The effect of time on the weathering of silicate minerals: why do weathering rates differ in the laboratory and field?

摘要: Abstract The correlation between decreasing reaction rates of silicate minerals and increasing duration chemical weathering was investigated for both experimental field conditions. Column studies, using freshly prepared Panola Granite, produced ambient plagioclase that decreased parabolically over 6 years to a final rate 7.0×10 −14 mol m −2 s −1 . In contrast, the corresponding partially kaolinized after reaching steady-state 2 months reactions, significantly less (2.1×10 −15 ). Both were normalized content BET surface area. Extrapolation fresh with time indicated several thousand would be required replicate naturally weathered under identical remain orders magnitude faster than previously measured profile in Granite. Additional trends established from tabulation reported plagioclase, K-feldspar, hornblende biotite. Discrepancies literature, by normalization respect areas gas absorption (BET) geometric methods, overcome developing time-dependent roughness factor. Regression curves through corrected strong correlations similar four minerals. average R (mol ) described power function R=3.1×10 −13 t −0.61 which relationship describing decrease suggesting control transport-limited reaction. dependency is discussed terms processes intrinsic mineral those extrinsic environment. Intrinsic area, increases weathering, shown account third exponential above equation. Other factors, including progressive depletion energetically reactive surfaces accumulation leached layers secondary precipitates, must explain differences reacting Extrinsic controls, low permeability, high mineral/fluid ratios increased solute concentrations, produce reactions close thermodynamic equilibrium conditions compared highly unsaturated during plagioclase. These difference lab rates.