96 97 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 D I A M O N D L I G H T S O U R C E A N N U A L R E V I E W 2 0 2 0 / 2 1 Spectroscopy Group Beamline I18 Searching for new sources of rare earth elements Related publication: Borst A. M., SmithM. P., Finch A. A., Estrade G.,Villanova-de-Benavent C., Nason P., Marquis E., Horsburgh N. J., Goodenough K. M., Xu C., Kynický J. & Geraki K. Adsorption of rare earth elements in regolith-hosted clay deposits. Nat. Commun. 11 , 4386 (2020). DOI: 10.1038/ s41467-020-17801-5 Publication keywords: Rare earth elements; Critical metals; Clayminerals; Resources T he Rare Earth Elements (REE) are essential for high-tech industries. Most of the world’s heavy REE come from clay deposits formed by weathering of granites in China, and the extraction of rare earths is often performed with little care for the environment. A lack of understanding of how these clay deposits form and how REE are held in them hampers the search to identify alternative resources outside China. Many metals are loosely stuck (‘adsorbed’) to the outer surfaces of clay crystals. An international team of researchers led by the Universities of St Andrews and Brighton used synchrotron techniques to determine where the REE were in the deposit and what surrounded them. They investigated deposits from China and a prospective site in Madagascar. Using Diamond Light Source’s Microfocus beamline (I18) allowed them to study the samples micron by micron. Combining X-ray Absorption Spectroscopy (XAS) with Scanning X-ray Fluorescence (SXRF) element mapping showed what surrounded the metals and their distribution in the sample. Although the rock samples from the two areas are different, the REE in both stick to the clay surfaces identically. At the atomic level, the Madagascan clay deposits are the same as those currently exploited in China. Understanding how the metals stick to the clay crystals will allow us to develop easier, more environmentally friendly ways to extract them. These results hint that deposits like those currently found in China may be more widespread than we thought. The hunt is now on to find other rare earth deposits to provide alternative supplies for the technologies that underpin a carbon-free future. Rare earth elements (REE, the lanthanides and Y) are critical metals for modern technologies 1 . Their importance comes from uses in high strength magnets and other high-tech applications such as phosphors, catalysts and rechargeable batteries. At present, ~80% of rare earths produced globally are mined in China. China’s dominance over the REE market has led to global concerns because restricted access to these metals would be a bottleneck to high-tech industries elsewhere. The most valuable rare earths (i.e. Nd and the heavy REE, or HREE, Gd-Lu) are dominantly mined from Chinese clay deposits which form by intense subtropical weathering of granitic bedrock. Despite relatively low concentrations of REE (0.05 - 0.2 wt.% REE oxides) compared to some hard-rock REE ores, the metals can easily be extracted via in situ or heap leaching 2 . The key requirement for this to work is that the majority of REE (>50%) are loosely stuck, or ‘adsorbed’to the surfaces of clay minerals. Similar weathering profiles occur outside China and might provide alternative sources for REE, allowing a wider supply network. However, the exact nature of these deposits remained unclear and the detailed mechanisms by which they form was still poorly understood. This study combines X-ray Absorption Spectroscopy with detailed geochemicalandmineralogicalworktoidentifythedistributionandcoordination of REE in economically mineralised weathering profiles from Southern China, and compares these to clay deposits from Madagascar. Synchrotron X-ray Fluorescence (SXRF) maps, X-ray Absorption Near Edge Structure (XANES) and Extended X-ray Absorption Fine Structure (EXAFS) data were collected on the I18 beamline to determine inwhich phases the REE are hosted at different levels along the weathering profile, and to determine the local bonding environment of the easily leachable REE fraction as well as the non-recoverable REE fraction that is built into mineral structures. Yttrium K-edge (17038 eV) and Nd L 3 -edge (6208 eV) X-ray absorption spectraweremeasured as proxies for light and heavy REE, respectively.The overall aim is to identify whether clay deposits in northern Madagascar represent direct structural analogues to the easily leachable Chinese deposits. TheweatheringprofilesfromChinaweredevelopedonabiotitegranitefrom Zhaibei, Yiangxi province 3 , which are economically mined for REE. The sample selected for detailed study was a strongly weathered sample (pedolith) with the highest concentration of ammonium sulphate leachable REE (1000 mg/kg REE, of which 37% are HREE and Y), from the upper 2 m of a 12 m thick weathering profile. Rare earths in the granitic bedrock are hosted in minerals such as zircon, apatite, monazite, REE-fluorcarbonates, xenotime and fluorapatite 3 . Intense weathering along the soil profile replaced the original minerals with Fe-Mn oxides and clay minerals. Scanning Electron Microscopy (SEM) and X-ray Diffraction (XRD) examination show the clays dominantly consist of kaolinite, K-feldspar, quartz and iron oxides (goethite) and minor illite (Fig 1a). Samples from Madagascar were taken from weathering profiles over peralkaline granite and syenite from the Ambohimirahavavy complex caldera complex 4,5 . The rocks underlying these profiles generally have higher REE concentrations and are more enriched in heavy REE than the granitic rocks that underlie the Chinese deposits. The hypothesis was that weathering profiles developed over better precursors may be more favourable for the development of high grade and HREE rich clay-hosted REE deposits, and provide more attractive and economically viable alternatives to the Chinese deposits. Samples selected for this study were previously characterised by Estrade et al 5 , and chosen to reflect a range in exchangeable REE grades. This included a strongly weathered sample (pedolith) with high grades in leachable REE (1962 mg/ kg REE, of which 29% HREE) developed over bedrock with REE-rich pegmatite veins 5 . The mineralogy was similar to the Chinese pedolith sample, dominantly composed of kaolinite, minor halloysite, K-feldspar, Fe and Mn oxyhydroxides with accessory zircon and cerianite (Fig 1b). We also analysed partially weathered rock (saprolite) from deeper parts of the profile with more primary minerals still preserved, including K-feldspar, zircon and a range of primary Nb- Ti-Zr minerals. Synchrotron SXRF element maps of selected areas demonstrate local enrichment of Y (in red, Fig 1) along boundaries of clay minerals (kaolinite and halloysite) in both the Chinese and Madagascar pedolith and saprolite samples. The results summarised here focus on the easily leachable (economic) REE fraction. XANES data for the clay-hosted REE all show identical Y K-edge XANES and are compared to mineral standards in which the REE occupy differently coordinated sites (Fig 2a).The XANES spectra forY associated with clay minerals in the pedolith and saprolite samples are near-identical toY in aqueous solution, and similar to the rare earthminerals parisite-(Ce) and bastnasite-(Ce), in which the REE occupy sites surrounded by 8 to 9 other elements (mostly oxygen). ThestructuralstateofYassociatedwithkaoliniteinthepedolithandsaprolite samples was further constrained from quantitative EXAFS refinements. Best fits were obtained for a configuration where Y occurs in simple hydration spheres surrounded by 8.1 - 8.6 ± 0.9 oxygens (of water molecules), at average distances of 2.35 – 2.38 ± 0.01 Å. The fit obtained for this configuration of Y in the clays was within error of the best fit obtained for Y in aqueous solution (Fig 2b). This suggests that Y are indeed fully hydrated, and not directly bound to the clay structure. Furthermore, the radial distribution functions (Fig 2b) show no evidence for a scattering peak at 4 Å, suggesting there are no atoms of Al or Si within a radial distance of 4 Å as would be expected for inner-sphere adsorption mechanisms (Fig 3). These results indicate that REE in both the Chinese and Madagascar samples are present as 8- to 9-coordinated, fully hydrated, outer sphere complexes which are loosely adsorbed onto the flat basal surfaces of kaolinite (Fig 3), and not tightly bound as inner sphere 5-6 coordinated edge or surface complexes (Fig 3). It can therefore be concluded that REE-adsorbed kaolinites from Madagascar and China are direct structural analogues. Our data explain the easily leachable nature of these economically important ore types and suggest a common adsorption mechanism at both locations. The identification of a common adsorption mechanism confirms that this process operates globally, and further supports the search for critical rare earth deposits in the weathering environment. References: 1. Goodenough K. M. et al. The Rare Earth Elements: Demand, Global Resources, and Challenges for Resourcing Future Generations. Nat. Resour. Res. 27 , 201–216 (2018). DOI: 10.1007/s11053-017-9336-5 2. Wall F. et al. Responsible sourcing of critical metals. Elements 13 , 313–318 (2017). DOI: 10.2138/gselements.13.5.313 3. Wang L. et al. Petrological and geochemical characteristics of Zhaibei granites in Nanling region, Southeast China: Implications for REE mineralization. Ore Geol. Rev. 64 , 569–582 (2015). DOI: 10.1016/j. oregeorev.2014.04.004 4. Estrade G. et al. Unusual evolution of silica-under- and -oversaturated alkaline rocks in the Cenozoic Ambohimirahavavy Complex (Madagascar): Mineralogical and geochemical evidence. Lithos 206 – 207 , 361–383 (2014). DOI: 10.1016/j.lithos.2014.08.008 5. Estrade G. et al. REE concentration processes in ion adsorption deposits: Evidence from the Ambohimirahavavy alkaline complex in Madagascar. Ore Geol. Rev. 112 , 103027 (2019). DOI: 10.1016/j.oregeorev.2019.103027 Funding acknowledgement: This work was supported by the NERC SoSRARE consortium (grant numbers NE/ M010856/1, NE/M011267/1 and NE/M01116X/1) awarded to the University of St Andrews, University of Brighton and the British Geological Survey and analyses were carried out at the I18 beamline (grants SP14793 and SP15903). Corresponding author: Dr Anouk Borst, KU Leuven and Royal Museum for Central Africa, Belgium,
[email protected] Figure 1: SXRF elemental maps (500x500 µm) showing Y (red), Fe (blue), Mn (green) counts in (a) Chinese pedolith sample with high concentrations of leachable REE and (b) a REE-rich lower pedolith sample from the studied profiles in Madagascar. Note heterogeneous enrichment of Y along kaolinite grains . Adapted frommain article. Figure 2: (a) Normalized Y K-edge XANES for REE-mineral standards with REE in different coordination states (Coordination Numbers: CN=9 in green, CN=8 in red, CN=6 in blue). Black lines represent Y XANES for clay-hosted Y in the Malagasy and Chinese pedolith and saprolite samples. (b) Radial distribution functions and EXAFS fits for clay-hosted Y in the samples and Y in aqueous solution, showing best fit coordination numbers for Y and average bond distances of its nearest neighbours. Measured spectra shown in black and fits shown in red. Figure 3: Schematic adsorption model of rare earth elements onto kaolinite. Crystal structure shows the 1:1 stacking of Al-octahedral (O) and Si-tetrahedral (T) sheets in kaolinite. XAS data demonstrate that Y are dominantly present as 8-fold hydrated outer-sphere basal surface complexes on the aluminol surface (O sheet). The EXAFS data show no evidence for Al or Si scattering within 4 Å, thereby excluding the inner-sphere adsorption models. Adapted frommain article.