Kinetics of muscovite, phlogopite, and biotite dissolution and alteration at pH 1-4, room temperature

The reaction kinetics of dioctahedral muscovite and trioctahedral phlogopite and biotite were studied in aqueous solutions at pH 1-4 and room temperature. The experiments were performed in a reactor where mineral suspensions were separated from eluent solutions with dialysis membranes. Dissolution of muscovite was close to stoichiometric. In the experiments with trioctahedral micas, a solid residual of altered 2:1 layers was formed by preferential release of cations. A preferential release of K was found with phlogopite and biotite. Also, an excess release of octahedral cations compared to tetrahedral Si was obtained in biotite experiments. Describing the dissolution rates, R, by the equation R = k(H)+a(H)(n)+, apparent rate constants, k(H+), of 1.7 . 10(-2) for muscovite, 3.4 . 10(-10) for phlogopite and 3.2 . 10(-10) for biotite, in units of mol (O-20(OH)(4) formula units) m(-2) s(-1) were obtained. The corresponding exponents, it, were 0.14, 0.40, and 0.61. These values were calculated from rates that were normalized to the total mineral surface area at the start of experiments. Surface area changed during the experiments, partly due to delamination along basal surfaces, which affected the calculation of rate constants and the pi-I-dependence of rates. Rates that are normalized to total surface area may have little relevance for micas, since reactive sites probably are concentrated on edge surfaces which comprise a small fraction of total surface. Biotite Fe2+ was found to reduce dissolved Fe3+, confirming previous reports about the importance of Fe-rich silicates for the redox state of dissolved species. Possible effects of nonstoichiometric release of cations on the composition of altered products are discussed.