Structure
The space group and cell of Zussmanite are R*3, ahex 11.66 and chex28.69 Angstroms (Agrell et al., 1965). The structure of Zussmanite contains continuous sheets of rhombohedrally stacked layers of Fe-O octahedral parallel to (0001) (Lopes et al.,1967) and to either side of these are attached (Si,Al)-O tetrahedral in a way to produce a rhombohedral unit cell(Lopes et al.,1969). These layers are linked to one another by Potassium (K) atoms and also by three-member rings of tetrahedra that share oxygens with the six-members; displayed in figure 2 (Lopes et al.,1967). Zussmanite’s structure has a close affinity to that of the trioctahedral micas which have a layer of Fe-O octahedral sandwiched between inward pointing tetrahedral. It differs from the micas because its Si-O ratio is 9:21 which results in a sharing coefficient 1.83, as compared with 2.5 and 1.75 for micas, and 1.2 and 2.0 for framework silicates (Lopes et al.,1969). The Fe-(O,OH) mean distance in the first octahedron is 2.1 Angstroms, the second octahedron is 2.14 Angstroms, and in the third Octahedron is 2.17 Angstroms. The mean distance in the Si-O bonds in Zussmanite are 1.61 Angstroms for the first tetrahedron, 1.61 Angstroms for the second tetrahedron, and 1.65 Angstroms for the third tetrahedron; data given in table I (Lopes et al.,1969). The six-member rings are not directly linked to one another which allows for adjustment by tilting outwards of all tetrahedral, as opposed to many micas where rotations and tilts are used to achieve the larger dimensions of the octahedral layer. The flattening of the octahedral layer perpendicular to the layer is pronounced in Zussmanite due to shared and unshared edges. This flattening could be due to the tendency for shared oxygens to come closer and shields iron (Fe) atoms from other neighboring iron (Fe) atoms.
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