Contribution to the mineralogy of acid drainage of Uranium minerals: Marecottite and the zippeite-group

J. Brugger 1 , 2 , Peter C. Burns 3  and N. Meisser 4
  • 1 Department of Geology and Geophysics, The University of Adelaide, North Terrace, 5005 Adelaide, South Australia
  • 2 Division of Mineralogy, South Australian Museum, North Terrace, 5000 Adelaide, South Australia
  • 3 Department of Civil Engineering and Geological Sciences, 156 Fitzpatrick Hall, University of Notre Dame, Notre Dame, Indiana 46556-0767, U.S.A.
  • 4 Musée Géologique Cantonal & Laboratoire des Rayons-X, Institut de Minéralogie, UNIL-BFSH2, CH-1015 Lausanne-Dorigny, Switzerland

Abstract

Sulfate-rich acid waters produced by oxidation of sulfide minerals enhance U mobility around U ores and U-bearing radioactive waste. Upon evaporation, several secondary uranyl minerals, including many uranyl sulfates, precipitate from these waters. The zippeite-group of minerals is one of the most common and diverse in such settings. To decipher the nature and crystal chemistry of the zippeite-group, the crystal structure of a new natural hydrated Mg uranyl sulfate related to Mgzippeite was determined. The mineral is named marecottite after the type locality, the La Creusaz U prospect near Les Marécottes, Western Swiss Alps.

Marecottite is triclinic, P1, with a = 10.815(4), b = 11.249(4), c = 13.851(6) Å, a = 66.224(7), b = 72.412(7), and g = 69.95(2)°. The ideal structural formula is Mg3(H2O)18[(UO2)4O3(OH)(SO4)2]2(H2O)10. The crystal structure of marecottite contains uranyl sulfate sheets composed of chains of edge-sharing uranyl pentagonal bipyramids that are linked by vertex-sharing with sulfate tetrahedra. The uranyl sulfate sheets are topologically identical to those in zippeite, K(UO2)2(SO4)O2·2H2O. The zippeite-type sheets alternate with layers containing isolated Mg(H2O)6 octahedra and uncoordinated H2O groups. The uranyl sulfate and Mg layers are linked by hydrogen bonding only.

Magnesium-zippeite is redefined as Mg(H2O)3.5(UO2)2(SO4)O2, based on comparison of the powder X-ray diffraction pattern of micro-crystalline co-type material with the pattern of a synthetic phase. Magnesium-zippeite contains zippeite-type uranyl sulfate sheets with Mg located between the layers, where it is in octahedral coordination. In Mg-zippeite, distorted Mg octahedra are linked by sharing vertices, resulting in dimers. The apices of the Mg octahedra correspond to two O atoms of uranyl ions, and four H2O groups.

Magnesium-zippeite and marecottite co-exist, sometimes in the same sample, at Lucky Strike no. 2 mine, Emery County, Utah (type locality of Mg-zippeite), at Jáchymov, Czech Republic, and at La Creusaz. This study provides insight into the complexity of the zippeite-group minerals containing divalent cations, where different arrangements in the interlayers result in different unit cells and space groups.

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