Crystal chemistry of double-ring silicates; structures of sugilite and brannockite

The crystal structures of sugilite, Na₂K(Fe³⁺,Mn³⁺,Al)₂Li₃Si₁₂O₃₀, a = 10.009(2) Å, c = 14.006(3) Å, from the Wessels mine, Kalahari (South Africa), and brannockite, KSn₂Li₃Si₁₂O₃₀, a = 10.002(2) Å, c = 14.263(3) Å from the Foote mine, Kings Mountain (North Carolina, U.S.A.), were refined from X-ray single-crystal data. Both minerals crystallize in the space group P6/mcc and belong to the double-ring silicates. Li occupies strongly angularly distorted tetrahedra, sharing two edges with (Fe³⁺,Mn³⁺,Al) octahedra in sugilite and with Sn octahedra in brannockite. Na in sugilite exhibits distorted ninefold coordination, whereas the corresponding site in brannockite is vacant. Literature data of double-ring silicates are used to develop some general rules concerning the structural and chemical variation within this structure type.