The crystal structure of a single crystal of foitite, hydrothermally synthesized from a bulk composition of XY(Fe2Al)ZAl6[Si6O18|(BO3)3|(OH)3OH] at 700°C and 200 MPa, has been refined to an R(|F|) index of 0.023 for 950 unique reflections. The compound is trigonal with space group symmetry R3m, Z = 3, a = 15.964(1) Å, c = 7.119(1) Å, V = 1571.2(3) Å3, and Dcalc = 3.133 g cm−3. The crystal-chemical formula of this foitite derived by structure refinement is XY(Fe1.58 Al1.42) ZAl6 [Si6O18|(BO3)3|(OH)3.58O0.42]. This composition is in very good agreement with the data obtained by electron microprobe analysis. The two main results of the structure analysis are: (1) iron occupies exclusively the Y-position and (2) bond valence calculations clearly indicate a hydrogen-deficiency at the anion site O1 yielding less than 4 OH groups per formula unit.

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