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A Model for the Mechanism of Fe3+ to Fe2+ Reduction in Dioctahedral Smectites

Published online by Cambridge University Press:  28 February 2024

V. A. Drits  and
A. Manceau
V. A. Drits
Affiliation:
Environmental Geochemistry Group, LGIT-IRIGM, University Joseph Fourier and CNRS, 38041 Grenoble Cedex 9, France Geological Institute of the Russian Academy of Sciences, 7 Pyzhevsky Street, 109017 Moscow, Russia
A. Manceau
Affiliation:
Environmental Geochemistry Group, LGIT-IRIGM, University Joseph Fourier and CNRS, 38041 Grenoble Cedex 9, France
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Abstract

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A model to compensate the 2:1 layer having excess negative charge owing to the reduction of Fe3+ to Fe2+ by sodium dithionite buffered with citrate-bicarbonate in nontronite, beidellite, and montmorillonite is proposed. This model is based on reassessing published experimental data for Fe-containing smectites and on a recently published structural model for reduced Garfield nontronite. In the reduced state, Fe2+ cations remain six-fold coordinated, and increases of negative charge in the 2:1 layer are compensated by the sorption of Na+ and H+ from solution. Some of the incorporated protons react with structural OH groups to cause dehydroxylation. Also, some protons bond with undersaturated oxygen atoms of the octahedral sheet. The amount of Na+ (p) and H+ (ni) cations incorporated in the structure as a function of the amount of Fe reduction can be described quantitatively by two equations: p = m/(1 + K0mrel) and ni = K0mrel/(1 + K0mrel); with K0 = CEC (9.32 − 1.06mtot + 0.02mtot2), where mtot is the total Fe content in the smectite, m is the Fe2+ content, mrel is the reduction level (m/mtot), CEC is the cation-exchange capacity, and K0 is a constant specific to the smectite. The model can predict, from the chemical composition of a smectite, the modifications of its properties as a function of reduction level. Based on this model, the structural mechanism of Fe3+ reduction in montmorillonite differs from that determined in nontronite and beidellite owing to differences in the distribution of cations over trans- and cis-octahedral sites.

Keywords

CMS Source Clay SWa-1Dioctahedral SmectiteIron ReductionStructural Formula
Type
Research Article
Information
Clays and Clay Minerals , Volume 48 , Issue 2 , April 2000 , pp. 185 - 195
Copyright
Copyright © 2000, The Clay Minerals Society

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