Romanechite, structure

The crystal structure of romanechite (or psilomelane) is closely related to that of a-MnO,. Where as the or-modification of manganese dioxide consists of cornersharing double chains of MnO octahedra connected trough common edges, the romanechite structure is build up by crosslinking of chains of double and triple octahedra, as shown in Fig. 7(b). The resulting [2 X 3] tunnels, extending in the h direction of the monoclinic cell, are partially... 

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.

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)...

The first ordered microporous transition metal oxides were microporous manganese oxides, known as octahedral molecular sieves (OMS). The manganese oxide OMS are classified into three families the pyrolusite-ramsdellite family with a (1 x n) channel structure the hollandite-romanechite family with a (2 x n) channel structure ... 

Figure 3.1 Structure and pore of manganese oxide octahedral molecular sieves (OMS) (a) todorokite (3x3) channel (b) romanechite (2x3) channel and (c) hollandite (2 x 2) channel...

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...

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