An Interferometric Study of the Dissolution Kinetics of Anorthite: the Role of Reactive Surface Area

An optical interferometry system has been used to study the dynamics of the dissolution of anorthite (010) cleavage surfaces. With this technique, it is possible to measure directly the surface retreat of alumino-silicates as a function of time and thereby the dissolution rate using a new application of interferometry. Vertical scanning white light interferometry (VSWLI) provides about 0.5 to 1.5 m lateral resolution and near atomic scale vertical resolution on the order of 2 nm for surface features up to 100 microns high. Dissolution rates are determined directly from the surface-normal retreat of the anorthite surface over time. The dissolution experiments are carried out in a flow-through cell system with a near endmember anorthite (An98) from Miyake-jima, Tokyo, Japan. Perchloric acid solutions (pH 3) were used at a constant temperature of 25°C. After having measured the topography of the original pristine anorthite surface, measurements of the surface normal retreat were taken after 48, 84, 120, and 168 hrs of run duration at 15 different regions on the surface. An internal-reference technique allows absolute measurements of the changes in surface height for the very first time. Three-dimensional height maps produced from the interferometric raw data are used to derive the overall dissolution mechanism of the (010) anorthite cleavage surface. In addition, the evolution of individual etch pit depths is measured versus the neighboring ‘‘flatlands’’. Measured rates of surface-normal retreat occurring in many etch pits roughly fall in two categories with mean dissolution rates of 2.7 10 12 and 6.6 10 12 [moles/cm2/sec]. Subsequent surface profiles (height maps) of different regions of the anorthite surface are compared to produce our so-called ‘‘difference profiles’’. These profiles are the tool to study quantitatively the differences of the reaction rates, that is, the heterogeneity of the anorthite dissolution process. The spatial extent of the heterogeneity are explored by carrying out running averages of the dissolution rates using a sliding square area of varying length. During the experiment, a part of the pristine surface was protected from the solution by using a high temperature silicon rubber. This protected area serves later as an internal reference after the silicon rubber is removed at the end of the experiment. The comparison of the pristine reference surface with the dissolved surface area allows us to measure the absolute surface normal retreat with high precision. From these measurements, an average bulk rate for dissolution of the (010) anorthite surface is calculated to be 5.7 10 13 [moles/cm2/sec]. Finally, our directly determined bulk rate for the (010) face is compared with the bulk rates calculated from the rate law obtained from powder experiments and using the BET or total surface area.