Todorokite

There are approximately 250 known manganese minerals. The primary ores which typically have a Mn content >35%, usually occur as oxides or hydrated oxides, or to a lesser extent as siUcates or carbonates. Table 5 Hsts the manganese-containing minerals of economic significance (10). Battery-grade manganese dioxide ores are composed predominately of nsutite, cryptomelane, and todorokite. 

A thermally stable, pure todorokite has been synthesized by autoclaving a layered stmctured manganese oxide, initially generated from the reaction of MnO and Mn " under alkaline conditions. The synthetic manganese oxide molecular sieve (11) was shown to have a tunnel size, ie, diameter of 690 pm. This material was thermally stable to 500°C just as natural todorokite is (68). 

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... 

Kim, H.-S., Pasten, P.A., Gaillard, J.-F. and Slair, P.C. (2003) Nanocrystalline todorokite-like manganese oxide produced by bacterial catalysis. Journal of the American Chemical Society, 125, 14284-14285. 

From a mineralogical perspective, Fe-Mn nodules are composed of layers of manganese oxides (vernadite, todorokite, and birnessite), hydrous Fe oxide FeOOH H2O), aluminosilicates, quartz, and feldspar. Like the rest of the Fe-Mn oxides, the nodules... 

Oxic Diagenesis Metals remobilized from sediments lying in the oxic zone. Remobilization likely occurs in anoxic microzones adjacent to nodules. Bioturbation is an important metal transport agent. Some nodules now found in oxic sediments were likely formed during times when the redox boundary was closer to the seafloor. 10-50 Todorokite (high Cu and Ni content) 32% 5-10 15-20... 

Suboxic Diagenesis Metals remobilized from reducing sediments. Upward diffusive transport through pore waters supplies metals to nodule bottoms. Accretion is episodic, occurring only when the depth of the redox boundary rises close to the sediment-water interface. 100-200 Todorokite/Birnessite (low Cu and Ni content) 48% 20-70 60-200... 

Many mineral species are known to be selectively crystallized by the presence of bacteria. Carbonate minerals, such as calcite, aragonite, hydroxycalcite, and siderite oxide minerals, such as magnetite and todorokite oxalate minerals, such as whewellite and weddellite sulfide minerals, such as pyrite, sphalerite, wurtzite, greigite, and mackinawite and other minerals, such as jarosite, iron-jarosite, and g3q>sum, are known to precipitate in the presence of bacteria. Therefore, investigations have been developed to analyze the formation of banded iron ore by the action of bacteria, and to analyze the ancient environmental conditions of the Earth through the study of fossilized bacteria. 

Siegel, M.C. and S. Turner Crystalline Todorokite Associated with Biogenic Debris in Manganese Nodules Science. 219, 172-174 (1983). 

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. 

Most of the Mn(IV) oxide minerals listed in table 8.3 occur in weathered continental rocks, and often constitute important manganese ore deposits. However, several of the minerals, notably todorokite, bimessite, vemadite and, perhaps, buserite and asbolane, are major constituents of seafloor hydrothermal crusts near spreading centres and in manganese nodule deposits. 

Therefore, four types of atomic substitution may contribute to the crystal chemistry of todorokite- and vemadite-bearing manganese nodules. 

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