On the complex H-bonding network in paravauxite, Fe2+Al2(PO4)2(OH)2 · 8 H2O

Phosphate minerals represent the major host for transition metals and H2O in pegmatitic rocks, playing an essential geochemical role in the evolution processes of pegmatites. A good knowledge of their crystal chemistry is therefore necessary to better understand the genesis of pegmatites. Paravauxite is a mineral found in hydrothermal tin veins and granite pegmatites. Its ideal chemical formula is Fe2+Al2(PO4)2(OH)2·8H2O. Its crystal structure was solved and refined by Baur in 1969 on the basis of single-crystal X-ray diffraction data. This structure model appears to be consistent. However, due to the technical limitations of X-ray diffraction, the refinement only provided the isotropic displacement parameters, and the positions of nine independent proton sites were assigned but not refined. This led to a poor description of (the expected) complex H-bonding scheme in the paravauxite structure. In light of this, the crystal structure of a natural paravauxite was reinvestigated using electron microprobe analysis in wavelength dispersive mode (EPMA-WDS) and single-crystal neutron diffraction in an attempt to resolve these open questions.