The HOH layer is the main structural unit in the crystal structures of Fe3+-disilicates ericssonite-2O, ideally Ba2Fe3+2Mn4(Si2O7)2O2(OH)2, ferroericssonite, ideally Ba2Fe3+2Fe2+4(Si2O7)2O2(OH)2, and yoshimuraite, ideally Ba4Ti2Mn4(Si2O7)2(PO4)2O2(OH)2, a TS-block mineral of Group II. The chemical compositions of the core part of the HOH layer in ericssonite-2O and ferroericssonite, Fe3+2Mn4(Si2O7)2O2(OH)2 and Fe3+2Fe2+4(Si2O7)2O2(OH)2, are similar to the chemical composition of the core part of the HOH layer in yoshimuraite, Ti4+2Mn4(Si2O7)2O2(OH)2, except for the cation species at the -coordinated MH site in the H sheets: Fe3+ and Ti4+, respectively. Despite this similarity, the topology of the HOH layer in ericssonite-2O and ferroericssonite is different from that in yoshimuraite. In TS-block minerals, different distortions of MO octahedra correspond to specific types of linkage of H and O sheets. Topological consideration of Fe3+-disilicates ericssonite-2O and ferroericssonite and yoshimuraite, a TS-block mineral of Group II, shows that different topologies of the chemically identical HOH layer are due to a difference in the bond-valence contributions of Fe3+ and Ti4+ at the MH site in the H sheet (i.e., inability of Fe3+ to contribute sufficient bond-valence to the XOM anion) and subsequent different distortions of MO octahedra in the O sheet, where MO = Mn2+, Fe2+.