Crystal-chemical study of wavellite from Zbirov, Czech Republic

摘要

The crystal chemistry of wavellite from Zbirov (Czech Republic), ideally Al_3(PO_4)_2(OH,F)_3?5H_2O, was addressed by means of a multi-methodological approach based on electron microprobe analysis (EMPA) using wave-dispersive spectroscopy, single-crystal X-ray diffraction, powder and single-crystal infrared spectroscopy and Raman spectroscopy. The EMPA data showed the presence of significant F replacing OH in the sample studied. The structure was solved in the Pcmn orthorhombic space group, with the following unit-cell constants: a = 9.6422(7), b = 17.4146(15), c = 7.0094(2) A, V - 1176.98(10) A3. Phosphorus atoms display tetrahedral (PO_4) coordination, while Al cations display octahedral coordination. The mineral framework can be viewed as the repetition of cationic arrays made up of AlO_6 polyhedra, bridged by PO_4 groups and further joined by O-H---O hydrogen bonds. The single-crystal unpolarized Fourier transform infrared (FTIR) spectrum shows combination bands indicating the presence of both OH and H_2O in the structure. Both FTIR and Raman spectra show a broad absorption extending from 3600 to 2800 cm~(-1) resulting from the overlapping of several components due to the water molecules and the OH group. The frequencies observed are comparable to those expected on the basis of the Libowitzky relationship for the range of D-K---A bond systems in the structure.