The crystal structure of zunyite

Zunyite, a rare aluminosilicate mineral of composition (OH, F)18Al13Si5O20Cl, has an unusual structure built up of Si5O16 groups of linked silicon tetrahedra and aluminium-oxygen groups of linked octahedra. The structure proposed by Pauling is verified and refined by X-ray methods, using 163 (hkO) reflections from single-crystal photographs with Mo Ka radiation. Refinement of positional parameters is carried out by the least-squares method, with inclusion of off-diagonal terms in the normal equation matrix because of overlap of oxygen atoms in the (100) projection. Refinement of isotropic temperature parameters for separate atoms is carried out with the use of difference syntheses, by methods differing somewhat from published ones. A general discussion of isotropic temperature factor refinement from differerence syntheses is given. One stage of least-squares refinement using 410 (hhl) reflections is carried out, for comparison with the (hkO) refinement. The corresponding accuracy of interatomic distances is ± 0.02 A. A large apparent temperature factor discrepancy between the two refinements is attributed to a deviation in the contrast of one of the X-ray photographs. The refined structure differs from the trial structure in distortion of coordination polyhedra, as found in other similar structures. The arrangement of protons in the structure is proposed from structural arguments, and the proposed arrangement requires the inclusion of at least two fluorine atoms per stoichiometric molecule.