AUTHIGENIC REE-PHOSPHATES IN A LATERITIC WEATHERING PROFILE, SALINE COUNTY, ARKANSAS

Rare Earth Elements (REEs) are an integral part of modern technology, and they occur in our study area. The researchers previously identified accessory minerals such as titanite/sphene and fluorapatite in the alkaline igneous parent rock from this locality to be particularly enriched in REEs. We hypothesize that these accessory minerals release the REEs during weathering to form authigenic REE-bearing phases. Here we are identifying the mineral phases present in the weathering profile and determining where REEs are concentrated. We collected six grab samples every 1.5 meters up the weathering profile in an inactive open-pit bauxite mine. We size-fractionated the bulk samples and manually separated specific mineral grains from the sand-size fraction. These were identified using X-ray powder diffraction supplemented by scanning electron microscopy (SEM) equipped with an energy dispersive spectrometer (EDS). Chemical analyses (EDS) were used to differentiate minerals that contained REEs. We found flat mats of acicular REE-phosphates intercalated with slightly altered biotite (not vermiculite or chlorite). These REE-phosphates have been interpreted to be authigenic and generally contain more Ce than Nd with some containing La and Y and possibly other LREEs. These REE distributions are consistent with REE distributions in the parent rock accessory minerals. The biotite substrate does not contain detectable REEs. Florencite ((REE)Al₃(PO₄)₂(OH)₆) in association with kaolinite was identified with X-ray powder diffraction higher in the weathering profile. Decomposition of the accessory minerals in the parent rock supply the REEs that are being concentrated in authigenic phases up the weathering profile.