Oxidation of Iron in Biotite by Different Oxidizing Solutions at Room Temperature

Abstract

Biotite samples (10–20 µm lepidomelane) were mixed with solutions that contained various oxidants (O2, NaOCl, H2O2, or Br2) and salts (NaCl, KCl, NaCl‐NaBPh4, or none) to compare the effectiveness with which these solutions oxidize structural Fe2+ at 25°C when customary procedures and mica samples at different stages of expansion are used. In general, the treatments that minimized interlayer K exchange did not oxidize detectable amounts of Fe2+, whereas those causing mica expansion oxidized equivalent fractions of the Fe2+ if they were given enough time. The rates of Fe2+ oxidation by the O2 and NaOCl‐O2 solutions were very slow in even the expanded mica and were similar enough to suggest the OCl‐ ions did not contribute to the process under these conditions. The Br2 and H2O2 treatments, on the other hand, oxidized 97 and 85% of the structural Fe2+ in a fully expanded mica, respectively, in a 24‐h period. The relative effects of Br2 and H2O2 solutions probably reflect differences in the oxidants themselves but are confounded by the low pH of the Br2 solution, NaBPh4 interactions and H2O2 decomposition. Indeed, all the results make it clear that more distinction needs to be made between the role of the oxidants and the side efects of the solutions and procedures that are used in oxidation experiments.

Natural Weathering Rates of Silicate Minerals

“…Biotite exhibits a progressive loss of K and an increase in the Al/ Si ratio that correlates with increasing weathering intensity and decreasing depth in the profile (Figure 22(a)). These changes are driven by the relatively rapid oxidation of ferrous Fe in biotite (Amonette et al, 1988). As shown by the electron backscatter image in Figure 22(b), this reaction produces epitaxial replacement of biotite by kaolinite.…”

“…The first stage of biotite weathering, for example, involves the oxidation of ferrous atoms and the concurrent release of K to form oxybiotite (Amonette et al, 1988;Buss et al, 2008). Depending on the weathering environment, biotite subsequently undergoes direct transformation to kaolinite with congruent release of Mg or it can weather to vermiculite with Mg partially retained in the solid (Fordham, 1990).…”

Chemical weathering rates of a soil chronosequence on granitic alluvium: III. Hydrochemical evolution and contemporary solute fluxes and rates

“…This apparent discrepancy reflects selective or nonstoichiometric elemental release from mineral phases. For example, biotite contains ferrous ion, which during the early stages of weathering, oxidizes to ferric ion with the concurrent release of interlayer K (White and Yee, 1985;Amonette et al, 1988). This process is responsible for elevated solute K associated with young weathering environments such as glacial terrains (Anderson et al, 1997;Blum and Erel, 1997).…”

The ubiquitous nature of accessory calcite in granitoid rocks: Implications for weathering, solute evolution, and petrogenesis

“…A significant number of experimental and field studies have documented that biotite weathers by two mechanisms: a fast reaction involving the oxidation of structural Fe and the release of interlayer K and Rb and a much slower reaction involving structural breakdown and the conversion of biotite to clay or vermiculite (White and Yee, 1985;Amonette et al, 1988). These reactions impact Sr isotopic release with Rb-derived radiogenic Sr being released relatively fast to solution followed by slower release of structural Sr of lower radiogenic content (Taylor et al, 2000).…”