BIOTITE DISSOLUTION AT 25OC: THE PH DEPENDENCE OF DISSOLUTION RATE AND STOICHIOMETRY

Abstract

The rate and stoichiometry of biotite dissolution were studied in the pH range 2-10 using thin-film continuous flow reactors. The release of interlayer K is relatively fast and becomes diffusion-controlled within a few days. The release rates of framework ions (Mg, Al, Fe, Si) are much slower and reach an apparent steady-state within ten days. The stoichiometry and rate of dissolution vary greatly with pH. Consistent with surface reaction control of release rates, an empirical rate law, R = kH[H+]m + k0 + kOH[H+]n (moles m-2 h-1) describes proton- and hydroxyl-catalysed dissolution for each ion.Rapid K+ release provides a tracer for the extent of the hydrated reacting layer on the biotite surface and within interlayers. An altered reaction layer composition, calculated from mass balances for released ions, results from preferential leaching of some ions and is consistent with that of vermiculite. X-Ray powder diffractometry confirmed the formation of both vermiculite and kaolinite during the weathering reaction. The pH dependence of release rates, normalised to the corresponding ion concentrations in the reacting layer, correlate with those for the respective binary oxides (SiO2, Al2O3, Fe2O3, MgO).Release rates for Al, Mg, and Fe at neutral pH are much slower when the mineral has been previously reacted at low pH where these ions are released rapidly. Model simulations suggest that, for ions that initially dissolve rapidly, release rates will decrease as the ion is depleted in the reacting layer. Rates will eventually approach those of the most slowly dissolving ion. At 25°C and pH 7, this process would lead to stoichiometric dissolution within 50 y.