X-ray Structure Refinement and Vibrational Spectroscopy of Metavauxite FeAl2(PO4)2(OH)2•8H2O

In this paper, we provide a crystal-chemical investigation of metavauxite, ideally FeAl2(PO4)(2)(OH)(2)8H(2)O, from Llallagua (Bolivia) by using a multi-methodological approach based on EDS microchemical analysis, single crystal X-ray diffraction, and Raman and Fourier transform infrared (FTIR) spectroscopy. Our new diffraction results allowed us to locate all hydrogen atoms from the structure refinements in the monoclinic P2(1)/c space group. Metavauxite structure displays a complex framework consisting of a stacking of [Al(PO4)(3)(OH)(H2O)(2)](7-) layers linked to isolated [Fe(H2O)(6)](2+) cationic octahedral complex solely by hydrogen bonding. The hydrogen-bonding scheme was inferred from bond-valence calculations and donor-acceptor distances. Accordingly, strong hydrogen bonds, due to four coordinated H2O molecules, bridge the [Fe(H2O)(6)](2+) units to the Al/P octahedral/tetrahedral layer. The hydroxyl group, coordinated by two Al atoms, contributes to the intra-layer linkage. FTIR and Raman spectra in the high-frequency region (3700-3200 cm(-1)) are very similar, and show a complex broad band consisting of several overlapping components due to the H2O molecules connecting the isolated Fe(H2O)(6) and the adjacent Al/P octahedral/tetrahedral layers. A sharp peak at 3540 cm(-1) is assigned to the stretching mode of the OH group. The patterns collected in the low-frequency region are dominated by the stretching and bending modes of the PO43- group and the metal-oxygen polyhedra.