The long-term alteration rate of Na-smectite in natural bentonite formation

Alteration of the natural bentonite in Kuroishi ore deposit located in the north Japan was estimated by the vertical element profiles of Na, Ca and Mg in the drilled core samples. The exchangeable Na depleted from the ground surface to 20 m in depth and the total loss of Na coincided with the accumulation of Ca and Mg. This suggests the Na depletion was caused by the ion exchange reactions. A simple analytical calculation using the steady state approximation indicated the average alteration rate was about 1 cm/1000 years. This value is equivalent to that by geomorphological studies.