HR: 12:05h
AN: B11E-08    [PDF]
TI: Connecting the Molecular- With the Macro-scale: Evidence for the Stepwave Dissolution Model
AU: * Fewless, T A
EM: tfewless@rice.edu
AF: Rice University, 6100 Main Street Dept. of Earth Science - MS126, Houston, TX 77005 United States
AU: Davis, K
EM: kjdavis@rice.edu
AF: Rice University, 6100 Main Street Dept. of Earth Science - MS126, Houston, TX 77005 United States
AU: Luttge, A
EM: aluttge@rice.edu
AF: Rice University, 6100 Main Street Dept. of Earth Science - MS126, Houston, TX 77005 United States
AB: A general crystal dissolution model by Lasaga and Luttge (2001, 2003) has shown promise in furthering our understanding of nanoscale dissolution processes at crystal surfaces. The model predicts trains of steps ("stepwaves") generated at the outskirts of etch pits radiating across the crystal surface. Each step will lower the crystal surface causing surface normal retreat and thus control the overall dissolution rate. This model is fundamentally based on a modified Gibbs-Thomson equation. It utilizes Monte Carlo techniques to explore the behavior of crystal dissolution (and precipitation) including alumino-silicates, carbonates, and sulfates. Here, we test some key model predictions using the sulfate mineral barite (BaSO$_{4}$). Atomic force microscopy (AFM) and vertical scanning interferometry (VSI) were used to look at the (001) surfaces of freshly cleaved samples. Experiments were conducted in a flow-through cell that allowed monitoring the evolution of the sample surface with AFM. Results show that etch pits appear within minutes and indeed generate steps at their outskirts that radiate away from the growing pit. Complementary investigations were conducted with VSI. This technique has a much larger field of view than the AFM, typically up to mm$^{2}$, but provides comparably high vertical resolution of up to 0.7 angstroms. VSI measurements show that the entire barite surface retreats during the dissolution process as predicted by the theoretical model. Within the last ten years biologically produced chelating agents have been discovered naturally occurring in the environment. The computer model is used to determine the effects of chelating agents and our experiments using VSI also show a marked, as much as two orders of magnitude, increase in dissolution rate of barite due to the presence of chelating agents. Our presentation will discuss a quantitative attempt to directly link molecular-scale processes with observations at the nano-, micro- and macro-scale to better understand dissolution kinetics of crystalline matter.
DE: 1045 Low-temperature geochemistry
DE: 1055 Organic geochemistry
SC: Biogeosciences [B]
MN: 2003 Fall Meeting