Distinguishing xenotime and zircon in ores and estimating the xenotime content for on-site analysis

Abstract

Estimation of the content of the major minerals containing rare earth element (REE) (e.g., xenotime, monazite, and bastnäsite) to extract metal REEs is a critical task for efficient exploration of mines with REE reserves. X-ray-excited optical luminescence (XEOL) imaging is a promising method for estimating the REE-bearing mineral content on-site. However, distinguishing between xenotime and zircon in ores via XEOL imaging is difficult owing to their similar luminescence colors and intensities. This study reveals that XEOL images of ores before and after annealing at 1300 °C can distinguish xenotime and zircon by investigating images obtained via cathodoluminescence (CL), which is the same phenomenon as XEOL except that it used electron bombardment instead of X-ray irradiation. After annealing, zircon exhibits a luminescence intensity stronger than that of xenotime in the CL images. In these images, zircon corresponds to an area with green luminescence whose CL intensity is drastically enhanced by annealing; in contrast, xenotime corresponds to an area with green luminescence whose CL intensity does not change much. The xenotime content in ores can be estimated from the area corresponding to xenotime in the CL images. The exposure time for CL images, which is comparable to XEOL images, is obtained in 30 s. Therefore, the proposed method can be applied to XEOL imaging and used to on-site prescreen ores before precise quantitative analyses, such as inductively coupled plasma mass spectrometry, electron-probe microanalysis, or scanning electron microscopy based on automated mineralogy, which require a large amount of time; thus, the adoption of the proposed method can lead to a drastic reduction in the time required to explore mines reserving REEs.