Bridging the gap between laboratory measurements and field estimations of silicate weathering using simple calculations |
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Authors: | Jiwchar Ganor Peng Lu Zuoping Zheng Chen Zhu |
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Institution: | (1) Department of Geological and Environmental Sciences, Ben-Gurion University of the Negev, P. O. Box 653, Beer-Sheva, 84105, Israel;(2) Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405-1405, USA |
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Abstract: | Weathering rates of silicate minerals observed in the laboratory are in general up to five orders of magnitude higher than
those inferred from field studies. Simple calculations show that even if the field conditions were fully simulated in standard
laboratory experiments, it would be impossible to measure the slow rates of mineral dissolution that are observed in the field.
As it is not possible to measure the dissolution rates under typical field conditions, one should extrapolate the available
data to the field conditions. To do this, a rate law for the dissolution of plagioclase in the field was formulated by combining
the far from equilibrium dissolution rate of weathered natural oligoclase at 25°C with the effect of deviation from equilibrium
on dissolution rate of fresh albite at 80°C. In contrast to the common view that laboratory experiments predict dissolution
rates that are faster than those in the field, the simulation based on this rate law indicates that laboratory dissolution
experiments actually predict slower rates than those observed in the field. This discrepancy is explained by the effect of
precipitation of secondary minerals on the degree of saturation of the primary minerals and therefore on their dissolution
rate. Indeed, adding kaolinite precipitation to the simulation significantly enhances the dissolution rate of the plagioclase.
Moreover, a strong coupling between oligoclase dissolution and kaolinite precipitation was observed in the simulation. We
suggest that such a coupling must exist in the field as well. Therefore, any attempt to predict the dissolution rate in the
field requires knowledge of the rate of the secondary mineral precipitation. |
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Keywords: | Aqueous geochemistry Ground water Waste water |
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