Spectroscopic studies of Mssbauer, infrared, and laser-induced luminescence for classifying rare-earth minerals enriched in iron-rich deposits

Aoyagi, Noboru   ; Nguyen, T. T.*; Kumagai, Yuta   ; Nguyen, T. V.*; Nakada, Masami  ; Segawa, Yukari; Nguyen, H. T.*; Le, B. T.*

Rare-earth phosphates often appear as an accessory phase in igneous or metamorphic rocks; however, these rocks are composed of myriad chemical elements and nuclides that interfere with the qualitative or quantitative analyses of the rare-earth phosphates over a range of concentrations in the absence of a pretreatment. In addition, the limit of each analytical methodology constrains the approach as well as the usefulness of the results in geoscience applications. Here, we report the specific mineral characterization of rare-earth (RE)-containing ores from Yen Phu mine, Vietnam, using a range of state-of-the-art spectroscopic techniques in conjunction with microscopy: Mssbauer spectroscopy, infrared microspectroscopy, time-resolved laser-induced fluorescence spectroscopy (TRLFS), and scanning electron microscopy with energy-dispersive X-ray spectroscopy. Because the distribution of each element in the deposit differs, such combinatorial works are necessary and could lead to more plausible answers to questions surrounding the point of origin of rare-earth elements. The results of our Mssbauer spectroscopic analysis indicate that the three ores sampled at different locations all contain magnetite-like, hematite-like, and iron(III) salts other than hematite. In addition, we confirmed the presence of phosphate around the grain boundary in the magnetite-like mineral phase by infrared microspectroscopic analysis. The present analytical findings of trace amounts of europium(III) using TRLFS suggest that the europium ions generate identical luminescence spectra despite being embedded in three different matrices of iron minerals. This demonstration highlights the benefits of combinatorial spectroscopic analyses to gain insights into the effects of the environment of REs on their solid-state chemistry and shows the potential utility of TRLFS as a resource mining tool. Further applications of this approach in the analytical screening of rocks and minerals are feasible.