A short outline of the tunnel oxides

“Tunnel oxides” is a rather generic term which historically refers to a number of minerals which, from a chemical point of view, are (mainly) manganese oxides. In nature manganese occurs in three different oxidation states - Mn2+, Mn3+ and Mn4+ - with the latter being the dominant form in tunnel oxides. Tetravalent manganese typically has octahedral coordination, and using only [Mn4+O6] building modules linked together via corner- and edge-sharing, it is feasible to construct many framework structures. A smaller set of titanate minerals display the same feature, as might be expected due to the similar crystal-chemical behaviour of manganese and titanium. Starting from the basic formula of manganese dioxide, Mn4+O2, incorporation of mono- and divalent cations (primarily alkali and alkali earths) within the tunnels of the structures, can be accommodated by partial reduction of manganese to Mn3+, or through substitution of tetravalent Mn (and Ti) by other trivalent cations with typically display octahedral coordination. The substitutions can be summarized in the generic formulae: A+x (M3+x M4+1-x) O2 or A2+x (M3+2x M4+1-2x) O2 where A = Na+, K+, Rb+, Mg2+, Ba2+, Pb2+; M3+ = Mn, Fe, V, Cr, Al; and M4+ = Mn or Ti. Water molecules can also enter the tunnels. In all these compounds Mn4+O6 or Ti4+O6 octahedra are arranged in edge-sharing columns, which in turn link together, again by edge-sharing, to construct ribbons, with widths ranging from 1 to . Cross-linking by corner-sharing of columns or ribbons in near perpendicular directions, gives rise to a number of different tunnel structures that may be square or rectangular, depending on the dimensions of the ribbons. The ideal topological symmetry of the frameworks is either tetragonal (for structures with the same dimensionality in the two directions, e.g., 1 x 1, 2 x 2, …) or orthorhombic. In most cases the real symmetry is lower, where the deviation arises from minor distortions of the framework and/or ordered distributions of cations. The unit cell axis along the direction in which the octahedral columns or ribbons run is 2.8-2.9 Å, corresponding to a single octahedron repeat, whereas the two unit cell parameters in the orthogonal plane depend on the width of the ribbons. Among tunnel oxides, several mineral and synthetic compounds are known which display a wide range of structure topologies. For their use in several important applications (e.g., heavy metals sorption in contaminated waters or soils, or immobilisation of radioactive cesium), emphasis has been given to the dimensions of the tunnels, for which these compounds can act as selective molecular sieves. A selection, sorted by dimensions of the tunnels, includes. 1 x 1 tunnels: pyrolusite, rutile, cassiterite, plattnerite, argutite, manganite, stishovite. 1 x 2 tunnels: ramsdellite, groutite, goethite, diaspore. 1 x 3 tunnels: nsutite (with 1 x 3 tunnels occurring at the unit cell scale). 2 x 2 tunnels: hollandite, cryptomelane, coronadite, priderite, redledgeite, strontiomelane, henrymeyerite, mannardite, ankangite, manjiroite, and a lot of synthetic compounds, including among others the high pressure form of feldspar (KAlSi3O8) with octahedrally coordinated Si4+. 2 x 3 tunnels: romanechite. 2 x 4 tunnels: synthetic Rb0.25(Mn4+,Mn3+)O2, synthetic Na0.33(Mn4+,Mn3+)O2•xH2O. 2 x 5 tunnels: synthetic Rb0.27(Mn4+,Mn3+)O2. 3 x 3 tunnels: todorokite. 3 x 4 tunnels: woodruffite. Tunnel oxides show a high degree of structural flexibility: the substitution of VIM4+ cations with trivalent cations is often coupled with octahedral distortion: such a distortion is more pronounced when the substituting cation is Mn3+, which is known to display Jahn-Teller effect, and is also related to the local bond valence balance. From the crystallographic point of view, these structures can be described as the members of a polysomatic family built up by two sets of nearly perpendicular ribbons. In general, each member of the family will be described by a symbol of the kind “M x N”, with both M and N potentially ranging between 1 and , being M  N. This is purely formal, since of course the structures, e.g., 2 x 3 and 3 x 2 are topologically identical between each other, and should not be considered twice (cf. Fig. 1). The capability of tunnel oxides to incorporate larger cations within the tunnels is limited to those polysomes with M  2. Polysomes with N =  are those structures in which ribbons degenerate into infinite octahedral layers and therefore do not belong, strictly speaking, to the family of tunnel oxides, although some of the typical features of microporous material (e.g., cation exchange capabilities) have been described for these layered polysomes as well. The 1 x  (lithiophorite), 2 x  (birnessite), and 3 x  (‘buserite’) polysomes are known to occur so far. The correspondence between polysomatic symbol and the tunnel structures is fairly good, with one exception only. Ramsdellite has been considered as the 1 x 2 tunnel structure and included together with the other tunnel oxides, but in strict polysomatic terms this is incorrect. In fact the common feature of all tunnel oxides berlonging to the polysomatic family is the occurrence of two sets of ribbons formed by edge-sharing octahedral chains with dimensions ranging from 1 to  that develop in orthogonal directions. This is not the case for ramsdellite, where the ribbons (with width corresponding to two octahedral chains) are parallel (cf. Fig. 2). Confusingly, in ramsdellite the notation “1 x 2” refers to the opening of the tunnels, but not to the width of the ribbons. Consequently 1 x 1 tunnels found in all other members of the polysomatic family – but those with N =  – do not occur in ramsdellite. The notation “1 x 2”, when referred to the polysomatic description and not to the tunnels, denotes a different structure, which has not been yet reported for any mineral or synthetic compound, but only recognized as local domains within nsutite.