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

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 SiO$_{2 am}$ were additionally performed toconstrain the effect of Si-rich surface layer formation on dissolution rates. The face-specificdissolution rates of the crystalline albite and of the albite glass were determined from elementbudget in solution and surface retreat measured by vertical scanning interferometry. Theresults 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 itstopography and its dissolution rate was observed. The (001), (010) and (101) flat facescontaining the strongest bonds dissolved the most slowly and their dissolution rates remainedconstant throughout the experiments. In contrast, the stepped (111) face was characterized bythe 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 ofvery thin (100-200 nm) Si-rich surface layers in acidic conditions, which weakly affected thedissolution 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 byinterfacial dissolution-reprecipitation for albite crystal, molecular dynamic calculationssuggest 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 43 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.
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Anne Perez, Damien Daval, Maxime Fournier, Mélanie Vital, Jean-Marc Delaye, et al.. Comparing the reactivity of glasses with their crystalline equivalents the case study of albite. Geochimica et Cosmochimica Acta, Elsevier, 2018, 254, pp.122-141. ⟨10.1016/j.gca.2019.03.030⟩. ⟨cea-02339619⟩

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