SURFACE-REACTIONS DURING THE EARLY STAGES OF WEATHERING OF ALBITE

The aim of this study is to gain a better understanding of the nature and distribution of surface species occurring during the initial stage of weathering of albite. Instead of the classical acid-base titration experiments used extensively in previous work, the surface of freshly ground mineral was titrated by adding increasing amounts of solid to pure water. The aqueous phase was analyzed for all the elements present after a well-defined short reaction time. Compared to the classical titration experiment, this approach has the advantage of defining the entire composition of the system and limits the interferences of the dissolution process with surface reactions. The results confirm the rapid exchange of Na+ by H+ at the surface of albite, leading to the formation of an H-feldspar and a deprotonation producing negatively charged surface species. Addition of a large excess of Na+ to the aqueous solution demonstrates the reversibility of the exchange reaction. The exchange reaction and the deprotonation can be depicted by apparent equilibrium constants which describe the density of the negatively charged species as a function of pH. These results strongly suggest that the negatively charged surface species are closely related to Na sites at the feldspar surface where three species can be identified: NaAlSi3O8, HAlSi3O8, and AlSi3O8-. The fractional dependence of pH obtained (0.35) for the density of the surface charge is close to the fractional order of the dissolution rate in the same pH region (pH > 5.5). Additional experiments show that the fresh albite surface is able to react with gaseous CO2 which results in a more disturbed surface layer.