Short- and long-term weathering rates of a feldspar fraction isolated from an arid zone soil

Numerous studies have established that laboratory determined weathering rates are several orders of magnitude greater than rates derived from field studies at the catchment or basin scale. The laboratory rates have been determined on specimen minerals or rock isolates which were subsequently crushed. Some samples were pretreated to remove organic coatings and possible carbonate contamination. Using magnetic, density, and size separation, we isolated a plagioclase (An(20))-K-feldspar fraction from an arid zone soil (classified as Pachappa, Mollic Haploxeralf). Silica release rates decreased with time and after up to 3.7 years were in the range 0.02 x 10(-12) mol m(-2) s(-1), when surface area is expressed with BET surface area or 0.6 x 10(-12) mol m(-2) s(-1), when expressed with geometric surface area. These Si rates are in good agreement with published field rates, as are the individual mineral dissolution rates calculated from mineral abundance and cation release. These data are in contrast to faster laboratory determined rates based on crushed or ground samples, or shorter reaction times. Based on laboratory studies with and without various pretreatments, we conclude that organic and Fe oxide surface coatings due to natural weathering are not the explanation for our low reaction rates. Apparently even mild treatment such as sonification and sieving results in enhanced weathering which persists for more than 100 days.