Crystal structure of bassetite and saléeite: new insight into autunite-group minerals
Dal Bo, Fabrice; Hatert, Freédéric; Mees, Florias; Philippo, Simon; Baijot, Maxime; Fontaine, François
European Journal of Mineralogy Volume 28 Number 3 (2016), p. 663 - 675
published: Jun 1, 2016
ArtNo. ESP147052803015, Price: 29.00 €
The crystal structures of two autunite-group minerals have been solved recently. The crystal structure of bassetite, Fe2+ [(UO2)(PO4)]2 (H2 O)10, from the type locality in Cornwall, United Kingdom (Basset Mines) was solved for the first time. Bassetite is monoclinic, space group P 21 /n, a = 6.961(1), b = 20.039(2), c = 6.974(1) Å and b = 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 P 21 /n, a = 6.951(1), b = 19.942(1), c = 6.967(1) Å and b = 90.58(1). The crystal structure investigation of bassetite (R 1 = 0.0658 for 1879 observed reflections with |F o|≥4 σ F ) and saléeite ( R 1 = 0.0307 for 1990 observed reflections with | F o |≥ 4σ s F ) confirms that both minerals are topologically identical and that bassetite contains ten water molecules per formula unit. Their structure contains autunite-type sheet, [(UO2)(PO4)]-, consisting of corner-sharing UO6 square bipyramids and PO4 tetrahedra. Iron and magnesium are surrounded by water molecules to form Fe(H2 O)6 or Mg(H2 O)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.