HR: 1340h
AN: T23B-0560    [Abstracts]
TI: Classical Nucleation Theory Explains Mineral Growth and Dissolution Within a Mechanistic and Quantitative Framework
AU: * Dove, P M
EM: dove@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24060 United States
AU: Han, N
EM: nhan@vt.edu
AF: Virginia Tech, Department of Geosciences, Blacksburg, VA 24060 United States
AU: De Yoreo, J J
EM: deyoreo1@llnl.gov
AF: Lawrence Livermore National Laboratory, Dept. Chemistry and Materials Sciences, Livermore, CA 94551 United States
AB: The dissolution and precipitation of silicate minerals exert strong controls on the spatio-temporal evolution of geothermal and faulted subsurface systems. Geochemical principles dictate that these processes must be dependent upon thermodynamic driving force (extent of fluid undersaturation or supersaturation), however this dependence is not well understood. A complete physical picture that explains widely observed variations in dissolution behavior is lacking and some data show ongoing inconsistencies that cannot be explained by the largely-empirical kinetic `laws'. Here we show that the dissolution of silica and silicate minerals can be understood across broad conditions through the same nucleation rate theory that was originally developed for crystal growth. This theory should, in principle, also apply to dissolution but, before now, has never been tested. Methods: Kinetic measurements were conducted at 200°C using quartz sand (Destin, FL) using established flow-through reactor methods for measuring H4SiO4 production rate at steady state. Undersaturated solutions were prepared with and without reagent grade NaCl or CaCl2·2H2O and additions of silicic acid. All solutions had circumneutral pH and calculated pHT,200 = 5.7. Parallel experiments exposed natural (100) surfaces of a euhedral quartz crystal to four experimental conditions that, according to theory, should give distinct dissolution mechanisms. Durations of each treatment were determined from measured rates to calculate reaction time necessary to give equal silica production at 200°C. Thus, etching times ranged from 28 days to four hours. Resulting nanoscale structures were examined under a drop of water using atomic force microscopy. Findings: By generalizing nucleation theory across the potential energy continuum of growth to include dissolution, we present a quantitative and mechanistic model that explains how quartz dissolution processes change with increasing undersaturation from simple step edge retreat, to dislocation and defect-driven pit nucleation. We further show that the origin of the so-called salt effect that was recognized almost 100 years ago arises from increases in surface energy to activate dissolution by two-dimensional nucleation of vacancy islands, to greatly increase site density. This process has not been heretofore recognized as possible for oxide or silicate minerals. Nucleation rate theory also describes the dependence of dissolution kinetics on undersaturation and supersaturation for the dominant aluminosilicates, kaolinite and K-feldspar. In doing so, it resolves discrepancies between data sets reported for kaolinite at 80° and 200°C by revealing the existence of a temperature-activated transition in the dissolution and growth processes. Nucleation theory may be the missing link to unifying mineral growth and dissolution into a mechanistic and quantitative framework across the continuum of driving force.
DE: 1011 Thermodynamics (0766, 3611, 8411)
DE: 1034 Hydrothermal systems (0450, 3017, 3616, 4832, 8135, 8424)
DE: 3625 Petrography, microstructures, and textures
SC: Tectonophysics [T]
MN: Fall Meeting 2005