SPECTROSCOPIC INVESTIGATIONS OF REE HOSTED HEAVY MINERAL SANDS AND PHOSPHATES

The current demand on many of our natural resources requires that we reassess our geologic models and expand our search for rare earth element (REE) resources in the United States. Currently, there is a lack of sufficient spectroscopic investigations characterizing surface materials associated with marine phosphates and paleo-beach placers. Understanding the expected spectral properties of these deposits using visible to mid infrared spectroscopic methods will add significant information about how we assess such deposits in the future using both laboratory and imaging spectroscopy.

Spectra of REE minerals show fine structure in the visible (VIS) to near-infrared (NIR) that are caused by crystal field f-f transitions of rare earth ions present in the minerals. These sharp spectral absorptions allow accurate identification of these minerals when found in significant concentrations and can also help identify the type of REEs based on the position of their absorptions.

Diagnostic spectral features related to rare earth ions are observed in many common phosphates such as apatite, monazite and xenotime. In heavy-mineral sands, monazite will usually dominate the spectral signiture, but additional absorptions from zircon and xenotime may be present if significant amounts of these minerals occur in the sand. While these pure endmembers show crystal field f-f transition features, low concentrations and abundance of REEs within the host rock can make identification of these bands challenging in natural environments. To better understand these limitations, phosphorites and phosphates containing a wide range of REE concentrations and heavy mineral sands bearing various amounts of monazite were examined. Using laboratory spectroscopy, phosphorite and phosphates from various geologic environments that contain total REE from the percent level down to hundreds of ppm were examined to better understand the lower detection limits. Analysis of phosphate minerals and phosphorite rocks show that both REE concentration and the matrix of the materials play an important role in detection limits. Natural and constructed mixtures of heavy-mineral sands were also characterized to estimate the lowest concentration of monazite in sand that is detectable with VIS to NIR spectroscopy.