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

Abstract

Schubnelite from the U deposit of Mounana, Gabon, crystallizes in space group P1̅ 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) ų. The structure of schubnelite [Fe³⁺(V⁵⁺O₄)(H₂O)] contains isolated (VO₄) tetrahedra and edgesharing (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 compound [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.