Nature and role of natural alteration gels formed on the surface of ancient volcanic glasses (Natural analogs of waste containment glasses)

This paper provides an overview of the geochemical alteration of basaltic glass, considered for over 20 years as a suitable natural analogue for nuclear borosilicate glasses. The available data show that natural basaltic glasses may survive for million of years under subsurface conditions. Mineralogical studies show that an alteration layer called palagonite forms on the surface of the basaltic in response to the chemical attack by water. Under some environmental conditions, the alteration layer consists of an amorphous gel-like material, leading many authors to suspect hydration of the glass by water permeation and alkali interdiffusion. In other cases, the alteration layer is crystallized to some extent and contains clay minerals (smectite). Such layers are formed mainly on the younger natural glass samples (<1 My) by a process of coprecipitation of the elements dissolved from the glass. On samples older than 1 My, the alteration layers also contain zeolites. In the presence of these hydrated aluminosilicates, a hydrated residual glass is systematically observed, which thus forms as a consequence of interdiffusion processes. Leach tests conducted under controlled laboratory conditions at temperatures up to 200 °C on both natural glass samples and synthetic basaltic glass provide identical kinetic results and alteration mechanisms. When compared with the data for SON68 nuclear borosilicate glass, the initial dissolution rates show the same activation energy (about 72 kJ mol−1) and consequently similar dissolution mechanisms. Moreover, when altered under static conditions at high reaction progress, both basaltic glass and nuclear glass have similar behavior characterized by a significant drop in the dissolution rate, up to three to five orders of magnitude lower than the initial dissolution rate. The time-dependence of the thickness of the altered layers measured on natural glass samples confirms this kinetic trend over time: the long-term dissolution rate is very low. This decrease may be related to diffusion mechanisms involving key chemical species and controlled by the mineralogy of the palagonite layer.