Calcite (1Â 0Â 4)-water interface structure, revisited

The structure of the calcite (1 0 4)-water interface is reassessed using a new set of high-precision specular X-ray reflectivity measurements. In situ measurements of the specular reflectivity signal to a vertical structural resolution of 0.45 Å are used to define the interfacial structure, including vertical displacement patterns of the Ca and CO3 groups as well as the nature of interfacial water. These new data show two organized interfacial water layers, consistent with previous results, and distortion of the interfacial calcite structure to a depth of four to six unit cells, at least two deeper than previously reported. These results are in reasonable agreement with recent computational studies, at least in terms of the locations of the first and second water layers. The difference between the interfacial structure derived from previous X-ray reflectivity results and that presented here emphasizes the need for high-precision measurements to provide a robust understanding of the interfacial structures of reactive minerals in aqueous environments.