ALBERT

Description: The HOH layer is the main structural unit in the crystal structures of Fe 3+ -disilicates ericssonite-2 O , ideally Ba 2 Fe 3+ 2 Mn 4 (Si 2 O 7 ) 2 O 2 (OH) 2 , ferroericssonite, ideally Ba 2 Fe 3+ 2 Fe 2+ 4 (Si 2 O 7 ) 2 O 2 (OH) 2 , and yoshimuraite, ideally Ba 4 Ti 2 Mn 4 (Si 2 O 7 ) 2 (PO 4 ) 2 O 2 (OH) 2 , a TS-block mineral of Group II. The chemical compositions of the core part of the HOH layer in ericssonite-2 O and ferroericssonite, [5] Fe 3+ 2 Mn 4 (Si 2 O 7 ) 2 O 2 (OH) 2 and [5] Fe 3+ 2 Fe 2+ 4 (Si 2 O 7 ) 2 O 2 (OH) 2 , are similar to the chemical composition of the core part of the HOH layer in yoshimuraite, [5] Ti 4+ 2 Mn 4 (Si 2 O 7 ) 2 O 2 (OH) 2 , except for the cation species at the [5]-coordinated M H site in the H sheets: [5] Fe 3+ and [5] Ti 4+ , respectively. Despite this similarity, the topology of the HOH layer in ericssonite-2 O and ferroericssonite is different from that in yoshimuraite. In TS-block minerals, different distortions of M O octahedra correspond to specific types of linkage of H and O sheets. Topological consideration of Fe 3+ -disilicates ericssonite-2 O 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 Fe 3+ and Ti 4+ at the M H site in the H sheet ( i.e. , inability of Fe 3+ to contribute sufficient bond-valence to the X O M anion) and subsequent different distortions of M O octahedra in the O sheet, where M O = Mn 2+ , Fe 2+ .