『(Abstract)
Qualification 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 non-stoichiometric 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.』
(Text)
(References)
Acknowledgements