Crystal-chemical investigation of Al-2(PO4)(OH)(3) augelite from Rapid Creek, Yukon, Canada

The structural characterization of Al-2(PO4)(OH)(3) augelite, a natural orthophosphate coming from Rapid Creek, Yukon, Canada, was carried out by means of a multi-methodological approach based on WDS-EMPA chemical analysis, single crystal X-ray diffraction, and infrared spectroscopy. The structure was solved in the C2/m monoclinic space group, with the following unit cell constants: a = 13.0740(10), b = 7.9690(7), c = 5.0910(7) angstrom, beta = 112.282(5), V = 490.81(9) angstrom(3). Phosphorus atoms display tetrahedral (PO4) coordination, while aluminum cations display octahedral and unusual five-fold coordination. The framework of the mineral can be viewed as the repetition of cationic arrays made up by AlO6/AlO5 polyhedra, bridged by PO4 groups, and further joined by O-H..O hydrogen bonds: such interactions were also correlated to IR spectroscopy analysis, by means of Libowitzky formula applied on OH stretching vibrations.