DISSOLUTION RATES OF CALCITE (10(1)OVER-BAR4) OBTAINED BY SCANNING FORCE MICROSCOPY - MICROTOPOGRAPHY-BASED DISSOLUTION KINETICS ON SURFACES WITH ANISOTROPIC STEP VELOCITIES

This paper reports on our use of scanning force microscopy (SFM) to st udy calcite dissolution rates. Until now, calculation of rates has bee n limited to surfaces on which steps retreat at an isotropic velocity. More commonly, on surfaces where steps in different directions retrea t at different velocities, the rate depends on the velocities and the densities of the differently oriented steps. Here, we show that calcul ating rates from SFM image sequences is possible for anisotropic surfa ces if the ratio of step lengths in different directions is constant i n the analyzed images (e.g., by considering steps at nonintersecting p its exclusively). In contrast to nonintersecting pits, high velocity s teps are formed at intersecting pits on the calcite (10 (1) over bar 4 ) surface. At these steps, material can be removed without the slow nu cleation of kink sites. The morphology of these steps is not straight and they become easily pinned by particles or impurities. Therefore, m easuring the velocity of step retreat directly in the images fails, bu t calculating the dissolution rate of surface regions with high veloci ty steps is still possible. Dissolution rate is roughly similar in bot h the deep etch pits and the areas with high velocity steps at interse cting pits. Consequently, we suggest that the formation of high veloci ty steps contributes considerably to the weak enhancement of the rate with increasing dislocation density because additional etch pits withi n the intersectional regions do not significantly increase the rate. T he calculated dissolution rate of similar to 1.5 x 10(-6) mol m(-2) s( -1) on the calcite (10 (1) over bar 4) surface in water at 24 degrees C and pCO(2) = 10(-3.5) atm corresponds well to the rates obtained fro m batch experiments. Thus, SFM can be regarded as an instrument capabl e of acquiring rates even on surfaces with anisotropic step velocities . Copyright (C) 1998 Elsevier Science Ltd.