Rates of dissolution of biotite in 0.1, 0.01, 0.001 M HCl decrease as the proportion of ferric iron in octahedrally coordinated sites increases. This is interpreted as being partly due to a decrease in the rate of K exchange from oxidized biotite so that a smaller surface area is exposed to acid attack. The availability of structural cations (Mg, Mn, Zn, K, etc.) to plants growing on biotite rich soils will be a function of the oxidation state of structural iron in the biotite.
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Gilkes et al. (1973) studied this question.