Fission Particle Tracks in Micas and Micaceous Vermiculites as Related to Chemical Weathering and Cation Exchange Properties

Abstract

AbstractFission particle tracks from 238U impurities in natural micas and micaceous vermiculites were observed by scanning electron microscopy after hydrofluoric acid etching. Track frequency was increased by neutron activation in a thermal nuclear reactor. Without HF etching, the average rate of interlayer K+ released from micas saturated with UO22+ and activated, during 6 days depletion in 0.5N Mg(OAc)2 solutions at pH 7 renewed every few min, were 40 to 50% higher than that from nonactivated samples. Activated UO22+ saturated micaceous vermiculites from Colorado (CO) and Wisconsin (WI) liberated more K, Fe, and Si(OH)4 compared to that from the controls, in 0.5N Mg(OAc)2 at pH 4, renewed frequently. Weathering of micaceous minerals in nature would thus be accelerated by the additional diffusion of interlayer and structural cations through the natural fission particle tracks. The cation exchange capacity (CEC) of the activated micaceous vermiculites after UO22+ saturation increased from 16 to 25 meq/100g (CO) and from 6 to 12 meq/100g (WI) after the extensive salt treatments involved in CEC measurement, while negligible change in CEC occurred in the CO Na controls and one‐third as much change occurred in the WI Na control. The observed higher K+ selectivity (relative to Ca2+) of the activated samples (0.86 for CO and 1.39 for WI), compared to the controls (0.43 and 1.21, respectively), indicates that activation provided an increase in edge wedge sites for the selective adsorption of fixing cations including the fission product 137Cs+.