Surface speciation of calcite observed in situ by high-resolution X-ray reflectivity

High-resolution, in situ X-ray reflectivity measurements were made of the c alcite (104)-water interface in calcite-saturated aqueous solutions at pH v alues ranging from 6.8 to 12.1 and low P-CO2. The X-ray reflectivity data, taken over a momentum transfer range of 6 Angstrom(-1), indicate that the c alcite surface does not vary significantly over this range of experimental conditions. From an analysis of the data at pH 8.3, the best-fit reflectivi ty model requires the presence of 1.0 +/- 0.4 monolayer of a hydroxyl speci es (OH or OH2) at 2.50 +/- 0.12 Angstrom above the surface Ca ions and invo lves 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 invoki ng changes other than protonation reactions in the surface speciation of te rrace areas. This is consistent with scanning force microscopy studies of c alcite 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 us ed for direct, in situ measurement of mineral surface structure, to provide strong constraints on chemical speciation and reactivity at the mineral-fl uid interface. Copyright (C) 2000 Elsevier Science Ltd.