The new minerals levinsonite-(Y) [(Y, Nd,Ce)Al(SO4)(2)(C2O4) center dot 12H(2)O] and zugshunstite-(Ce) [(Ce,Nd,La)Al(SO4)(2)(C2O4) center dot 12H(2)O]: Coexisting oxalates with different structures and differentiation of LREE and HREE

Two new minerals, levinsonite-(Y) [(Y,Nd,Ce)Al(SO4)(2)(C2O4) . 12H(2)O] and zugshunstite-(Ce) [(Ce,Nd,La)Al(SO4)(2)(C2O4). 12H(2)O], occur in an evapo rite assemblage at Alum Cave Bluff, Great Smoky Mountains National Park, Te nne:jsee. Electron microprobe analyses with oxalate and water assumed as st oichiometric yield the formula Y0.33La0.02Ce0.12Pr0.03Nd0.23Sm0.12Eu0.02Gd0 .08Dy0.04Er0.01Al(SO4)(2)(C2O4). 12H(2)O for levinsonite-(Y) and Y0.00La0.0 9Ce0.54Pr0.07Nd0.26Sm0.03Eu0.01Gd0.01Dy0.00Er0.00Al(SO4)(2)(C2O4). 12H(2)O for zugshunstite-(Ce). Smaller ions such as Y and several MREE to HREE are enriched in the former phase, whereas the LREE are concentrated:ed in the l atter phase. The two minerals occur in intimate contact, with low-temperatu re conditions favoring MREE in levinsonite-(Y) and LREE in zugshunstite-(Ce ). The crystal structures are based on the same fundamental building blocks , chains of alternating REE polyhedra and oxalate groups, with two sulfate tetrahedra attached to each REE polyhedron. The REE polyhedron in levinsoni te-(Y) is a square antiprism defined by eight ligands (< HREE-O > = 2.404 A ngstrom), whereas that in zugshunstite-(Ce) is a nine-fold cooordinated cap ped square antiprism (< LREE-O > = 2.507 Angstrom. The polyhedra resemble t hose in xenotime and monazite, respectively. The addition of one ligand in zugshunstite(Ce) into the capping position results in relative shifts of th e chains, isolated water molecules, and isolated Al(H2O)(6) octahedra relat ive to their positions in the levinsonite-(Y) structure. The source of the REE elements is likely to be the weathering of pyritiferous schist, which i s reported to contain monazite and xenotime. Chelation of REE and Al by oxa late under hydrous conditions at surface temperature permits: formation of the minerals. Copyright (C) 2001 Elsevier Science Ltd.