DISSOLUTION KINETICS OF BASALTIC GLASSES: CONTROL BY SOLUTION CHEMISTRY AND PROTECTIVE EFFECT OF THE ALTERATION FILM

Abstract

Basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses. Alteration experiments were conducted in closed and open systems at 90°C with a synthetic basalt glass doped with 1% lithium (dissolution tracer). The evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments. The maximum dissolution rate (initial rate, r0) 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 account for the alteration kinetics of basaltic glasses, nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity: the chemical 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 inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon-enriched solutions. The basaltic glass dissolution rate dropped by a factor not exceeding 200 (or about two orders of magnitude) compared with r0 at the high imposed silicon concentrations (130 ppm). A protective effect of the alteration film was advanced to account for the four-orders-of-magnitude rate drop observed in closed system experiments, based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years. The mean alteration rates can be estimated from the measured palagonite thicknesses, taking into account the age of the natural glasses; the rates were comparable to those measured in the laboratory for high reaction progress.