Comparing the reactivity of glasses with their crystalline equivalents: The case study of plagioclase feldspar

Abstract

To evaluate the impact of atomic short-and long-range orders on silicate dissolution kinetics, the dissolution of amorphous and crystalline albite was investigated at pH 1.5 and 10 at 90°C. Experiments in solution saturated with respect to SiO2 am were additionally performed to constrain the effect of Si-rich surface layer formation on dissolution rates. The face-specific dissolution rates of the crystalline albite and of the albite glass were determined from element budget in solution and surface retreat measured by vertical scanning interferometry. The results show that atomic ordering primarily impacts solid reactivity, irrespective to the pH of the solution. A strong relation between the crystal surface orientation, the evolution of its topography and its dissolution rate was observed. The (001), (010) and (10-1) flat faces containing the strongest bonds dissolved the most slowly and their dissolution rates remained constant throughout the experiments. In contrast, the stepped (1-11) face was characterized by the highest initial dissolution rate, but progressively decreased, suggesting that the preferential dissolution of stepped sites expose afterwards more stable planes. The differences in terms of etch pit density from one surface to another also allowed to explain the difference in dissolution rates for the (001) and (010) faces. The fluid chemistry suggested the formation of very thin (100-200 nm) Si-rich surface layers in acidic conditions, which weakly affected the dissolution rate of the pristine crystal. At pH 1.5, albite glass dissolves at a rate similar to that of the fastest studied faces of the crystal. Whereas Si-rich surface layers likely formed by interfacial dissolution-reprecipitation for albite crystal, molecular dynamic calculations suggest that the open structure of the glass could also allow ion-exchange following water diffusion into the solid. This different mechanism could explain why the surface layer of the glass is characterized by a different chemical composition. Results at pH 10 are strikingly different, as the albite glass dissolves 50 times faster than its crystalline equivalent. This non-linear response of the material upon pH was linked to the density of critical bonds in albite which is indeed pH-dependent. In acidic pH, the preferential dissolution of Al leaves a highly polymerized and relaxed Si-rich surface, whereas in basic pH the preferential dissolution of Si leads to a complete de-structuration of the network because of the lack of Si-O-Al bonds.

A Statistical Approach for Analysis of Dissolution Rates Including Surface Morphology

“…The fact that they did not see any polishing artifacts on the miscut sample could reflect AFM's smaller field of view, but in our study the polishing artifacts were also ephemeral features (hours). The prevalence of mechanical damage due to polishing effects has been reported for oligoclase in a study by [36], where polishing scratches resulted in dissolution rates that almost doubled the mean surface retreat.…”

Can a simple topological-constraints-based model predict the initial dissolution rate of borosilicate and aluminosilicate glasses?

“…Ten sodium borosilicate glasses and six aluminosilicate glasses were selected for this study. The first three glasses displayed in Table 5 have been subject to previous investigations by Gin et al 32 , Pacaud et al 37 , and Perez et al 38 . Note that NBSACZ corresponds to the International Simple Glass (ISG) 4 , which has been subject to many studies since 2013 39 .…”

Thermodynamic equilibrium and kinetic fundamentals of oxide dissolution in aqueous solution

“…For instance, the activation energy for the dissolution of quartz and silica glass is approximately the same [148], suggesting a similar mechanism of breaking the Si-O bond in the dissolution of the two phases. Differences in their dissolution rate are related to differences in the distortions of local structure in the glass and different energetics [149,150], which are associated with broader bond lengths and angle distributions, larger molar volumes, and other structural features in glasses that are not present in crystals [151,152]. Structural features in oxide glasses such as water-accessible free volume may lead to dissolution mechanisms that are different from those in crystals [151,152].…”

“…Differences in their dissolution rate are related to differences in the distortions of local structure in the glass and different energetics [149,150], which are associated with broader bond lengths and angle distributions, larger molar volumes, and other structural features in glasses that are not present in crystals [151,152]. Structural features in oxide glasses such as water-accessible free volume may lead to dissolution mechanisms that are different from those in crystals [151,152]. In addition, the stepwave dissolution mechanism that well describes the dissolution kinetics of crystals with dislocation defects may not be applicable to glasses [56].…”