On the labyrinthine crystal chemistry of beudantite, PbFe3+3[(As,S)O4]2(OH)6

Beudantite (ideally Pb(Fe3+,Al)3[(As,S)O4]2(OH)6, a=7.31, c=17.03 , sp gr R-3m) is a secondary mineral that occurs in the oxidized zones of polymetallic deposits. Its principal constituents are Pb, Fe, As and S, but with a labyrinthine composition reported since its discovery. The X-ray structural model previously reported, although providing a crystalline edifice with overall geometric consistency (in terms of bond-distances and angle), has a number of limitations: Recent data on the chemical composition of beudantite have further complicated matters, showing how this mineral can act as a multicomponent solid solution system, with wide variations in the As, S, Fe, Cu, and Sb contents. Having available well-characterised single-crystals of various size, we would like to perform a neutron diffraction experiment, in order to obtain a full structural model of beudantite, including positions and anisotropic displacement parameters for all atomic sites. The neutron diffraction is the only technique able to unambiguously reveal: 1) whether the previously reported symmetry of beudantite structure is correct; 2) if the disorder around the Pb site is due to the presence of Cu and Zn or not; 3) if Al replaces only Fe; 4) if As and S share the same site, or whether a lowering of the symmetry can realistically give symmetry-independent tetrahedral sites; 5) if F atoms replace OH in the crystal; 6) whether the structure contains OH groups or even solvation H2O.