The influence of natural mineral coatings on feldspar weathering

Quantification of the rate of weathering of feldspar, the most abundant mineral in the Earth's crust, is required to estimate accurately carbon dioxide fluxes over geological timescales and to model groundwater chemistry. Laboratory dissolution rates, however, are consistently found to be up to four orders of magnitude higher than the 'natural' rates measured in the field. Although this discrepancy has been attributed to several factors, previous research has tended to suggest that the underlying mechanism of feldspar dissolution under acidic pH may differ between the field and the laboratory. Here we demonstrate that weathered albite surfaces, like laboratory-dissolved samples, are sodium- and aluminium-depleted, indicating that the dissolution mechanism in acidic soils is similar to that in acidic laboratory solutions. We find that microtopography images are consistent with dissolution occurring at specific surface sites-indicative of surface- controlled dissolution dominated by a nonstoichiometric layer. Elevated aluminium and silicon ratios reported previously, and used to suggest a mechanism for field weathering different from laboratory dissolution, can alternatively be explained by a thin, hydrous, patchy, natural coating of amorphous and crystalline aluminosilicate. This coating, which is largely undetected under scanning electron microscopy after cleaning, but visible under atomic force microscopy, alters surface chemistry measurements and may partially inhibit the field dissolution rate.