KINETICS OF DISSOLUTION AND SR RELEASE DURING BIOTITE AND PHLOGOPITE WEATHERING

Abstract

The dissolution kinetics and isotopic composition of Sr released during the weathering of biotite and phlogopite were measured under experimental conditions at pH ~ 3 and 25°C. Although the overall release of Sr was essentially stoichiometric, the 87Sr/86Sr ratios of weathering solutions were generally higher than the reacting mineral. The initial phase of dissolution was characterized by the simultaneous rapid release of Sr from the sheet silicate and from trace amounts of calcite inclusions. During the initial phase, the 87Sr/86Sr ratios of the output solutions were lower than either the bulk mineral or the later, steady-state ratios. After the early stages of the experiments, calcite dissolution became limited by the rate at which it was exposed to the weathering solution and the 87Sr/86Sr of the output solutions increased to values above those of the biotite and phlogopite (after correction for the calcite inclusions). Differences in the location of Sr and Rb within the mica structure may cause the Sr isotopic ratio of the effluent to increase above the mineral ratio. The log values of the steady-state Sr release rate constants (moles per meter per second) were determined to be -15.1 for biotite and -15.9 for phlogopite under our experimental conditions. Cation release rates indicate that different sites within the sheet silicate structure reacted at different rates. Interlayer cations were generally released more rapidly than octahedral cations, which reacted slightly faster than the tetrahedral cations. Interlayer potassium release was considered to be controlled by diffusion with a moving boundary condition. Diffusion coefficients calculated from this model were 3.5 x 10-19 cm2/s for biotite and 1.9 x 10-18 cm2/s for phlogopite. Iron oxidation appeared to limit the overall biotite dissolution rate. Iron-free phlogopite reacted nearly twice as fast as biotite. The log values of the overall dissolution rates (moles per meter per second) were determined to be -11.6 for biotite and -11.2 for phlogopite. Cation release during phlogopite dissolution was also much less stoichiometric than during biotite weathering.