The role of the coordination defect: A new structural description of four fluorite-related sesquioxide minerals, bixbyite (Mn2O3), braunite (Mn7SiO12), braunite II (CaMn14SiO24), parwelite (Mn10Sb2As2Si2O24), and their structural relationships

The anion-deficient, fluorite-related structures of the manganese-based minerals bixbyite (Mn2O3), braunite (Mn7SiO12), braunite II (CaMn14SiO24) and parwelite (Mn10Sb2As2Si2O24) are reinterpreted in terms of the coordination defect (CD) theory to gain new insights into their structural interrelationships. CDs are extended, octahedral defects centred by an anion vacancy and including its immediate atomic environment: it is represented as □M4O6, where the symbol □ is the anion vacancy. The bixbyite motif is a CD dimer (two edge-sharing octahedra), and this motif repeats, by further edge-sharing, around the 2-fold screw axes of the cubic structure. These same dimers are present in each of the other structures, but the presence of Si4+ in braunite and braunite II, together with that of other foreign cations such as As5+ and Sb5+ in parwelite, leads to different juxtapositions of these motifs. Moreover, the structure of braunite, Mn2+(Mn3+)6SiO12, reflects the clustering of 12 Mn3+-centred octahedra (MnO6) around a central SiO4 tetrahedron to generate a structure for the [(Mn3+)6SiO12]2− anion which is almost identical to that of the well-known cuboctahedral structure of the PO4-centred heteropolytungstate anion, [(W6+)12PO40]3−. The structure of braunite II, [Ca(Mn3+)14SiO24], is simply an intergrowth of slabs of bixbyite- and braunite-type structures, linked by the CaO8 cubes of the latter.