Theoretical Insights into the Hydrated (10.4) Calcite Surface: Structure, Energetics, and Bonding Relationships

Roothaan-Hartree-Fock molecular orbital methods were applied to investigate the ground-state structural, energetic properties, and bonding relationships of the hydrated (10.4) calcite surface. The adsorption of water molecules was modeled at the 6-31G(d,p) level of theory using Ca-n(CO3)(n) slab cluster models (4 <= n <= 18) with a varying number of H2O monomers (2 <= (H2O)(n) <= 6) interacting with the surface. Modeling results add fresh insights into the detailed 3D structural registry of the First and second hydration layers and the reconstructed (10.4) calcite surface, complementary to the information acquired from earlier atomistic, density functional, X-ray scattering, and grazing incidence X-ray diffraction studies. Both the modeled energies and geometries agree best with results of earlier density functional calculations, supporting the associative character of adsorbed water molecules. Two adsorption configurations are postulated: (i) H2O molecules interacting with surface Ca through ionic bonding and by hydrogen bonding to a surface O with their dipole slightly oblique above the surface (1st hydration layer), and (ii) H2O molecules that hydrogen bond to surface O and to H2O molecules in the first hydration layer with their dipole nearly parallel to the surface (2nd hydration layer). These interactions are consistent with the chemisorption and physisorption of H2O on calcite surfaces, proposed on the basis of previous thermogravimetric and Fourier-transformed infrared studies. Most significant is the distortion of the surface Ca-O octahedra caused by the relaxation (and possibly rupture) of some Ca-O bonds upon hydration, weakening the topmost atomic layer. These findings are consistent with interpretations of Xray photoelectron spectroscopy, density functional theory, and electrokinetic studies that suggest the preferential release of surface Ca atoms over surface CO3 groups upon hydration of the cleavage surface. These insights will help to elucidate mechanisms of carbonate mineral dissolution, the rearrangement of surface layers, ion replacement, charge development, and solute transport through subsurface lattice layers.