Hollandite (Post et al., 1982) is a manganese oxide mineral within the hollandite supergroup (Biagioni et al., 2012) with a general chemical composition of A_0-2B₈(O,OH)₁₆, where A = Ba²⁺ and B = (Mn⁴⁺₆Mn³⁺₂) in hollandite. The B cations form edge sharing double chains of B-O octahedra, with each double chain forming a wall of a four-sided tunnel. The large A cations, in ideally eight-coordinated sites, reside in the tunnel and offset any undersaturated charge on the octahedra. The supergroup is divided further into the coronadite group (Mn⁴⁺ dominates the B cations and includes hollandite) and the priderite group (Ti⁴⁺ dominates). Other minerals in the coronadite group include (A cations) K⁺ in cryptomelane [ideally B = (Mn⁴⁺₇Mn³⁺)], Sr²⁺ in strontiomelane [ideally Sr(Mn⁴⁺₆Mn³⁺₂)O₁₆], Pb²⁺ in coronadite [ideally B = (Mn⁴⁺₆Mn³⁺₂)], and Na⁺ in manjiroite [ideally B = (Mn⁴⁺₇Mn³⁺)]. Ferrihollandite is BaMn⁴⁺₆Fe³⁺₂O₁₆. Other B cations in natural samples can include Ti⁴⁺, Fe³⁺, Al³⁺, Si⁴⁺, Mg²⁺ or additionally, in synthetic phases, Zn²⁺, In⁴⁺, Ni, Cr, and many others. Likewise, A-cation substitutions include Ca, Sr, H₂O, vacancies, etc. Priderite is (K,Ba)_0-2(Ti⁴⁺₇Fe³⁺)₈O₁₆ and other members of the priderite group are redledgeite [Ba(Ti⁴⁺₆Cr³⁺₂)O₁₆], mannardite [Ba(Ti⁴⁺₆V³⁺₂)O₁₆ . H₂O], henrymeyerite [Ba(Ti⁴⁺₇Fe²⁺)O₁₆]. The presence or absence of H₂O does not define a hollandite-like species. Hollandite-supergroup mineralization occurs in oxidized zones of manganese ores, in hydrothermal deposits, and in some soils. The tunnel topology allows for these minerals to be good ionic conductors for batteries.