REE phosphates vs REE carbonates in the Bahoruco karst bauxites, Dominican Republic

The mineralogy and mineral chemistry of REE phosphates and REE carbonates has been investigated in three localities of the Bahoruco karst bauxites, Dominican Republic, by means of powder X-ray diffraction (XRD), petrographic microscopy, scanning electron microscopy (SEM-EDS) and electron microprobe (EMP). The identified REE minerals include primary REE phosphates (monazite) and secondary REE phosphates, carbonates, and oxides. This work is focused on the secondary REE phosphates and carbonates. Phosphates are mostly enriched in Y and HREE (Gd, Dy), whereas carbonates display a wide range of compositions in terms of LREE (La, Ce, Nd, Sm) and HREE (Gd) contents. Geochemical stability data suggest that the dominant REE solid phase present depends on the total concentration of carbonate and/or phosphate in the solution, carbonates being more stable at pH around 7.