Feedbacks in silicate glass alteration studied by in operando fluid-cell Raman spectroscopy

The alteration of silicate glasses (and minerals) is involved in most biogeochemical cycles and poses challenges in several technical applications. The mechanism(s), kinetics and rate-determining factors have been extensively studied in an attempt to eventually arrive at a quantitative, predictive process description. However, this has proven challenging, because of the non-linear progression of the alteration process, which is affected by several reaction-inherent feedbacks. We investigated the interplay of such feedbacks through Raman spectroscopic fluid-cell experiments, enabling in operando visualization and rate measurements of reactions at the altering glass surface. One experiment with a low surface area-to-solution volume (SA/SV) ratio exhibited a transient acceleration of initial dissolution rates, followed by continuously rapid glass dissolution along with slow growth of a smectite-based surface alteration layer (SAL), apparently limited by glass-derived Mg. Contrastingly, in a high SA/SV experiment, glass dissolution monotonically decreased after the onset of rapid SAL growth, which was followed by the formation of carbonate coating and pore-space fillings, as well as by zeolite precipitation. Growth conditions and resulting properties of the SAL seem to exert dominant, non-linearly decelerating effects on glass dissolution, but are opposed by (re-)accelerating effects of glass cracking, pitting, and SAL delamination. In turn, SAL formation depends on precipitation kinetics and the accumulation of glass-derived solutes at the reaction front. However, dissolution and precipitation may also feedback with solution chemistry and transport processes. Such mechanistic insights from in operando experiments help understanding silicate alteration processes and can assist the improvement of predictive models.