Comparing the reactivity of glasses with their crystalline equivalents the case study of albite

To evaluate the impact of atomic shortand 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. Suggested Reviewers: Jonathan Icenhower Sandia National Laboratories, USA jpicenh@sandia.gov Dr Icenhower is a specialist of glass reactivity and has a strong expertise regarding the experimental study of dissolution kinetics of glass, ceramics, minerals (e.g. Icenhower and Steefel, GCA, 2015). Jose Godinho University of Manchester, UK jose.godinho@manchester.ac. uk Dr Godinho is a mineralogist who conducted several innovative studies related to the face-specific dissolution of minerals (e.g. Godinho et a., GCA, 2014). Jincheng Du North Texas University, USA jincheng.du@unt.edu Dr. Du’s research expertise concerns atomistic computer simulations of glass, ceramics, and nanostructured materials (e.g. Mauro and Du, 2012). Inna Kurganskaya Institute of Geological Sciences, Switzerland inna.kurganskaya@geo.unibe. ch Dr Kurganskaya is an expert of kinetic Monte Carlo simulations and has a strong expertise in the field of solid-fluid interaction with a focus on surface reactivity and dissolution kinetics (e.g. Kurganskaya and Luttge, GCA, 2013). Sebastian Kerisit Pacific Northwest National Laboratory, USA sebastien.kerisit@pnnl.gov Dr Kerisit is a geochemist with a broad knowledge on mineral nucleation and growth and also in developing numerical models for nuclear glass alteration (Kerisit et al., 2015). Anne Pérez Laboratoire Géomatériaux et Environnement (LGE) Univ. Paris-Est Marne-la-Vallée 5 boulevard Descartes, 77454 Marne-la-Vallée France anne.perez@u-pem.fr Paris, 28 May 2018