Dissolution kinetics of basaltic glasses: control by solution chemistry and protective effect of the alteration film

Basaltic glasses are considered as natural analogs for industrial nuclear a luminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 90 degreesC with a synthetic basalt glass doped with 1 % lithium (dissolution tracer). The evolution of the alteration kinetics ov er time was assessed by comparison of reaction progress at different degree s in closed system experiments. The maximum dissolution rate (initial rate, r(0)) was comparable to the value observed For an SON68-type nuclear glass ; the basaltic glass alteration rate subsequently dropped by four orders of magnitude. The kinetic models currently proposed in the literature to acco unt for the alteration kinetics of basaltic glasses, nuclear glasses or alu minosilicate minerals are based on the concept of chemical affinity: the ch emical affinity alone is assumed to control the dissolution kinetics. When applied to the experimental data for the closed system tests with basaltic glass, these models failed to account for the low measured rates. An inhibi ting effect of dissolved silica was then investigated through open system b asaltic glass alteration experiments with silicon-enriched solutions. The b asaltic glass dissolution rate dropped by a factor not exceeding 200 (or ab out two orders of magnitude) compared with r(0) at the high imposed silicon concentrations (130 ppm). A protective effect of the alteration film was a dvanced to account for the four-orders-of-magnitude rate drop observed in c losed system experiments, based notably on an examination of natural basalt ic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thickn esses, taking into account the age of the natural glasses: the rates were c omparable to those measured in the laboratory for high reaction progress. ( C) 2001 Elsevier Science B.V. All rights reserved.