SPECTRALLY DISTINGUISHING BETWEEN REE-BEARING MINERALS BASED ON DIFFERENCES IN THEIR CRYSTAL FIELD F- F TRANSITION ABSORPTIONS

Remote sensing recognition of rare earth element (REE)-bearing minerals can be accomplished using visible to near-infrared absorptions when they contain trivalent Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, or Yb in spectrally detectable quantities. Although f-orbital coupling with molecular vibrations is traditionally viewed as “weak,” it is apparently strong enough to cause measurable differences in crystal field splitting and wavelength positions of f-f transition absorptions depending on the associated anion. Spectra of synthetic lanthanide compounds (e.g., carbonates, chlorides, nitrates, oxides, phosphates, and sulfates) show small but diagnostic spectral changes. These diagnostic absorptions are also present in REE-bearing carbonate and phosphate minerals such as bastnaesite, parasite, apatite, monazite, and xenotime. Crystal field f-f transitions are not present in trivalent La, Ce, Gd, and Lu compounds due to their 4f-orbital configurations. In situ reflectance spectra of ore rocks can be used to identify the minerals that contribute REE spectral signatures (i.e., Nd in bastnaesite looks spectrally different than Nd in monazite). This suggests that light REE deposits can be spectrally differentiated from heavy REE deposits and their mineralogy can be determined in the field or by using imaging spectroscopy. Nd is a pathfinder for light REEs and Dy a pathfinder for heavy REEs because they are the more abundant of the spectrally detectable lanthanide ions in REE-bearing minerals. The combination of a hand-held visible and near-infrared spectrometer covering wavelengths from 0.35 to 1 micron and a portable radiation meter were used to find local concentrations of monazite and xenotime in REE-enriched gneisses in the Front Range of Colorado. Imaging spectroscopy data collected over the Mountain Pass light REE deposit in California were used to map the location of Nd- and Sm-bearing carbonate rocks. Future work will concentrate on determining detection thresholds in both light and heavy REE deposits.