Within the large and comprehensive group of oxide minerals (class 4 in the Strunz classification; Strunz and Nickel 2001), the most relevant subgroup in the frame of microporous materials is that of the so-called “tunnel oxides.” This rather generic term 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...

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