An EFTEM/HRTEM high-resolution study of the near surface of labradorite feldspar altered at acid pH: evidence for interfacial dissolution-reprecipitation |
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| Authors: | R Hellmann J-M Penisson R L Hervig J-H Thomassin M-F Abrioux |
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| Institution: | (1) LGIT, CNRS UMR C5559-OSUG-Université J. Fourier, 38041 Grenoble Cedex 9, France e-mail: hellmann@obs.ujf-grenoble.fr, FR;(2) DRFMC, CEA-Grenoble, 38054 Grenoble Cedex 9, France, FR;(3) Center for Solid State Science, Arizona State University, Tempe, Arizona 85287–1704, USA, US;(4) UMR CNRS 6532, LMGE – ESIP- Université de Poitiers, 86022 Poitiers Cedex, France, FR |
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| Abstract: | Using an approach combining high-resolution and energy-filtered transmission electron microscopy (HRTEM and EFTEM), we have studied with Å to nm-spatial resolution the interfacial region that delimits the near-surface altered zone and non-altered labradorite feldspar after dissolution under acid pH conditions. The interface is characterized by extremely sharp and spatially coincident changes in structure and chemistry. The 500-nm-thick altered zone is depleted in interstitial cations (Ca, Na, K) and Al, a framework element, whereas it is enriched in H, O, and Si. Modeling H+-alkali interdiffusion within a 500-nm-thick altered zone shows that volume interdiffusion cannot reproduce the sharp chemical interfaces measured by EFTEM. Based on these new data, we propose that the near-surface altered zone is a result of interfacial dissolution-reprecipitation, and not of preferential leaching of cations and interdiffusion with H+. This implies an intrinsic dissolution process that is stoichiometric, where the breaking of bonds and release of interstitial cations and framework elements (Al, Si, and O) to solution occur contemporaneously at equal relative rates from the original fluid–mineral interface. |
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| Keywords: | Feldspar dissolution EFTEM and HRTEM Interfacial dissolution – reprecipitation mechanism Leached layers and preferential cation release |
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