Spectroscopic Investigation of Cation Migration In Smectite Clay Minerals

Abstract

Structural changes arising from the dehydration of smectites containing small exchange-cations have been investigated through a number of techniques including: ESR, NMR and Mössbauer spectroscopy. Dehydration results in the penetration of exchange-cations into the pseudohexagonal cavities in basal surfaces of montmorillonite, hectorite and nontronite. Because of differences in the type of isomorphic substitution of each smectite the reactions between the exchange-cations and lattice defects in the octahedral sheet are different and give rise to different physical properties. In montmorillonite small cations irreversibly collapse the clay interlayers after thermal dehydration and the exchange-cations are no longer exchangeable. The present results contradict the hypothesis that exchange-cations migrate into vacant octahedral sites and instead favor their fixation within the pseudohexagonal cavities. A mechanism has been proposed in order to account for the exchange-cation fixation in terms of the reorientation of structural OH groups of the octahedral sheet so that the proton of the OH group points into the vacant octahedral sites. The "locking" of protons in these sites explains the lack of change in the dimensions of the octahedral sheet and interstratification of dehydrated and expandable layers previously observed for heat treated Li+-montmorillonite. Large cations unable to recess deeply into the cavities do not repel the proton of structural OH groups sufficiently to overcome the reorientation barrier and therefore interlayer collapse does not obtain. Hectorite, which has no octahedral vacancies, does not irreversibly collapse although an electronic interaction is observed between the exchange-cations in the pseudohexagonal cavities and the octahedral sheet which results in the formation of structural defect centers observed by ESR spectroscopy. In nontronite the tetrahedral charge is delocalized over defect tetrahedra and adjoining octahedra. Nonetheless, exchange-cations migrate into the pseudohexagonal cavities of nontronite but do not result in irreversible collapse of the structure. When the tetrahedral substitution is Al3+ for Si4+ a proportion of the layer charge is localized on the octahedral sheet and exchange-cations migrate into hexagonal cavities in the neighborhood of such substitutions. This results in preferential distortion of certain cis octahedral Fe3+ sites. Layer collapse does not occur in nontronite because even relatively mild heating causes protons to become mobile and migrate through the structure rather than becoming fixed in octahedral vacancies. Exchange-cations remain localized above tetrahedral sites where Si4+ has been substituted by Fe3+ with the consequence that these tetrahedral sites become partially resolved in the Mössbauer spectrum of the dehydrated nontronite. This phenomenon represents a method of testing for the presence of this type of substitution in nontronite and perhaps other layer silicates.