Structure of the hydrated (1014) surface of rhodochrosite (MnCO ₃)

The three-dimensional structure of the hydrated (1014) surface of MnCO ₃ at 90% relative humidity and 295 K is determined from measurements of X-ray scattering along ten crystal-truncation rods (CTRs). The scattering data provide both vertical and lateral information about the interfacial structure. The model that best fits the scattering data is a surface having a first layer of manganese carbonate and an overlayer of oxygen (as water). Within the measurement uncertainty, the overlayer of oxygen (O_w) and the first-layer of manganese (Mn₁) have equal occupancies of 0.84. The Mn₁-O_w distance between these layers is 2.59 ± 0.04 Å. The overlayer O atoms are displaced laterally by 0.157 Å in the x- and 0.626 Å in the y-direction relative to the first-layer Mn atoms. The first-layer carbonate groups tilt by -4.2 ± 2.1° in phi (toward the surface plane) and -2.6 ± 1.2° in chi (an axis perpendicular to phi). The second-layer carbonate groups do not tilt, at least within measurement uncertainty. The spacing between Mn atom layers remains unchanged within measurement error whereas the spacing between layers of C atoms in carbonate contracts for the top three layers. Knowledge of the detailed atomic structure of the hydrated (1014) surface of MnCO₃ provides a structural baseline for the interpretation of chemical reactivity.