A comparative study of the dissolution mechanisms of amorphous and crystalline feldspars at acidic pH conditions

Abstract

The dissolution of amorphous and crystalline oligoclase, a Na-feldspar with traces of Ca and K, was investigated at 90 °C and acidic pH of 1.5 and 3 to unravel the impact of long- and short-range orders on silicate dissolution mechanisms. Experiments were conducted in solutions spiked with 29SiO2(aq) and saturated with respect to SiO2(am). Through morphology, structural, and composition characterizations, we showed that on the amorphous samples (glass samples), the altered layer was mostly formed by leaching, while a combination of both interfacial precipitation and leaching explains the layers formed on the crystalline sample. As expected, the altered layer was thicker at the most acidic pH and it became passivating on crystalline sample at pH 3. The alteration was faster on amorphous oligoclase than on its crystalline equivalent due to the more open structure of the glass. The preferential release of Al was suggested to play a key role, by weakening the silicate network of both substrates. Finally, in this study, a large overestimation of the global alteration of the materials was noticed based on the solution analyses. Discrepancies with solid analyses were attributed to an underestimation (≥2-fold factor) of the total reactive surface area.