THE NEW MINERALS LEVINSONITE-(Y) OXALATES WITH DIFFERENT STRUCTURES AND DIFFERENTIATION OF LREE AND HREE

Abstract

Two new minerals, levinsonite-(Y) [(Y,Nd,Ce)Al(SO4)2(C2O4) · 12H2O] and zugshunstite-(Ce) [(Ce,Nd,La)Al(SO4)2(C2O4) · 12H2O], occur in an evaporite assemblage at Alum Cave Bluff, Great Smoky Mountains National Park, Tennessee. Electron microprobe analyses with oxalate and water assumed as stoichiometric yield the formula Y0.33La0.02Ce0.12Pr0.03Nd0.23Sm0.12Eu0.02Gd0.08Dy0.04Er0.01Al(SO4)2(C2O4) · 12H2O for levinsonite-(Y) and Y0.00La0.09Ce0.54Pr0.07Nd0.26Sm0.03Eu0.01Gd0.01Dy0.00Er0.00Al(SO4)2(C2O4) · 12H2O for zugshunstite-(Ce). Smaller ions such as Y and several MREE to HREE are enriched in the former phase, whereas the LREE are concentrated in the latter phase. The two minerals occur in intimate contact, with low-temperature 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 levinsonite-(Y) is a square antiprism defined by eight ligands ( = 2.404 Å), whereas that in zugshunstite-(Ce) is a nine-fold cooordinated capped square antiprism ( = 2.507 Å). The polyhedra resemble those in xenotime and monazite, respectively. The addition of one ligand in zugshunstite-(Ce) into the capping position results in relative shifts of the chains, isolated water molecules, and isolated Al(H2O)6 octahedra relative to their positions in the levinsonite-(Y) structure. The source of the REE elements is likely to be the weathering of pyritiferous schist, which is reported to contain monazite and xenotime. Chelation of REE and Al by oxalate under hydrous conditions at surface temperature permits formation of the minerals.