Abstract

Schubnelite from the U deposit of Mounana, Gabon, crystallizes in space group P1 with a = 5.466(1), b = 5.675(2), c = 6.610(1) Aa, alpha = 101.02(1), beta = 95.10(1), gamma = 107.31(1) degrees , and V = 189.8(2) Aa 3 . The structure of schubnelite [Fe (super 3+) (V (super 5+) O 4 )(H 2 O)] contains isolated (VO 4 ) tetrahedra and edge-sharing (Fephi 6 ) octahedra (phi = unspecified anion) and is isostructural with [M(TO 4 )(H 2 O)] compounds with M = Mg,Mn and T = Mo, W. The topology of the schubnelite framework can be described as an arrangement of mutually orthogonal 6 3 and 4 4 nets. The fundamental building block (FBB) of the schubnelite structure does not occur in any other M (super [6]) T (super [4]) phi mineral. Many stoichiometrically similar compounds [M(TO 4 )(H 2 O)] crystallize in the kieserite structure-type, including the synthetic compounds V (super 3+) (PO 4 )(H 2 O) and Mn (super 3+) (PO 4 )(H 2 O). The kieserite arrangement has a (super [6]) M (super 3+) -(H 2 O)- (super [6]) M (super 3+) bridge. Both V (super 3+) (3d 2 ) and Mn (super 3+) (3d 4 ) have electronic degeneracies that drive spontaneous distortions resulting in satisfaction of the incident bond-valence requirements around the bridging H 2 O group. For Fe (super 3+) (3d 5 ) in schubnelite, there is no electronic degeneracy and hence no spontaneous local distortion of the environment around the Fe (super 3+) cation. Hence, an Fe (super 3+) -(H 2 O)-Fe (super 3+) bridge cannot form and schubnelite is forced to crystallize in a different arrangement.

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