TY - JOUR

T1 - Diffusive transport of water in porous feldspars from granitic saprolites

T2 - In situ experiments using FTIR spectroscopy

AU - Simonyan, A. V.

AU - Behrens, H.

AU - Dultz, S.

N1 - Copyright: Copyright 2011 Elsevier B.V., All rights reserved.

PY - 2009/12/1

Y1 - 2009/12/1

N2 - A novel experimental cell was developed for in situ measurements of transport phenomena in porous media using Fourier-Transform Infrared (FTIR) Spectroscopy. The technique was employed at ambient pressure in the temperatures range of 11-44 °C to study the H2O → D2O exchange between water-saturated weathered feldspars (bulk porosity of 5-19 vol% for feldspar) from granitic saprolites and a surrounding aqueous liquid. Such measurements are an important step for understanding internal weathering reactions of feldspars in soils and aquifers. Effective diffusion coefficients Deff for water in water-saturated porous feldspars were determined assuming one-dimensional diffusion in a quasi-homogeneous medium. The values of Deff vary from 7.2 × 10-10 to 1.9 × 10-11 m2/s and are 1-2 orders of magnitude lower than the diffusion coefficients (D) of protons and molecular H2O in liquid water. The activation energy for the H2O → D2O exchange process in porous feldspars ranges from 7.8 to 18.8 kJ/mol. The results imply that the effective diffusivity of water is mainly controlled by physical properties of the feldspars like porosity, pore connectivity, pore geometry and distribution. Perthitic feldspars with homogeneous pore distribution in the albitic lamellas have diffusional tortuosity factors X = D/Deff between 3 and 10 while alkali feldspars with inhomogeneously distributed and disconnected pores have much higher X values up to 129. Diffusion anisotropy has been verified for a vein perthite with diffusion perpendicular to the lamellas being faster by 0.3-0.5 log units than within the lamellas. It has to be emphasized that the study is based only on few selected feldspars, including perthitic feldspar, and additional work on samples with different weathering stages is needed to test the importance of the different parameters controlling diffusive transport in the pore system.

AB - A novel experimental cell was developed for in situ measurements of transport phenomena in porous media using Fourier-Transform Infrared (FTIR) Spectroscopy. The technique was employed at ambient pressure in the temperatures range of 11-44 °C to study the H2O → D2O exchange between water-saturated weathered feldspars (bulk porosity of 5-19 vol% for feldspar) from granitic saprolites and a surrounding aqueous liquid. Such measurements are an important step for understanding internal weathering reactions of feldspars in soils and aquifers. Effective diffusion coefficients Deff for water in water-saturated porous feldspars were determined assuming one-dimensional diffusion in a quasi-homogeneous medium. The values of Deff vary from 7.2 × 10-10 to 1.9 × 10-11 m2/s and are 1-2 orders of magnitude lower than the diffusion coefficients (D) of protons and molecular H2O in liquid water. The activation energy for the H2O → D2O exchange process in porous feldspars ranges from 7.8 to 18.8 kJ/mol. The results imply that the effective diffusivity of water is mainly controlled by physical properties of the feldspars like porosity, pore connectivity, pore geometry and distribution. Perthitic feldspars with homogeneous pore distribution in the albitic lamellas have diffusional tortuosity factors X = D/Deff between 3 and 10 while alkali feldspars with inhomogeneously distributed and disconnected pores have much higher X values up to 129. Diffusion anisotropy has been verified for a vein perthite with diffusion perpendicular to the lamellas being faster by 0.3-0.5 log units than within the lamellas. It has to be emphasized that the study is based only on few selected feldspars, including perthitic feldspar, and additional work on samples with different weathering stages is needed to test the importance of the different parameters controlling diffusive transport in the pore system.

UR - http://www.scopus.com/inward/record.url?scp=70349736127&partnerID=8YFLogxK

U2 - 10.1016/j.gca.2009.08.031

DO - 10.1016/j.gca.2009.08.031

M3 - Article

AN - SCOPUS:70349736127

VL - 73

SP - 7019

EP - 7033

JO - Geochimica et cosmochimica acta

JF - Geochimica et cosmochimica acta

SN - 0016-7037

IS - 23

ER -