Schubnelite, [Fe3+(V5+O4)(H2O)], a novel heteropolyhedral framework mineral

Abstract

Schubnelite from the U deposit of Mounana, Gabon, crystallizes in space group P 1 with a = 5.466(1), b = 5.675(2), c = 6.610(1) Å, α = 101.02(1), β = 95.10(1), γ = 107.31(1)°, and V = 189.8(2) Å3. The structure of schubnelite [Fe3+(V5+O4)(H2O)] contains isolated (VO4) tetrahedra and edge-sharing (Feφ6) octahedra (φ = unspecified anion) and is isostructural with [M(TO4)(H2O)] compounds with M = Mg,Mn and T = Mo,W. The topology of the schubnelite framework can be described as an arrangement of mutually orthogonal 63 and 44 nets. The fundamental building block (FBB) of the schubnelite structure does not occur in any other M([6])T([4])φ mineral. Many stoichiometrically similar compounds [M(TO4)(H2O)] crystallize in the kieserite structure-type, including the synthetic compounds V3+(PO4)(H2O) and Mn3+(PO4)(H2O). The kieserite arrangement has a ([6])M3+-(H2O)-([6])M3+ bridge. Both V3+(3d2) and Mn3+(3d4) have electronic degeneracies that drive spontaneous distortions resulting in satisfaction of the incident bond-valence requirements around the bridging H2O group. For Fe3+ (3d5) in schubnelite, there is no electronic degeneracy and hence no spontaneous local distortion of the environment around the Fe3+ cation. Hence, an Fe3+-(H2O)-Fe3+ bridge cannot form and schubnelite is forced to crystallize in a different arrangement.