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Investigation of the {104} surface of calcite under dry and humid atmospheric conditions with grazing incidence X-ray diffraction (GIXRD)

¹ Fakultät für Geowissenschaften, Institut für Mineralogie/Kristallographie, Ruhr-Universität Bochum, Universitätsstr. 150, D-44780 Bochum, Germany
² Institut de Ciencia de Materials de Barcelona (CSIC), Campus de la UAB, E-08193 Bellaterra, Catalunya, Spain

^* Corresponding author, e-mail: uta.magdans{at}rub.de

Using grazing incidence X-ray diffraction (GIXRD) the three-dimensional surface and interface structure of the {104} calcite surface was investigated under dry and humid atmospheric conditions. The measurement of specular and non-specular crystal truncation rods (CTR) provided information about the periodic order of the vertical and lateral surface structure on atomic scale. The calcite surface is nearly ideally terminated with relaxation of the atoms in the topmost layers down to about 12 Å depth. In dry environment the calcite surface structure is dominantly determined by the large shift, rotation and tilt of the surface carbonate groups. In humid atmosphere a laterally ordered monolayer of water is formed on the surface. The truncated coordination site of the surface Ca²⁺ ions are filled by a water molecule in a distance of (2.1 ± 0.2) Å. The relaxation of the structure is significantly changed, which results in a strong distortion of the Ca-O coordination octahedron. The refinement of water as complete rigid H₂O species positioned flat on the surface with one rotation parameter resulted in the formation of hydrogen bonds to the adjacent surface carbonate groups. No evidence for a surface reconstruction was found.

Key-words: calcite surface, surface structure, surface X-ray diffraction, GIXRD, crystal truncation rod, CTR, solid-water interface.