⁵⁷Fe Mössbauer Spectroscopic Study of Structural Changes during Dehydration of Nontronite: Effect of Different Exchangeable Cations

Abstract

Abstract--Dehydration-induced migration of different exchangeable cations toward the layers of nontronite has been studied by M6ssbauer spectroscopy. As interlayer water is removed exchangeable cations migrate toward Fe 3+ sites in the tetrahedral sheets of the nontronite (WFe3+) causing them to distort. The amount of distortion is linearly related to the ionic potential (IP) of the exchangeable cations and is greatest for cations with highest IP. Octahedral Fe 3+ sites (VIFe3+) are also affected by migration of cations into the pseudohexagonal cavities. As exchangeable cations move into the pseudohexagonal cavities, interaction with WFe3+ sites increases. The intensity of the outer WFe3 § M6ssbauer doublet increases with respect to the inner WFe3+ doublet as the IP of the exchangeable cation increases. It appears that the exchangeable cations play a significant role in determining the thermal stability of nontronite.

Redox Properties of Structural Fe in Clay Minerals. 2. Electrochemical and Spectroscopic Characterization of Electron Transfer Irreversibility in Ferruginous Smectite, SWa-1

“…The newly formed phase in the oxidized (i.e., re-oxidized and re-re-oxidized) spectra with a large QS value has been previously interpreted as both distorted cis -octahedral Fe 3+ ,,− and trans -octahedral Fe 3+ . − Regardless of the exact structural coordination of this Fe phase, the formation of this new phase can be linked to reduction of oxidized SWa-1 occurring at more positive E H -values than for native SWa-1. This observation is consistent with previous studies that showed structural Fe 3+ in nontronites with a larger QS value was preferentially reduced over structural Fe 3+ with smaller QS values. ,, As a result, QS values may be indicative of what E H -value(s) are required to reduce structural Fe 3+ in a clay mineral.…”

Redox Properties of Structural Fe in Clay Minerals. 1. Electrochemical Quantification of Electron-Donating and -Accepting Capacities of Smectites

“…The NAu-2 spectrum was more complex: ≈2% of the structural Fe 3+ was tetrahedrally coordinated ( Tet Fe 3+ ) and a second octahedral Fe 3+ site (i.e., Oct Fe 3+ [2], 32% of area) was observed. The Oct Fe 3+ [2] site has previously been interpreted as both trans -octahedral Fe 3+ (i.e., two hydroxyl groups opposite each other) − and distorted cis -octahedral Fe 3+ near a tetrahedral Fe 3+ atom ,, (Figure ).…”

Mössbauer spectrometry insights into the redox reactivity of Fe-bearing phases in the environment

“…The first group consists of Montmorillonites [51][52][53][54][55][56][57][58][59][60][61], where the two components attributed to oct-Fe 3+ expose a notable split in quadrupole splitting values. The second main group include most studied Nontronites [51,53,[62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78], for which the split in in quadrupole splitting values (Fig. 1, sites A + B) is less pronounced than for Montmorillonites (sites M1 + M2).…”

“…This implies that Fe can occupy both trans-OH (or M1) and cis-OH (or M2) octahedral sites in Montmorillonites, while Fe can only sites in M2 sites in Nontronites. As a result, in most studied Nontronites [51,53,[62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77][78], it is generally assumed that the A + B sites accounts for a continuum of M2-Fe 3+ facing varying environment in the structure (i.e., representing a sum of even more sub-doublets, [90] and refs therein). In addition, in the case of Montmorillonites [51-61, 91, 92], it is generally accepted that the lower quadrupole splitting component represents M2 sites, while the higher quadrupole splitting component would represent larger and more distorted M1 sites.…”