Characterization of Mica Dissolution in the Presence of Bulk and Confined Solutions: Implications for Natural Weathering Processes

Abstract

A significant portion of silicate weathering reactions taking place in soils and rocks occurs in the presence of thin films of water, for example within microcracks and micropores, and along grain boundaries. Here, the properties of aqueous solutions (e.g. atomic structure, solvent properties, viscosity, dielectric constant) can be quite different than they are in bulk solution, and the dominant transport mechanism shifts from advection to diffusion (Hochella and Banfield, 1995). It is unclear how these differences may influence dissolution and precipitation reactions. In this study we examine the characteristics and rates of mica dissolution at room temperature both in the presence of bulk solutions and thin, confined aqueous films. Mica minerals are ideally suited for this study because they can be cleaved to yield atomically smooth surfaces to produce films of uniform thickness. both the open and closed experiments, the samples were removed, thoroughly rinsed, and blown dry with N2. The same regions of the sample were then reimaged using SFM. Several scans of each region were taken to ensure that observed changes were not caused by tip induced artifacts. The thickness of fluid films formed between mica surfaces can accurately be measured over a range of a few nanometers to tens of micrometers with an optical interferometry technique known as FECO (fringes of equal chromatic order). Measurements using this technique have been made in our lab to measure film thicknesses between 800 nm and 25 ~tm, with an accuracy of ___ 80 nm. Our implementation of the technique is currently being refined so that it will achieve sub-nanometer resolution down to film thicknesses in the nanometer range as previously demonstrated by Israelachvili (1973).