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.
Bibliographical noteFunding Information:
Material in this paper is based upon work supported by the U.S. Department of Energy under Award No. DE-FG26-04NT42125 to CZ and WES and partially by the National Science Foundation under Award No.'s EAR0423971 and EAR0509775 to CZ. Any opinions, findings, and conclusions or recommendations expressed in this material, however, are those of the authors and do not necessarily reflect the views of the United States Government or any agency thereof. We thank Rick Haasch and John Baltrus for assistances with XPS analyses that were carried out in the Center for Microanalysis of Materials, University of Illinois and the National Energy Technology Laboratory. We thank Rick Knurr at University of Minnesota for chemical analyses of fluid samples, and Kyle Jones at US EPA for BET surface analysis. We also thank Arndt Schimmelmann at Indiana University for helping with techniques in sample preparation protocols and Sheila Hedges at NETL for reading an earlier version of the manuscript. The paper greatly benefited from comments and suggestions made by John Kaszuba as well as comments from an anonymous reviewer and editor Jeremy Fein.
- Mass transfer
- Secondary minerals