Romanechite, Todorokite, and Related Compounds

For a long time the structural classification of the mineral todorokite was uncertain, until Turner and Buseck [4] could demonstrate by HRTEM investigations that the crystal structure of that mineral consists of triple chains of edge-sharing octahedra, which form [3 x 3] tunnels by further corner-sharing. These tunnels are partially filled by Mg2+, Ca2+, Na+, K+, and water (according to the chemical analysis of natural todorokites). In 1988 Post and Bish could perform a Rietveld structure determination from XRD data taken for a sample of natural todorokite [25], This diffraction study confirmed the results of Turner and Buseck. The cations 

31 tunnels). Therefore they proposed a classification scheme (see Table 2) which describes the crystal structures as a system of tunnels T(m, rt) with a common dimension m and a variable dimension rt. For example, pyrolusite, which contains only -[1 x 1] tunnels, is denoted as 7(1, 1), and a compound that contains [2 x 2] (hollan-dite-like) and [2 x 3] (romanechite-type) tunnels can be considered as an intergrowth of 7(2, 2) and 7(2, 3) types. With increasing n the structures approach the layered compounds (e.g., 7(2, 4) and 7(2, 5) structures with broad channels). Finally, structural features denoted as 7(1,°o) and 7(2, co) can be regarded as representatives of phyllomanganates. 

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