Birnessite (nominally MnO2.nH2O) is a hydrous manganese dioxide mineral with a chemical formula of Na0.7Ca0.3Mn7O14·2.8H2O. It is the main manganese mineral species at the Earth's surface, and commonly occurs as fine-grained, poorly crystallized aggregates in soils, sediments, grain and rock coatings (e.g., desert varnish), and marine ferromanganese nodules and crusts. It was discovered at Birness, Aberdeenshire, Scotland. Formation Its precipitation from the oxidation of Mn(II) in oxygenated aqueous solutions is kinetically hindered and slow on mineral surfaces. Biological Mn(II) oxidation is generally fast relative to abiotic Mn(II) oxidation processes, and for this reason the majority of natural birnessites is believed to be produced by microorganisms, especially bacteria but also fungi. Composition and structure Birnessite is a non-stoichiometric compound, in which variable amounts of Mn4+ ions in the nominal MnO2·nH2O formula either are missing, or are replaced primarily by Mn3+ ions and secondarily by Mn2+ ions. Because a solid is overall electrically neutral, birnessite contains foreign cations to balance the net negative charge created by Mn4+ vacancies and heterovalent Mn substitutions. Two crystallographic structures are known, triclinic birnessite (TcBi) and hexagonal birnessite (HBi). The two of them consist of layers of edge-sharing MnO6 octahedra separated by one or two layers of water molecules. The one-water layer compounds have a characteristic ~7 Å repeat in the layer stacking direction, and addition of a second water layer expands the layer spacing to ~10 Å. The 10 Å form is named buserite. The layer composition of TcBi is typically Mn4+0.69Mn3+0.31. The Mn3+O6 and Mn4+O6 octahedra are fully ordered in raws in the MnO2 layers, such that every Mn3+-rich row alternates with two Mn4+-rich rows.10,11 The layer charge is offset by alkaline and alkali-earth cations (e.g, Na, K, Ca, Ba) into the interlayer region along with water molecules, and therefore TcBi has a cation-exchange capacity. A typical chemical formula of Na-exchanged TcBi is Na0.31(Mn4+0.69 Mn3+0.31)O2·0.4H2O. The layer structure of HBi differs from that of TcBi by the presence of octahedral Mn4+ vacancies (Vac). The chemical formula of synthetic HBi depends on pH. The generic formula is H+x Mn3+y Mn2+z (Mn4+u Mn3+vVacw)O2, with x + 3y + 2z = v + 4w for neutrality. The MnO2 layers are stacked periodically in synthetic triclinic and hexagonal birnessite crystals. It is, however, rarely the case in natural materials. In addition to being chemically complex, natural birnessite crystals are structurally disordered with respect to the layer stacking and the