title: Synthetic aenigmatite analog Na2(Mn5.26Na0.74)Ge6O20: structure and crystal chemical considerations.
abstract: Disodium hexamanganese(II,III) germanate is the first aenigmatite-type compound with significant amounts of manganese. Na(2)(Mn(5.26)Na(0.74))Ge(6)O(20) is triclinic and contains two different Na positions, six Ge positions and 20 O positions (all with site symmetry 1 on general position 2i of space group P1). Five out of the seven M positions are also on general position 2i, while the remaining two have site symmetry 1 (Wyckoff positions 1f and 1c). The structure can be described in terms of two different layers, A and B, stacked along the [011] direction. Layer A contains pyroxene-like chains and isolated octahedra, while layer B is built up by slabs of edge-sharing octahedra connected to one another by bands of Na polyhedra. The GeO(4) tetrahedra show slight polyhedral distortion and are among the most regular found so far in germanate compounds. The M sites of layer A are occupied by highly charged (trivalent) cations, while in layer B a central pyroxene-like zigzag chain can be identified, which contains divalent (or low-charged) cations. This applies to the aenigmatite-type compounds in general and to the title compound in particular.
authors.affiliation: Department of Materials Engineering and Physics, Division of Mineralogy, University of Salzburg, Hellbrunnerstrasse 34, A-5020 Salzburg, Austria. guenther.redhammer@aon.at
authors.forename: Günther J
authors.initials: GJ
authors.lastname: Redhammer
authors.forename: Georg
authors.initials: G
authors.lastname: Roth
authors.forename: Dan
authors.initials: D
authors.lastname: Topa
authors.forename: Georg
authors.initials: G
authors.lastname: Amthauer