Schubnelite, [Fe (super 3+) (V (super 5+) O ₄ )(H ₂ O)], a novel heteropolyhedral framework mineral

University of Manitoba, Department of Geological Sciences, Winnipeg, MB, Canada

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.

This record provided courtesy of AGI/GeoRef.

This article has been cited by other articles:

				M. Schindler, F. C. Hawthorne, and W. H. Baur A CRYSTAL-CHEMICAL APPROACH TO THE COMPOSITION AND OCCURRENCE OF VANADIUM MINERALS Can Mineral, December 1, 2000; 38(6): 1443 - 1456. [Abstract] [Full Text] [PDF]

				T. Witzke, R. Wegner, T. Doering, H. Pollmann, and W. Schuckmann Serrabrancaite, MnPO4{middle dot}H2O, a new mineral from the Alto Serra Branca pegmatite, Pedra Lavrada, Paraiba, Brazil American Mineralogist, May 1, 2000; 85(5-6): 847 - 849. [Abstract] [Full Text] [PDF]