Development of layer charge and kinetics of experimental smectite alteration. Clays & Clay Miner

Abstract--Sodium-saturated Wyoming bentonite was hydrothermally reacted at 150 ~ and 250 ~ for 30 to 180 days to determine smectite alteration rates that might be applied to nuclear-waste repository design. Na-Ca solutions deficient in K were used to determine the role of interlayer cations in the creation of high layer-charge in the smectites. The results provide insight into the mechanism and timing of various steps in the diagenetic alteration of smectite to illite. X-ray powder diffraction (XRD) analyses of the reacted clay showed little effect on the character of the 17-/ ~ reflection even after 180 days at 250~ Potassium saturation of these reacted clays and re-examination by XRD indicated collapse of some smectite layers, leaving at most only 60 % expandable layers. The development oflayer charge sufficient to cause collapse on saturation with a low hydration energy exchange-cation does not require K in the reacting fluid. Rate constants for the illitization reactions as determined by K-saturated collapse are between 1.0 x 10-3 and 2.8 x 10-3/day with activation energies <3.5 kcal/mole. Ca in a Na-silicate-bicarbonate solution slightly reduced the illitizafion rate constants. These rate constants are higher than expected from extrapolation fstudies of beidellite-composition glasses at higher temperatures, but lower than values obtained in studies of natural clays in artificial sea water. The release of Si, A1, and Mg in the 150 ~ experiments suggests congruent dissolution of the smectite. In contrast, at 250 ~ the releas