• journal_title：American Mineralogist
• Contributor：Michael Schindler ; Frank C. Hawthorne
• Publisher：Mineralogical Society of America
• Date：1999-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：84
• issue：4
• firstpage：665
• section：Articles

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.