Todorokite, tunnel structures

Figure 7. Crystal structures of (a) hollandite, (b) romanechite (psilomelane), and (c) todorokite. The structures arc shown as three-dimensional arrangements of the MnO() octahedra (the tunnel-tilling cations and water molecules, respectively, are not shown in these plots) and as projections along the short axis. Small, medium, and large circles represenl the manganese atoms, oxygen atoms, and the foreign cations or water molecules, respectively. Open circles, height z. = 0 fdled circles, height z = Vi.

Complementary structural techniques have recently helped to clarify the structure of bimessite and todorokite and to limit possible structural models for vemadite. High-resolution transmission electron microscopy (HRTEM) revealed the tunnel structure of todorokite in marine manganese deposits (30). Post and Bish (31) used these results to carry out a Rietveld refinement on powder diffraction data of todorokite, which confirmed the basic (3 X 3) tunnel structure of this mineral. 

Figure 7. Radial distribution functions (RDF), not corrected for phase shift from EXAFS spectra, of sediment-trap material from Lake Sempach and from reference oxides. Pyrochroite, Mn(OH)-, and bimessite [a Mn(IV) oxide] have the same layered structure with edge-sharing Mn octahedra. Todorokite is a Mn(IV) oxide with a 3 X 3 tunnel structure. A shift to longer distances occurs in going from the Mn(IV) oxide bimessite to the Mn(II) hydroxide pyrochroite. Contributions from double-comer Mn-Mn linkages are clearly seen in sediment-trap material and in todorokite and vemadite but not in the layered minerals bimessite and pyrochroite.

Generally, there are two major structural forms for these minerals chain or tunnel structures, and layer structures. All of these forms are comprised of MnOs octahedras. Water molecules and/or other cations (8) are ofien present at various sites in the structures. Mn oxides having a chain or tunnel structure include pyrolusite, ramsdellite, hollandite, romanechite, and todorokite. Typical structures for the chain or tunnel type Mn oxide mineral are presented in Figure 1. Lithiophorite, chalcophanite, and bimessite are examples of Mn oxide minerals havii a layer structure. Typical structural maps are shown in Figure 2. 

Figure 3.7 Crystal structures of (a) hollandite, (b) romanechite (psilomelane), and (c) todorokite. The structures are shown as three-dimensional arrangements of the MnOe octahedra (the tunnel-filling cations and water molecules, respectively, are not...

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... 

In the conventional precipitation and sol-gel routes, the resultant bimessite or buserite materials are usually Na type or K type, in which the cations are located in the interlayers, and MnOe octahedral units form the Cdl2-type structure in the layers. The stmcture and propaties of bimessite and buserite can be modified to load other cations, including alkali metals, alkaline earths, and transition metals, via two pathways. One pathway is interlayer substitution via ion exchange method at room temperature [14,22]. In this process, Na- or K-bimessite are stirred in a solution of dopants. Divalent metal cations, such as Mg ", Ni ", Co ", have been successfully ion exchanged with Na-birnessite mataials and used to prepare todorokite-type materials with tunnel stractures [22],... 

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