THE INFLUENCE OF NATURAL MINERAL COATINGS ON FELDSPAR WEATHERING

Quantification of the rate of weathering of feldspar, the most abundan t mineral in the Earth's crust, is required to estimate accurately car bon dioxide fluxes over geological timescales and to model groundwater chemistry. Laboratory dissolution rates, however, are consistently fo und to be up to four orders of magnitude higher than the 'natural' rat es(1,2) measured in the field. Although this discrepancy has been attr ibuted to several factors(2), previous research has tended to suggest that the underlying mechanism of feldspar dissolution under acidic pH may differ between the field and the laboratory(3). Here we demonstrat e that weathered albite surfaces, like laboratory-dissolved samples, a re sodium- and aluminium-depleted, indicating that the dissolution mec hanism in acidic soils is similar to that in acidic laboratory solutio ns. We find that microtopography images are consistent with dissolutio n occurring at specific surface sites-indicative of surface-controlled dissolution dominated by a nonstoichiometric layer. Elevated aluminiu m and silicon ratios reported previously(3,4), and used to suggest a m echanism for field weathering different from laboratory dissolution(3) , can alternatively be explained by a thin, hydrous, patchy, natural c oating of amorphous and crystalline aluminosilicate, This coating; whi ch is largely undetected under scanning electron microscopy after clea ning, but visible under atomic force microscopy, alters surface chemis try measurements and may partially inhibit the field dissolution rate.