X-ray-excited optical luminescence imaging for on-site identification of xenotime

The on-site identification of rare-earth-element (REE) bearing minerals prior to off-site quantitative analysis helps to efficiently develop mines that reserve REEs. This study examines a method for rapidly identifying xenotime, which is one of the principal REE bearing minerals, leading to such on-site analysis. It is based on acquiring X-ray-excited optical luminescence (XEOL) images of mineral ores that primarily comprise xenotime. Intense green or greenish-yellow luminescence originating from Sm, Eu, Tb, and Dy was detected for xenotime with a uranium content of less than 1.1 wt%, which includes over half of the xenotime in the world. Other minerals emitted no luminescence (e.g., cheralite, chlorite, and potassium feldspar), different colored luminescence from xenotime (e.g., albite and potassium feldspar), or weak green luminescence (e.g., zircon). Therefore, xenotime can in most circumstances be distinguished from other minerals by detecting intense green or greenish-yellow luminescence in XEOL images. The XEOL measurement was performed in air within 60 s for an area with the diameter of 3 mm, and the setup primarily required an X-ray tube of the type used in portable analyzers (e.g., X-ray fluorescence analyzers) and a digital camera. The results suggest that the XEOL imaging method may have potential to be used for the on-site and/or field identification of REE bearing minerals.