IDENTIFICATION OF RARE EARTH ELEMENTS IN A NEPHELINE SYENITE WEATHERING PROFILE, SALINE COUNTY, ARKANSAS

The Department of the Interior published a list of 35 minerals they considered critical to U.S. national security and the economy in May of 2018: that list included rare earth elements (REEs). Studies show alkaline igneous rocks like nepheline syenite are enriched in REEs. This research aims to determine if minerals in nepheline syenite from Saline County, Arkansas contain appreciable quantities of REEs as well. Samples were collected, during the fall of 2017, along a weathering profile from an abandoned bauxite mine. X-ray diffraction of different size fractions and scanning electron microscopy of polished thin sections revealed mineralogical changes typical of a bauxite weathering profile. Many accessory minerals contain appreciable concentrations of rare earth elements (La, Ce, Nd, Tb): semi-quantitative analyses of rare earth oxides reveal 3.2 to 3.9 weight percent REE₂O₃ in titanite and 7.0 to 7.3 weight percent REE₂O₃ in fluorapatite. Terbium was found in the weathering profile associated with authigenic iron oxides. Weathering of accessory minerals in the nepheline syenite may release REEs to be adsorbed on to iron oxides or clay mineral surfaces similar to the South China clay deposits to form economic deposits of REEs.