Short-range order in tunnel oxides: hollandite supergroup minerals

Hollandites are quite common oxides widespread in several geological settings; they are among the most common Mn minerals in soils, where they play a key role in a variety of cation-exchange reactions, and in Mn dendrites observed on rock surfaces and fractures. From a structural point of view, hollandites belong to the tunnel oxides and are characterized by channels partially occupied by large di- or mono-valent cations (Ba, Pb, K, Na) delimited by 2×2 octahedral walls. A lowering of symmetry is often observed, possibly related to framework distortions or ordering of cations and vacancies within the tunnels. Our aim is to investigate the local environment of tunnel cations in hollandite and assess the relationship between the disorder in their position and the nature and oxidation state of octahedral cations.