Abstract: The selectivity of K over Ca of Amelia biotite increased sharply upon oxidation by H2O2 at pH 6·0. This increase in K selectivity was only partially reversible upon reduction by Na2S2O4. Oxidation by H2O2 of the Ca-form of this biotite resulted in a loss of no more than 2·8 and 0·4 per cent of the total Fe and Al, respectively, and caused a small and perhaps insignificant decrease in layer charge. Although 95 per cent of the structural Fe2+ of the Ca-form of this biotite was ozidized by H2O2, only 17 per cent was reduced again by four treatments with Na2S2O4.
The evidence indicates that under the conditions of this experiment the loss of protons from structural hydroxyls was the dominant mechanism by which electroneutrality in the biotite was maintained during oxidation of structural Fe2+. Because this mechanism increases the bond strength of interlayer K, it explains the increased K selectivity of biotite upon H2O2 oxidation. The relatively small reduction by Na2S2O4 of structural Fe3+ to Fe2+, which implies an equally small reprotonation of structural hydroxyls, explains the incomplete reversibility of K selectivity by Na2S2O4 treatment.