Bermanite, Mn2+(H2O)4[Mn3+2(OH)2(PO4)2], a 5.446(3), b 19.25(1), c 5.428(3)Å, β 110.29(4)°, monoclinic, space group P21Z = 2, is a novel structure type based on densepacked [Mn3+2(OH)2(PO4)2]2− slabs parallel to the {010} plane linked by tetrahedral corner-sharing to insular trans−[Mn2+(Op)2(H2O)4] octahedra. Chains of edge-sharing Mn3+–O octa-hedra, expressed as1[Mn3+(OH)(Op)3], run parallel to the [101] direction and are corner-linked by the (PO4) tetrahedra to form the slabs. A compact slab approximates local [C.A.B.C] anion packing where the A and В layers are fully occupied and the C layers only one-quarter occupied.

R(hkl) = 0.059 for 1254 independent reflections. The average interatomic distances are Mn3+(l)–0 2.020Å, Mn3+(2)–O 2.023Å, Mn2+(3)–0 2.186Å, P(l)–O 1.550Å and P(2)–0 1.548Å. Local dense-packing and pseudo-orthogonal character of the structure allow several kinds of twinning, which are explicable on the basis of local structure geometry.

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