Fig. 2. (a) Projection of the crystal structure of fairchildite along [100]. Calcium dominated sites are coordinated by 12 oxygen atoms (light blue polyhedron). Potassium dominated positions are shown in yellow. Oxygen and carbon atoms are presented as red and black spheres, respectively. A part of the carbonate groups exhibits a pronounced disorder. Bicolored spheres indicate the corresponding partially occupied oxygen positions. The sizes of the bicolored segments refer to the percentages from the site-occupancy refinements as determined by Pertlik.²² (b) Distribution of the six possible disordered positions of carbonate groups around the carbon atom in (0, 0, 0). The partially occupied oxygen atoms define the 12 corners of a cube-octahedron. (c) Top view of a single layer (z ≈ ¼) containing the CaO₁₂-polyhedra and the non-disordered carbonate groups. (d) Top view of a single layer (z ≈ 0) containing the K-enriched sites and the strongly disordered carbonate moieties.