Visible and short-wave infrared reflectance spectroscopy of REE fluorocarbonates

Abstract

An understanding of the mineralogy and petrogenesis of rare earth element deposits has significant implications for their economic viability. Lanthanide-bearing compounds are known to produce sharp absorption features in the visible to short-wave infrared region (VIS-SWIR), however, a significant knowledge gap exists between the fields of hyperspectral reflectance spectroscopy and rare earth element mineralogy. Reflectance spectra were collected from four bastnäsite samples, two parisite samples, and one synchysite sample from the visible into the short-wave infrared. These REE fluorocarbonate mineral samples were characterized via scanning electron microscopy and electron probe microanalysis. Sharp absorptions of REE-bearing minerals are mostly the result of 4f-4f intraconfigurational electron transitions and for the light REE-enriched fluorocarbonates, the bulk of the features can be ascribed to Nd 3+ , Pr 3+ , Sm 3+ , and Eu 3+ . The lanthanide-related spectral responses of the REE fluorocarbonates are consistent across the group, supporting the notion that the REE cation site is very similar in each of these minerals. Carbonate-related spectral responses differed between these minerals, supporting the notion that the crystallographic sites for the carbonate radical differ between bastnäsite, synchysite, and parisite. Exploitable spectral differences include a distinct absorption band at 2243 nm that separates bastnäsite from synchysite and parisite. Similarly, for bastnäsite a dominantly Pr 3+ -related absorption band located is at 1968 nm, while in synchysite and parisite it occurs at 1961 nm.

Remote Sensing Exploration of Nb-Ta-LREE-Enriched Carbonatite (Epembe/Namibia)

“…Reflectance spectra of all carbonatite samples show carbonate absorption features that are typical for calcite at 2.34 µm [34,35]. Rowan et al [11] and Turner et al [33] showed that narrow absorption bands in the VNIR-and SWIR-range are related to specific REE-elements (mostly Nd, e.g., 0.741 and 0.798 µm [33]). The lower detection limit for Nd 3+ in high albedo samples is about 300 ppm [11].…”

“…Names and the location of exploration sectors are also indicated. Coordinate system: WGS 84/UTM 33S; (e) Reflectance spectra of carbonatite sample NA-Epembe-Sec B from the northern high-grade zone with distinct absorption in the VNIR due to Nd 3+ [33]; (f) Reflectance spectra of carbonatite sample NA-14-08-18-B10. Absorption features are marked at 0.741 and 0.798 µm for Nd 3+ , 2.2 µm for Al-OH, 2.3 µm (and 2.5 µm) for Mg-OH and 2.34 µm for calcite.…”

“…Selected samples from inside (e.g., sample NA-14-08-18-B2, -B5, -Sec B, see Figure 2) and outside (e.g., sample -B8, -B10) the high-grade zones were studied in the laboratory. Spectra from the high-grade samples exhibit distinct features at 0.741 µm and 0.798 µm that correspond to Nd 3+ absorption ( [33]; see Figure 2e,f). However, the overall REE content (∼0.5% TREO + Y) is low compared to other known carbonatite hosted deposits (e.g., up to 7.98 wt% TREO + Y as at Mountain Pass/USA [31]) and does not allow a direct detection of these absorption features in HyMap data, which will be described in Section 4.2.…”

Hyperspectral REE (Rare Earth Element) Mapping of Outcrops—Applications for Neodymium Detection

“…In both example spectra, the absorption bands at ~580 nm, ~740 nm, ~800 nm, and ~870 are resolved. These bands are attributed to neodymium based on the publications of [5,16,17,[20][21][22]. Figure 1.…”

“…Two reference spectra for monazite (a neodymium-bearing mineral) and a monazite-rich rock sample (a calcitic carbonatite from the Fen complex). The key spectral characteristic absorption bands in the VNIR range that are associated with neodymium [5,16,17,[20][21][22]. The graph is modified after Boesche et al 2014 [23].…”