Sverigeite, A Structure Containing Planar NaO₄ Groups and Chains of 3- and 4-membered Beryllosilicate Rings^*

Sverigeite, Na(Mn²⁺,Mg)₂Sn⁴⁺[Be₂Si₃O₁₂(OH)], is orthorhombic, Imma, with a = 10.815(8), b = 13.273(8), c = 6.818(6) Å, and Z = 4. Its crystal structure has been determined by direct methods and refined to an unweighted residual of 0.054 for all 571 reflections. Sverigeite is a single-chain beryllosilicate, which may be described in terms of three structural elements: (1) octahedral single-chains of alternating, edge-sharing SnO₆ distorted octahedra and planar Na0₄ groups running parallel to [001]; (2) tetrahedral single-chains of alternating, vertex-sharing Be₂SiO₈(OH) 3-rings and Be₂Si₂O₁₁(OH) 4-rings running parallel to [100]; and (3) discrete M₂O₁₀ groups of edge-sharing distorted MO₆ octahedra (M = Mn_0.63Mg_0.37), which link adjacent octahedral chains and also adjacent tetrahedral chains. Na displays a most unusual 4-fold, rectangular-planar coordination [Na-O = 2.433(5) Å], which may be regarded as a distorted octahedron with two vacant axial ligand sites. The next-nearest oxygen atoms are a distant 3.065(4) Å.