The crystal structure of zunyite

Abstract

The structure proposed by Pauling for the rare aluminosilicate mineral zunyite (Al_(13)(OH)_(18)Si_5O_(20)Cl) has been confirmed and refined with the use of 163 hkO reflections and 409 hhl reflections obtained with Mo Kα radiation from single crystal Weissenberg photographs. The structure is isometric (T^2_d) and is built up of Si_5O_(16) groups of linked silicon tetrahedra combined with Al_(12)O_(16)(OH)_(30) groups of linked aluminum octahedra. Refinement is carried out independently for the hkO and hhl data, and the final reliability factors are 0·12 for both sets of data. Positional parameters are refined by the least-squares method, and isotropic temperature parameters for separate atoms are adjusted with the help of difference syntheses. The refined structure differs from the trial structure by distortion of coordination polyhedra in a fashion similar to the distortions in related structures. The interatomic distance Al-O of 1·80 ± 0·016 Å is derived for tetrahedrally coordinated aluminum. The averaged Si-O distance is 1·64 ± 0·01 Å. The arrangement of protons in the structure is deduced from structural arguments. The proposed arrangement requires the inclusion of at least two fluorine atoms per stoichiometric molecule, modifying the chemical formula to (OH, F)_(16)F_2Al_(13)Si_5O_(20)Cl and explaining the importance of fluorine in the formation of zunyite.

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