Effect of composition and physical properties of natural kaolinitic clays on their strong acid weathering rates

Two samples of pure kaolinite with a significant difference in their crystallinity, a commercial kaolin sample with 20 wt.% of illitic component and one sample of illitic clay with up to 20 wt.% of kaolinitic component were used in dissolution kinetic experiments and the results compared to data obtained from the leaching of sandstone matrix kaolinites under laboratory conditions. The mineralogical and chemical compositions of all samples were characterised using X-ray diffraction and X-ray fluorescence techniques. Specific surface area was measured by the gas adsorption (BET) method. Structural disorder of kaolinite given by the extent of the sheet translations was calculated from the X-ray diffraction data (the ‘Hinckley index’). Dissolution experiments in continuously stirred suspensions (with solid/liquid ratio=1:100) were performed in 0.5 and 5 M solutions of H2SO4 at 25 and 40 °C, respectively, and the reaction rate was calculated from the measured concentrations of Al and Si in the solution. It was found that slight non-stoichiometry in the steady state stage of dissolution with preferential removal of aluminium is a common feature. It can be augmented by the presence of impurities, such as aluminium phosphates (crandallites). Acid weathering rates increase with structural disorder of kaolinite but observed differences cannot be fully explained by the diversity of crystal lattice structures. The contribution of illite to the dissolution process is very small; and thus, illitic admixtures cause the deceleration of the acid decomposition of natural kaolinitic clays. When calculating solute transport in acidified profiles it is suggested that the changes of the kinetic parameters of kaolinite due to the presence of poorly ordered structures (increasing the rate) and to the presence of 2:1 structures (such as illites, decreasing the rate) in mineral assemblages should be taken into consideration.