Geochemical and crystal-chemical processes of scandium enrichment from the mantle to lateritic contexts - Contribution to the understanding of the processes of critical metals enrichment

Abstract : The supply of rare metals has become a great concern due to the combination of geopolitical uncertainties and increasing use in new high-technology materials. However, to secure supply, progress on the understanding of the resource is needed. Among rare metals, there is a growing potential demand for scandium (Sc), resulting from its use in high-performance alloys and solid oxide fuel cells. The current supply is not guaranteed but lateritic deposits are a promising target. Scandium could become a by-product of many nickel laterites, and even the main product of high-grade lateritic Sc ores in eastern Australia. To understand the origin of lateritic Sc enrichment, we investigated the geochemical and crystal-chemical processes forming one of these deposits (Syerston–Flemington, New South Wales). A meta-analysis of geochemical studies on mantle-derived rocks and minerals, which form the parent rocks of all known Sc-enriched laterites reveals that the most favourable lithotypes are cumulates of clinopyroxene or amphibole. It also shows a drastic change in the compatibility of Sc with increasing depth in the mantle, ranging from incompatible in the spinel facies to compatible in the garnet facies. This dichotomy is of great interest for tracing the depth of partial melting of extrusive rocks.Integrated X-ray diffraction, whole-rock and microscopic chemical analyses and X-ray absorption spectroscopy have been used to understand the processes that concentrate Sc during lateritisation. Scandium-rich lateritic profiles result from the dissolution of primary minerals containing slight but significant Sc enrichment, followed by successive trapping by smectite and iron oxides after dissolution of the clay phases. Scandium grades reflect the high capacity of goethite to adsorb this element. Along with mechanisms of dissolution–precipitation forming successive generations of goethite, it preserves Sc immobility while most other elements are leached, leading to high residual concentrations of Sc.
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Mathieu Chassé. Geochemical and crystal-chemical processes of scandium enrichment from the mantle to lateritic contexts - Contribution to the understanding of the processes of critical metals enrichment. Earth Sciences. Université Pierre et Marie Curie - Paris VI; Macquarie university (Sydney, Australie), 2017. English. ⟨NNT : 2017PA066282⟩. ⟨tel-02282375v2⟩

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