A Short Outline of the Tunnel Oxides

Abstract

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 modules linked together via corner- and edge-sharing it is feasible to construct many framework structures. Besides, a set of titanate minerals display the same feature, as might be expected due to the similar crystal-chemical behaviour of Mn4+ and Ti4+ cations. Therefore, this note is devoted to describing the structural principles and arrangements of minerals—and a number of synthetic compounds as well—in which the dominant cations are Mn4+ and Ti4+. 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. Until recently, there was a considerable uncertainty and lively debate (e.g., Burns et al. 1983; Burns et al. 1985; Giovanoli 1985) concerning the valence of the reduced species and whether Mn2+ or Mn3+ was present. It is now commonly accepted on the basis of several high-quality structural studies that Mn3+ replaces Mn4+. In titaniferous phases, charge balance accompanying the inclusion of tunnel cations is not adjusted by reduction to Ti3+, but through incorporation of Fe3+, V3+ or …