SURFACE SPECIATION OF CALCITE OBSERVED IN SITU BY HIGH-RESOLUTION X-RAY REFLECTIVITY

Abstract

High-resolution, in situ X-ray reflectivity measurements were made of the calcite (104)-water interface in calcite-saturated aqueous solutions at pH values ranging from 6.8 to 12.1 and low PCO2. The X-ray reflectivity data, taken over a momentum transfer range of 6 #9-1, indicate that the calcite surface does not vary significantly over this range of experimental conditions. From an analysis of the data at pH 8.3, the best-fit reflectivity model requires the presence of 1.0 +/- 0.4 monolayer of a hydroxyl species (OH or OH2) at 2.50 +/- 0.12 #9 above the surface Ca ions and involves rotations of the surface carbonate groups toward the (104) plane. The X-ray reflectivity data for pH 6.8 and 12.1 can be explained without invoking changes other than protonation reactions in the surface speciation of terrace areas. This is consistent with scanning force microscopy studies of calcite growth and dissolution near equilibrium, which show that attachment and detachment of Ca and CO3 ions occurs primarily at step-edge kink sites. These results demonstrate how high-resolution X-ray reflectivity can be used for direct, in situ measurement of mineral surface structure, to provide strong constraints on chemical speciation and reactivity at the mineral-fluid interface.