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

Four Na 2 S 2 O 4 -reduced Na-vermiculites, each with some trioctahedral mica interstratified, were oxidized with H 2 O 2 at pH 6·5 and again reduced with Na 2 S 2 O 4 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 100g −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+ ${\{ {{{[ {Fe( {II} )} ]}_2}M{g_4}{O_4}{{( {OH} )}_4}} \}± 0}{\{ {{{[ {Fe( {III} )} ]}_2}M{g_4}{O_4}{{( {OH} )}_2}O_2^*} \}^ {± 0}} + 2{e^ - } + 2{H^ + }and by ejection of (dissolution of structural) octahedral metallic cations, (b) [Fe(II)]5MgO4(OH)4±0→[Fe(III)]4O4(OH)4±0+5e−+Fe3++Mg2+.{\{ {{{[ {Fe( {II} )} ]}_5}Mg{O_4}{{( {OH} )}_4}} \}± 0} → {\{ {{{[ {Fe( {III} )} ]}_4}{O_4}{{( {OH} )}_4}} \}± 0} + 5{e^ - } + F{e3 +} + M{g2 +}.$ 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 Mg 2+ ejected per Fe(II) oxidized was related to the degree of vermiculitization, being greatest with Na-degraded biotite [0·03 Fe 3+ and 0·11 Mg 2+ 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 100g (300°C basis) was lost, while 51 mmole of Fe(II) per 100g was oxidized in the presence of Na 2 S 2 O 4 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 ( r 2 = 0·04), the layer charge changes were concluded to be entirely independent of the oxidation or reduction of Fe in these minerals.