Abstract

The crystal structures of two autunite-group minerals have been solved recently. The crystal structure of bassetite, Fe2+[(UO2)(PO4)]2(H2O)10, from the type locality in Cornwall, United Kingdom (Basset Mines) was solved for the first time. Bassetite is monoclinic, space group P21/n, a = 6.961(1), b = 20.039(2), c = 6.974(1) Å and β = 90.46(1)°. The crystal structure of saléeite, Mg[(UO2)(PO4)]2(H2O)10, from Shinkolobwe, Democratic Republic of Congo, was also solved. Saléeite is monoclinic, space group P21/n, a = 6.951(1), b = 19.942(1), c = 6.967(1) Å and β = 90.58(1)°. The crystal structure investigation of bassetite (R1 = 0.0658 for 1879 observed reflections with |Fo| ≥ 4σF) and saléeite (R1 = 0.0307 for 1990 observed reflections with |Fo| ≥ 4σF) confirms that both minerals are topologically identical and that bassetite contains ten water molecules per formula unit. Their structure contains autunite-type sheets, [(UO2)(PO4)], consisting of corner-sharing UO6 square bipyramids and PO4 tetrahedra. Iron and magnesium are surrounded by water molecules to form Fe(H2O)6 or Mg(H2O)6 octahedra located in interlayer, between the autunite-type sheets. Two isolated independent water molecules are also located in interlayer. Energy-dispersive X-ray spectroscopy analysis confirmed the chemical composition obtained from structure refinement. These new data prompt a re-assessment of minerals of the autunite and meta-autunite groups.

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