Water in silicate glasses and melts of environmental interest: from volcanoes to cathedrals.
Résumé
n silicate glasses and melts, water acts according to two main processes. First, it can be dissolved in high temperature/high pressure melts. Second, it constitutes a weathering agent on the glass surface. A number of in-situ x-ray absorption fine structure (XAFS) studies for Fe, Ni, Zr, Th and U show that the more charged cations (Zr, Nb, Mo, Ta, Sn, Th and U) are little affected by the presence of dissolved water in the melt. In contrast, divalent iron and nickel are highly sensitive to the presence of water, which enhance nucleation processes, for example, of phyllosilicates at the Å-scale. Such information provides additional constraints on the role of water deep in the Earth, particularly in magmatology. By contrast, the weathering of glass surfaces by water can be studied from a durability perspective. Experimental weathering experiments of nuclear waste glasses performed in the laboratory show a variety of surface enrichments (carbon, chlorine, alkalis, iron) after exposure to atmospheric fluids and moisture. Mn-, and Fe-surface enrichments of analogous glasses of the XIVth century are related to the formation of Mn and Fe oxy/hydroxides on the surface. The impact on the glass darkening is considered in terms of urban pollution and mass tourism.