aerospace and automotive industries. In solid oxide fuel cells, the addition of Sc to the electrolyte improves conductivity and lowers the operating temperature, extending fuel cell life.Despite a crustal abundance of ca. 22 ppm (Rudnick and Gao, 2014), comparable to common elements such as copper or lead, current Sc production is limited to 10 t to 15 t per year (U.S. Geological Survey, 2016). The large ratio of ionic radius to charge of Sc 3+ hinders the concentration of scandium during most geochemical processes (Samson and Chassé, 2016). A notable exception concerns lateritic deposits developed over ultramafic-mafic rocks where Sc concentrations up to 100 ppm make it a potential by-product (Aiglsperger et al., 2016;Maulana et al., 2016). Recently, lateritic deposits with Sc concentrations high enough to mine as a primary product have been reported in Eastern Australia (Jaireth et al., 2014). Among these, the Syerston-Flemington deposit contains about 1350 t of Sc at an average concentration of 434 ppm Sc (Pursell, 2016), providing a century-long resource at the present levels of world consumption.We present the first data on Sc speciation in lateritic deposits by combining quantitative mineralogy, geochemical analysis and X-ray absorption near-edge structure (XANES) spectroscopy on drill-core samples from the lateritic profile of the Syerston-Flemington deposit. The results explain the geochemical conditions required to form such exceptional Sc concentrations and improve our understanding of the geochemical behaviour of this under-explored element. Geological ContextIn Eastern Australia, lateritic profiles developed under seasonally dry humid tropical climatic conditions that resulted in intensive weathering during the Tertiary; the present occurrences are often erosional remnants of fossil laterite (Milnes et al., 1987). The Syerston-Flemington deposit (Fig. 1a and Table S-1) is part of the lateritic cover developed over the Tout complex, an ultramafic-mafic 'Alaskan-type' intrusive complex in the Lachlan Fold Belt (Johan et al., 1989). Scandium anomalies have been found over a body of nearly pure clinopyroxenite (Fig. S-1), with Sc concentrations (ca. 80 ppm; Table S-3) twice as high as those in typical mantle clinopyroxenites (Samson and Chassé, 2016). Similar concentrations (60 ppm to 80 ppm) occur in other 'Alaskan-type' clinopyroxenites (Burg et al., 2009) and in clinopyroxenes from ocean-island basalts (Dorais, 2015). This suggests the accumulation of clinopyroxene in subvolcanic feeder conduits during fractional crystallisation of a mantle-derived melt. Anomalies of Sc concentration in the parent rock result from specific conditions of formation and are not shared by all ultramafic-mafic bedrocks. Scandium (Sc) has unique properties, highly valued for many applications. Future supply is expected to rely on unusually high-grade (up to 1000 ppm) lateritic Sc ores discovered in Eastern Australia. To understand the origin of such exceptional concentrations, we investigated Sc speciation in one of these d...

Scandium is often considered as immobile during chemical weathering, based on its low solubility. In contrast to other conservative (i.e. relatively immobile) elements incorporated into accessory minerals resistant to weathering (e.g. zirconium, thorium or niobium), the scarcity of scandium minerals indicates that the processes accounting for scandium's immobilisation are distinctive. However, the evolution of scandium speciation during weathering is unknown, limiting the understanding of the processes controlling its dynamics in the critical zone. Exceptional scandium concentrations in east Australian laterites provide the possibility of unravelling these mechanisms. We follow scandium speciation through thick lateritic profiles (> 30 m) using a multiscale mineralogical and spectroscopic approach involving electron microprobe, laser-ablation-inductively coupled plasma mass spectrometry, selective leaching and X-ray absorption near-edge structure spectroscopy, complemented by mass-transfer calculations. We show that the initial reservoir of scandium contained in the parent rock is preserved under reducing conditions occurring in the lowest horizons of the profiles. The dissolution of scandium-bearing clinopyroxene generates smectitic clays that immobilise and concentrate scandium. It is subsequently trapped in the lateritic duricrust by goethite. Scandium mobilisation appears in this horizon and increases upward as a result of the dissolution of goethite, possibly assisted by dissolved organic matter, and the precipitation of hematite. Molecular-scale analyses demonstrate that changes in speciation govern scandium dynamics, with substitution in smectitic clays and adsorption on iron oxyhydroxides playing a crucial role in scandium immobility in the saprolite and lower lateritic duricrust. The higher affinity of scandium for goethite relative to hematite drives scandium mobilisation in the upper lateritic duricrust, leading to its concentration downward in the lower lateritic duricrust. These successive mechanisms illustrate how the unique complexity of the critical zone leads to scandium concentrations that may form new types of world-class scandium deposits. Comparison with conservative elements and with rare-earth elements, expected to have similar geochemical properties, emphasizes the unique behaviour of scandium in the critical zone. While scandium remains immobile during the early stages of weathering, intense and long-term alteration processes, observed in lateritic contexts, lead to scandium mobilisation. This study highlights the dependence of scandium mobility on weathering conditions.