the positive charge of the hydrogens of the hydroxyl ions, one above and one below, closest to the interlayer space. The collapse of a single interlayer space (dehydration) brought the interlayer cation closer to the hydrogens of the hydroxyls causing the hydroxyls to rotate such that the hydrogens shifted toward the adjacent interlayer spaces. Collapse of these two interlayer spaces was therefore more difficult. This same mechanism helps explain ordering in illite/smectite. The difference is that hydration/dehydration is quick and reversible, whereas the change from smectite to illite is slow and irreversible.

Decomposition of X-Ray Diffraction Patterns: A Convenient Way to Describe Complex I/S Diagenetic Evolution

“…More recently, such a variability has been reported by Sato et al (1992) for smectites as a function of the layer charge, the charge location, the expansion energy, the nature of the interlayer cation, and the relative humidity. The effect of the latter parameter has also been shown by Moore and Hower (1986). By modeling low-angle X-ray scattering of smectite gels, Pons et al (1981Pons et al ( , 1982 demonstrated the coexistence of various hydration states in the same sample.…”

Hydration Properties and Interlayer Organization of Water and Ions in Synthetic Na-Smectite with Tetrahedral Layer Charge. Part 1. Results from X-ray Diffraction Profile Modeling

“…Similar ordered mixed-layer structures have already been reported upon dehydration of Na-and Sr-saturated low-charge dioctahedral montmorillonite. 20,65 Ordering was interpreted as resulting from the presence of octahedral vacancies leading to a specific orientation of hydroxyl groups. 65 The collapse of smectite interlayer and the insertion of the interlayer cation into the ditrigonal cavity make the OH group lean toward the vacant octahedral site, thus preventing a similar orientation of the OH group located on the other side of the octahedral sheet and the associated collapse of adjacent interlayer space.…”

Mechanism of Adsorption and Desorption of Water Vapor by Homoionic Montmorillonites: 2. The Li+, Na+, K+, Rb+ and Cs+-Exchanged Forms

“…In the first papers of this series (Cases et al 1992a(Cases et al , 1992b, it was shown that a multiple analytical approach allowed us to reconsider earlier models for hydration-dehydration of Na-montmorillonite in the range of intracrystalline swelling (Mamy 1968, Sposito and Prost 1982, More and Hower 1986, Kraehenbuehl et al 1987. The main features were: (1) interstratified hydrates persist at all water vapor relative pressure values higher than 0.25.…”