Structure Refinement and Vibrational Spectroscopy of Vauxite From the Type Locality, Llallagua (Bolivia)

A crystal-chemical investigation of vauxite, ideally FeAl2(PO4)2(OH)2-6H2O, from Llallagua (Bolivia) has been performed using a multi-methodological approach based on WDS-electron microprobe, single-crystal X-ray diffraction, and vibrational spectroscopies (Raman and FTIR). The structure was refined in the triclinic P1 space group, with the following unit-cell constants: a 9.1276(2), b 11.5836(3), c 6.15960(10) A, a 98.3152(10)8, fi 92.0139(10)8, y 108.1695(9)8, and V 610.05(2) A. The vauxite structure is based on a building unit oriented parallel to the c axis and composed of a chain of Fe2 and Al2 edge-sharing octahedra and two chains of corner-sharing P2 tetrahedra and Al1 octahedra, interconnected via corners and P1 tetrahedra. Neighboring building units are interconnected by Al3 octahedra and via Fe1 octahedra. The framework is completed with two non-coordinated water molecules. The latter, together with the two hydroxyl groups and the other four coordinated water molecules, form a complex hydrogen bonding network whose interactions further compact the whole framework. Both FTIR and Raman spectra show, in the H2O stretching region, a broad absorption consisting of several overlapping components due to the six water molecules plus the OH groups. The band multiplicity observed in the low-wavenumber region (,1400 cmT) is compatible with the presence of two distorted PO4 tetrahedra.