Iron Oxidation and Reduction Effects on Structural Hydroxyl and Layer Charge in Aqueous Suspensions of Micaceous Vermiculites

J. A. Veith* and M. L. Jackson
Department of Soil Science, University of Wisconsin, Madison, Wisconsin 53706, U.S.A.
* Present address, Institut für Bodenkunde der Technischen Universität, 1 Berlin 33, Englerallee 19-21, Germany.

Abstract: Four Na2S2O4-reduced Na-vermiculites, each with some trioctahedral mica interstratified, were oxidized with H2O2 at pH 6·5 and again reduced with Na2S2O4 in suspensions at pH 7·5–8·0. The layer charge (CEC + K+), measured at pH 6·50, did not change significantly when octahedral Fe was oxidized (7–92 mmole 100 g−1) or reduced (6–71 mmole 100 g−1). Electroneutrality was maintained within the octahedral sheet when Fe was oxidized or reduced. When Fe(II) was oxidized, electroneutrality was maintained by deprotonation of octahedral OH groups, (a){[Fe(II)]2Mg4O4(OH)4}±0 ⇄ {[Fe(III)]2Mg4O4(OH)2O2*}±0 + 2e + 2H+ and by ejection of (dissolution of structural) octahedral metallic cations, (b){[Fe(II)]5MgO4(OH)4}±0 → {[Fe(III)]4O4(OH)4}±0 + 5e + Fe3+ + Mg2+. When Fe(III) was reduced, electroneutrality was maintained by reprotonation of the deprotonated sites (O*, equation a). Reaction (b) was not reversible. Thus, reversibility of the reaction, Fe(II) ⇄ Fe(III), within the octahedral sheet decreased with increasing amount of ejected metallic cations. The amount of Fe(III) and Mg2+ ejected per Fe(II) oxidized was related to the degree of vermiculitization, being greatest with Na-degraded biotite [0·03 Fe3+ and 0·11 Mg2+ per Fe(II) oxidized] and lowest (nearly zero) with South African vermiculite. The number of deprotonated (O*) and reversible sites increased from 0·69 per Fe(II) oxidized with the K-depleted biotite to approximately 1·0 with South African vermiculite. The weathering increment was small since, of the total amount of Fe + Mg, less than 1·3 per cent was ejected from any of the four vermiculitic materials. When biotite was K-depleted, about 20 m-equiv of layer charge per 100 g (300°C basis) was lost, while 51 mmole of Fe(II) per 100 g was oxidized in the presence of Na2S2O4 and 82 mmoles in its absence in the aqueous suspensions. Since sequential reduction-oxidation-reduction treatments of K-depleted biotite and mica-containing vermiculites did not cause significant changes in layer charge (r2 = 0·04), the layer charge changes were concluded to be entirely independent of the oxidation or reduction of Fe in these minerals.

Clays and Clay Minerals; August 1974 v. 22; no. 4; p. 345-353; DOI: 10.1346/CCMN.1974.0220405
© 1974, The Clay Minerals Society
Clay Minerals Society (www.clays.org)