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 FeAl(PO)(OH)·8HO, 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 P2space group. Metavauxite structure displays a complex framework consisting of a stacking of [Al(PO)(OH)(HO)]layers linked to isolated [Fe(HO)]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 HO molecules, bridge the [Fe(HO)]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 HO molecules connecting the isolated Fe(HO)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 POgroup and the metal-oxygen polyhedra.