Resolving the structural state of heavy rare earth elements in lateritic ion adsorption clays

Lateritic Ion Adsorption Deposits (IADs) are the world's dominant source for heavy rare earth elements (HREE: Gd-Lu), currently mostly mined from China. IADs in Brazil, Madagascar and South East Asia may provide alternative supply for HREE. However, the exact nature of REE in the IADs is unclear; for example whether deposits elsewhere are directly analogous to the easily-leachable Chinese laterites, and whether the REE are truly adsorbed, structurally bound in clays or hosted in other mineral phases.

This study compares economically mineralized IADs from the Zhaibei granite, China to prospective IADs developed on peralkaline igneous rocks from Madagascar. We use synchrotron X-Ray Fluorescence (SXRF) element mapping and X-ray Absorption Spectroscopy (XAS) to study the distribution and coordination state of light and heavy REE. We explore sites of adsorption to clays and the presence of HREE in other mineral phases. The Malagasy and Chinese laterites have kaolinite as the dominant REE-hosting clay phase, with minor halloysite. XAS data demonstrate that the REE occur as 8 to 9-coordinated outer-sphere basal surface complexes on kaolinite, rather than 5- or 6-coordinated edge complexes, or 6- or 8-coordinated interlayer complexes, thus confirming the truly adsorbed nature of REE in lateritic IAD's from both localities.