flatness of the layers. A natural birnessite crystal may contain only a few layers, and they are often bent and always imperfectly stacked with orientational and translational loss of registry. The stacking disorder is referred to as “turbostratic” when the layers are oriented completely at random. Natural birnessite with turbostratically stacked layers is named vernadite, and the synthetic analog is named δ-MnO2. The layer spacing of vernadite can be also ~7 Å or ~10 Å, and interstratification of the two types of layers has been observed on quartz coatings and in ferromanganese crusts. Surface reactivity The +4 charge deficit of a vacancy can be balanced by a large variety of interlayer cations forming inner-sphere complexes above and below the vacancies (e.g., Ca, Cu, Zn, Pb, Cd, Tl). The relative stability of the interlayer cations has been evaluated experimentally and theoretically by surface complexation modeling and computational chemistry. Pb2+ has the highest stability at the HBi surface, and the high geochemical affinity of Pb2+ for birnessite probably explains its billion-fold enrichment in marine ferromanganese deposits compared to seawater, which surpasses those of all other elements. Transition metal cations sorbed on vacancies were also observed to enter the underlying vacancies and become incorporated into the MnO2 layer, when their effective ionic radius is close to that of Mn4+ (r = 0.53 Å). For example, Zn2+ (r = 0.74 Å) was never observed to enter a vacancy site, while Ni2+ (r = 0.69 Å) partly enters, and Co3+ (r = 0.54 Å) always does. Octahedral high-spin Co2+ (r = 0.74 Å) sorbed on a vacancy is transformed into a smaller tetrahedral complex (r = 0.58 Å) to penetrate into the octahedral Mn vacancy, and is subsequently converted to the low-spin state before being oxidized to Co3+ by Mn4+, which is reduced to Mn3+. The surface-catalyzed, redox-driven uptake of Co leads also to a billion-fold enrichment in marine ferromanganese deposits compared to seawater. The geochemical partitioning of Co between ferromanganese deposits and seawater is the second-highest of all elements after Pb2+. Properties and applications In nature, photosynthetic organisms use the high oxidative ability of birnessite-type Mn4CaO5 clusters to oxidize water into molecular oxygen through the photosystem II membrane protein complex. Because of their semiconducting properties, birnessite-type materials are used in a variety of areas, including catalysis and electrochemical energy storage (batteries and pseudocapacitors). The ordering of the Mn3+ cations in triclinic birnessite, and the