ARTICLE https://doi.org/10.1038/s41467-020-17801-5 OPEN Adsorption of rare earth elements in regolith-hosted clay deposits ✉ Anouk M. Borst 1 , Martin P. Smith 2, Adrian A. Finch 1, Guillaume Estrade 3, Cristina Villanova-de-Benavent 2, Peter Nason2, Eva Marquis 2, Nicola J. Horsburgh 1, Kathryn M. Goodenough4, Cheng Xu5,6, Jindřich Kynický7,8 & Kalotina Geraki 9 Global resources of heavy Rare Earth Elements (REE) are dominantly sourced from Chinese 1234567890():,; regolith-hosted ion-adsorption deposits in which the REE are inferred to be weakly adsorbed onto clay minerals. Similar deposits elsewhere might provide alternative supply for these high-tech metals, but the adsorption mechanisms remain unclear and the adsorbed state of REE to clays has never been demonstrated in situ. This study compares the mineralogy and speciation of REE in economic weathering profiles from China to prospective regoliths developed on peralkaline rocks from Madagascar. We use synchrotron X-ray absorption spectroscopy to study the distribution and local bonding environment of Y and Nd, as proxies for heavy and light REE, in the deposits. Our results show that REE are truly adsorbed as easily leachable 8- to 9-coordinated outer-sphere hydrated complexes, dominantly onto kaolinite. Hence, at the atomic level, the Malagasy clays are genuine mineralogical analogues to those currently exploited in China. 1 School of Earth and Environmental Sciences, University of St. Andrews, St. Andrews KY16 9AL, UK. 2 School of Environment and Technology, University of Brighton, Brighton BN2 4GJ, UK. 3 GET, CNRS, IRD, UPS, University of Toulouse, Toulouse, France. 4 British Geological Survey, The Lyell Centre, Research Avenue South, Edinburgh EH14 4AP, UK. 5 College of Earth Sciences, Guilin University of Technology, 541006 Guilin, China. 6 School of Earth and Space Sciences, Peking University, 100871 Beijing, China. 7 Department of Geology and Pedology, Mendel University, Zemedelska 1, 61300 Brno, Czech Republic. 8 BIC Brno Spol. s.r.o., Technology Innovation Transfer Chamber, Purkyňova 648/125, 61200 Brno, Czech Republic. 9 Diamond Light Source, Physical Science, ✉ Harwell Science Campus, Didcot OX11 0DE, UK. email: [email protected] NATURE COMMUNICATIONS | (2020) 11:4386 | https://doi.org/10.1038/s41467-020-17801-5 | www.nature.com/naturecommunications 1 ARTICLE NATURE COMMUNICATIONS | https://doi.org/10.1038/s41467-020-17801-5 egolith-hosted ion-adsorption deposits (IADs), formed by REEs. XAS was also used to study the speciation of Sc in laterite subtropical weathering of igneous rocks, are the world’s profiles of ultra-mafic rocks, where Sc was found to be adsorbed R 32 primary source for heavy rare earth elements (HREEs, Gd- dominantly to iron oxides . Although these studies provide Lu and Y)1,2. These elements, along with some of the other important insights into the geochemical behaviour and deport- rare earth elements (REEs), are considered critical to society ment of Ce and Sc during weathering, they do not constrain the because of their ubiquitous use in modern technologies and distribution and speciation of the economically significant ele- renewable energy solutions, and because limits on supply would ments HREE and Nd in association with lateritic clays developed hinder global economic and technological development3–5. on rare earth bearing igneous protoliths. Here, we measure Y K- Regolith-hosted IADs, also referred to as ion-adsorption clays, edge and Nd L3-edge X-ray absorption spectra, as proxies for weathering crust elution- or laterite-hosted rare earth deposits6–9, HREE and light REE, respectively, from regolith samples and readily liberate the metals following mild acidification during leachates to quantify the structural state of REEs associated with addition of ammonium sulfate leach solutions10. In such deposits, clay minerals as well as relict phases. Our results provide novel the REEs are inferred to be weakly adsorbed onto clay minerals constraints on the speciation and coordination of these elements (dominantly kaolinite and halloysite), as well as oxides, at a in regoliths of economic interest, and demonstrate that the rare range of structural sites, including broken edge sites, charged earths are genuinely adsorbed to clay minerals. aluminol or siloxane groups at defects and isomorphic substitu- 3+ 4+ 3+ 3+ tions (e.g. Al for Si ,Fe for Al ), the hydration shells of Results exchangeable cations or by direct substitution of exchangeable Locality descriptions. Regolith profiles in the Zhaibei region, cations 11,12. However, such behaviour is also consistent with the southern Jiangxi province, developed through subtropical dissolution of nanoparticulate secondary REE-bearing phos- weathering of Jurassic peraluminous biotite and muscovite phates, (fluor)carbonates, or colloid particles13. Based on the granites (188 ± 0.6 Ma)17. The laterites range in thickness between assumption that the exchangeable REEs are clay adsorbed, the 5 and 30 m26. The Zhaibei granite is dominantly composed term IAD is ubiquitously used in the literature to describe easily of quartz, K-feldspar, plagioclase and biotite, with small quantities leachable REE deposits associated with lateritic weathering. of muscovite and amphibole. The REEs are dominantly hosted However, despite their global economic and scientific importance, in the micas and amphibole, as well as accessory phases, such the adsorption mechanisms of REEs in lateritic deposits of as zircon, monazite, Ca-REE fluorcarbonates, fergusonite-(Y), commercial value remain poorly understood and the coordina- aeschynite-(Y), xenotime, titanite, rutile, ilmenite and tion of REEs in such deposits has never been measured in situ. fluorapatite17,26. Samples were obtained from surface-exposed The majority of economically exploited IADs occur in South- profiles in a 12-m-thick mineralised laterite (LJ316, Fig. 2). The ern China, where the REEs are hosted in the weathered crusts of exchangeable fraction of REEs in the LJ316 profile demonstrates a igneous, mostly granitic, bedrock7,8,12. They account for c. 35% of moderate LREE enrichment and negative Ce and Eu anomalies China’s total REE production and roughly 80% of global HREE (Supplementary Fig. 1), containing up to 29–44% HREEs (Sup- supplies4,6, despite being low grade (0.05–0.2 wt% total RE O , 2 3 plementary Fig. 2). incl. Y O ) and relatively low tonnage compared to hard-rock 2 3 The Malagasy regolith profiles developed on highly hetero- REE deposits associated with carbonatites and alkaline igneous geneous bedrock comprising silica-oversaturated and -under- rocks1,14. Economic exploitation of IADs is viable through REE saturated syenites, peralkaline granites and pegmatites, as well as extraction by low-cost in situ or heap leaching8,12,15, a process volcanic lithologies belonging to the Cenozoic Ambohimiraha- that has had significant environmental impact in China6. The key – vavy subvolcanic ring complex (c. 23 Ma)27 29,33,34. The country requirement is that the majority of REEs are readily liberated rocks (Mesozoic marl, limestone and mudstone) and the main using ionic solutions and are hence ion exchangeable10. syenites have low abundances of primary REE-bearing minerals, Weathering profiles developed on protoliths with a primary whereas the peralkaline granite, nepheline syenite and pegmatite enrichment in the mid and HREEs, such as peralkaline igneous dykes contain a wide range of REE-rich minerals, including REE rocks, have become a key target for exploration16,17. Many pro- fluorcarbonates, zirconosilicates, silicates, oxides and minor spective weathering profiles, some of which are HREE enriched, phosphates. Many of these comprise a secondary paragenesis have recently been studied outside China, including in Malawi, formed through late-magmatic hydrothermal alteration of Madagascar, USA, Brazil, the Philippines, Laos, Thailand and – primary igneous phases, such as eudialyte-group minerals Myanmar18 25 (Fig. 1). These may provide alternative supply of (EGMs) (Na-Ca-zirconosilicates)35, alongside Nb minerals, such HREE, but it is unclear whether the clays in these profiles are as members of the pyrochlore group and aeschynite-(Y)28,34. The direct structural analogues to the economically exploited Chinese Malagasy regolith profile is up to 30 m thick and comprises, from clay deposits and whether similar REE adsorption mechanisms the bottom up: unaltered bedrock, saprock, saprolite and a operate within them12. pedolith made up of a mottled zone, a ferruginous zone and a This work compares economically mineralised regoliths from topsoil (Fig. 2). The supergene mineralogy includes gibbsite, Mn Southern China, developed on the Zhaibei granite, Yiangxi and Fe oxyhydroxides and clay minerals, primarily kaolinite, Province17,26, to prospective regolith profiles on peralkaline minor halloysite (both 7 and 10 Å) and illite28. granites and syenites of the Ambohimirahavavy complex, northern Madagascar27–29 (Fig. 1). Using X-ray absorption spectroscopy (XAS), including X-ray absorption near-edge Sample characterisation. The Zhaibei pedolith sample chosen for structure (XANES) and extended X-ray absorption fine struc- XAS analyses was taken from the upper 2 m of the LJ316 laterite ture (EXAFS), micro synchrotron X-ray fluorescence (μSXRF) profile, which had the highest exchangeable REE fraction (1000.9 element mapping, and scanning electron microscopy (SEM), we mg kg−1, 37% HREEs; Supplementary Figs. 1 and 2; Supple- identify the microscale distribution and local bonding environ-