Coupled alkali-feldspar dissolution and secondary mineral precipitation in batch systems: 1. New experiments at 200 °C and 300 bars |
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| Authors: | Qi Fu Peng Lu Hiromi Konishi Robert Dilmore Huifang Xu WE Seyfried Chen Zhu |
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| Institution: | 1. ISTerre, Université Grenoble Alpes, France;2. CNRS, Université Clermont Auvergne, IRD, OPGC, Laboratoire Magmas et Volcans, F-63000 Clermont-Ferrand, France;3. Univ Lyon, UJM-Saint-Etienne, UCA, CNRS, IRD, LMV UMR 6524, F-42023 Saint-Etienne, France;4. Helmholtz Centre Potsdam, GFZ German Research Centre for Geosciences, 14473 Potsdam, Germany;5. Institut für Mineralogie, University of Münster, 48149 Münster, Germany;6. Lehrstuhl für Bodenkunde, TU München, Freising-Weihenstephan 85354, Germany;7. GeoRessources, Université de Lorraine-CNRS-CREGU, BP 70239, F-54506 Vand?uvre-lès-Nancy Cedex, France;1. Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario, K1A 0E8, Canada;2. Department of Earth Sciences, University of Oxford, South Parks Road, Oxford, OX1 3AN, United Kingdom;3. Institute of Geosciences, Johannes-Gutenberg University of Mainz, Mainz, 55128, Germany;1. Department of Geological Sciences, California State University of Fullerton, 800 North State College Blvd., Fullerton, CA 92831, United States;2. Department of Earth Sciences, University of Southern California, 3651 Trousdale Pkwy., Los Angeles, CA 90089, United States |
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| Abstract: | Batch reactor experiments were conducted to assess perthitic alkali-feldspar dissolution and secondary mineral formation in an initially acidic fluid (pH = 3.1) at 200 °C and 300 bars. Temporal evolution of fluid chemistry was monitored by major element analysis of in situ fluid samples. Solid reaction products were retrieved from two identical experiments terminated after 5 and 78 days. Scanning electron microscopy revealed dissolution features and significant secondary mineral coverage on feldspar surfaces. Boehmite and kaolinite were identified as secondary minerals by X-ray diffraction and transmission electron microscopy. X-ray photoelectron spectroscopy analysis of alkali-feldspar surfaces before and after reaction showed a trend of increasing Al/Si ratios and decreasing K/Al ratios with reaction progress, consistent with the formation of boehmite and kaolinite.Saturation indices of feldspars and secondary minerals suggest that albite dissolution occurred throughout the experiments, while K-feldspar exceeded saturation after 216 h of reaction. Reactions proceeded slowly and full equilibrium was not achieved, the relatively high temperature of the experiments notwithstanding. Thus, time series observations indicate continuous supersaturation with respect to boehmite and kaolinite, although the extent of this decreased with reaction progress as the driving force for albite dissolution decreased. The first experimental evidence of metastable co-existence of boehmite, kaolinite and alkali feldspar in the feldspar hydrolysis system is consistent with theoretical models of mineral dissolution/precipitation kinetics where the ratio of the secondary mineral precipitation rate constant to the rate constant of feldspar dissolution is well below unity. This has important implications for modeling the time-dependent evolution of feldspar dissolution and secondary mineral formation in natural systems. |
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