Abstract
Dissolution kinetics of phlogopite were studied at 50°, 80° and 120°C using 20ml and 100ml of 0.01 N HCl solutions containing 0.1 M NaCl (pH 1.9). The residual solid materials showed the formation of many etch pits and leached layers. The etch pits are formed more easily on the side surfaces than on the cleavage surfaces of phlogopite. The leached layer which was formed around of the flakes is composed of vermiculite and interstratified mica/vermiculite. These reveal that the dissolution of phlogopite crystal proceeds anisotropically and the dissolution rate is relatively greater along the a and b axis directions than along the c axis direction.
The release rates of cations other than Si from phlogopite are explained by a diffusion kinetic equation based on a contracting disk model. The Si release, which is controlled by a surface reaction mechanism, is much slower than the other cations, and is a rate-limiting process of the entire dissolution of phlogopite. The dissolution rates of phlogopite depend on water/solid ratios used in this experiment, and were about six times faster in the 100ml experiments than in the 20ml experiments.
The degree of incongruency of dissolution was also more significant in the 100 ml experiments than in the 20ml experiments. The calculated activation energies for release of cations in the 100 ml experiments, followed the order, Si>Fe, Mg, Al>K, which could be related to the priority of release of cations from phlogopite structure. The activation energiesfor the cations other than Si (45-55 kJ/mol) were higher than those reported previously for a diffusion process of cations from phyllosilicates into solution (-40 kJ/mol). This is probably due to the existence of vermiculite and interstratified mica/vermiculite which acts as a diffusion-inhibiting layer.
