Rietveld refinement of the todorokite structure

Rietveld refinements, using powder X-ray data, of the structures of todorokite samples from South Africa and Cuba have confirmed the basic [3 × 3] tunnel structure and, for the first time, have provided information about the positions of the tunnel cations and water molecules. Fourier-difference maps and subsequent refinements show a major tunnel site at about (0.36, 0, 0.35) and smaller more diffuse areas of electron density near (0.70, ½, 0.38) and (½, ½, ½). Structure-energy calculations suggest that the first two positions are occupied by water molecules and the third by cations such as Na and Ca. The difference maps indicate considerable positional disorder on the tunnel sites, probably caused by various tunnel contents and configurations of lower-valence octahedral cations in different unit cells. The Mn,0 octahedra at the edges of the triple chains have larger mean Mn–0 distances and probably accommodate the larger cations found in todorokite, e.g., Mg²⁺, Mn³⁺, Cu²⁺, Ni²⁺, etc.