Todorokite, structure

Post, J. E. Bish, D. L. (1988) Rietveld refinement of the todorokite structure. Amer. Mineral., 73, 861-9. 

Fig. 2. a The 2x2 structure of hollandite is formed hy edge sharing MOg octahedra arranged in a sheet-like manner, b In the todorokite structure the octahedral sheets have the 3 x 3 dimension and form higger channels... 

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.

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... 

The cation occupancies of todorokite may also account for the relative stability of this mineral towards oxidation. Todorokite is destabilized by the presence of substantial Mn2+ and Mn3+ ions in the structures, because these cations are vulnerable to oxidation. Thus, Mn2+-bearing todorokites show oxidation to manganite and vemadite. However, replacement of Mn2+ or Mn3+ by Mg2+, Ni2+, Zn2+ and Cu2+, which are not susceptible to oxidation, stabilizes todorokite, particularly in manganese nodule deposits. 

Turner, S., Siegel, M. D. Buseck, P. R. (1982) Structural features of todorokite intergrowths in manganese nodules. Science, 296, 841-2. 

Figure 25 Schematic diagram of tmmel structures built from rutile chains 1x1, rutile 2x2, hollandite 2x3, romanechite 3x3, todorokite. (Reprinted with permission from S. Turner and P.R. Buseck, Science, 1981, 212, 1024. 1981 AAAS)...

MnOj X HjO with a crystallographic layer structure, the interval between the layers is 10 A). This mineral is called Buserite or Todorokite . It displays strong ion exchange properties especially for transition metals. The ion exchange decreases in following order Cu, Co, Zn, Ni, Na, Ca, Mg . ... 

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. 

The sensitivity of EXAFS to changes in the local structure of MnOa polymorphs is illustrated in Figure 7, where the RDF of bimessite (a phylloman-ganate) and todorokite (a tectomanganate) are compared. The RDF of phyl-... 

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.

Figure 8. Local structures of bimessite and todorokite. The linkage of octahedra along edges and double comers is abbreviated by E and DC, respectively. Distances are from Rietveld refinements of powder diffraction data (31, 36). They can be compared with the average distances obtained from EXAFS in Table III.

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