The Canadian Mineralogist Vol. 54, pp. 163-176 (2016) DOI: 10.3749/canmin.1500060 STRUCTURE REFINEMENT AND VIBRATIONAL SPECTROSCOPY OF VAUXITE FROM THE TYPE LOCALITY, LLALLAGUA (BOLIVIA) GENNARO VENTRUTI, EMANUELA SCHINGARO, ALESSANDRO MONNO, AND MARIA LACALAMITA Dipartimento di Scienze della Terra e Geoambientali, Universit` a di Bari Aldo Moro, via E. Orabona 4, 70125 Bari, Italy GIANCARLO DELLA VENTURA AND FABIO BELLATRECCIA Dipartimento di Scienze, Universit` a Roma Tre, Largo S. L. Murialdo 1, 00146 Rome, Italy CORRADO CUOCCI Istituto di Cristallografia – CNR, Via G. Amendola, 122/O, 70126 Bari, Italy MANUELA ROSSI Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse, Universit` a di Napoli Federico II, Via Mezzocannone 8, 80134, Naples, Italy FRANCESCO CAPITELLI § Istituto di Cristallografia – CNR, Via Salaria Km 29.300, 00016 Monterotondo (Rome), Italy ABSTRACT A crystal-chemical investigation of vauxite, ideally FeAl 2 (PO 4 ) 2 (OH) 2 6H 2 O, 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 P 1 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, b 92.0139(10)8, c 108.1695(9)8, and V 610.05(2) ˚ A 3 . 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 H 2 O 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 cm 1 ) is compatible with the presence of two distorted PO 4 tetrahedra. Keywords: vauxite, phosphate, EMPA, single-crystal X-ray structure refinement, hydrogen bonding network, FTIR and Raman spectroscopy. INTRODUCTION Vauxite, ideally FeAl 2 (PO 4 ) 2 (OH) 2 6H 2 O, is a secondary aluminum phosphate mineral derived from the alteration of apatite. To date it has been reported only twice: from the type locality at Llallagua (Bolivia) where it occurs in hydrothermal tin veins (Gaines et al. 1997), and from a very peculiar occurrence at Teruel (Spain) as a weathering product of biogenic apatite in dinosaur teeth (Bauluz et al. § Corresponding author e-mail address: francesco.capitelli@ic.cnr.it 163