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

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. Grenthe et al. 1992; Shock et al. 1997) indicate that U sulfate complexes, mainly UO2(SO4)aq, are the principal aqueous species responsible for the high U solubility under those conditions (Fig. 1). In contrast, U carbonate complexes become dominant under neutral and alkaline conditions (Fig. 1). Uranium sulfate minerals commonly are widespread around U-bearing mine sites, where they usually form during the evaporation of acid sulfate-rich mine drainage waters (Finch and Murakami 1999). Uranopilite, (UO2)6(SO4)(OH)10·12H2O (Burns 2001); johannite, Cu(UO2)2(SO4)2(OH)2·8H2O (Cejka et al. 1988); schröckingerite, NaCa3(UO2)SO4)(CO3)3F·10H2O; coconinoite Fe2Al2(UO2)2(PO4)4(SO4)(OH)2·20H2O; and zippeite-group minerals are the most common uranyl sulfates. In addition, several new uranyl sulfates have been described recently: deliensite, Fe(UO2)2(SO4)2(OH)2·3H2O (Vochten et al. 1997); jáchymovite, (UO2)8(SO4)(OH)14·13H2O (Cejka et al. 1996); and rabejacite, Ca(UO2)4(SO4)2(OH)6·6H2O (Deliens and Piret 1993). The latter phases may be more widespread than