The structure of romanechite, (Ba,H2O)2(Mn4+,Mn3+)5O10, forms large, rectangular-shaped tunnels bounded by double- and triple-chains of edge sharing MnO6 octahedra. The Mn3+ cations are located in the octahedra at the edges of the triple chains, and the Ba and H2O are located in the tunnels, often in a ratio of 1 to 2. Romanechite occurs in oxidized zones in Mn-rich ore deposits.