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Behaviour of impurity elements during the weathering of ilmenite

¹ CSIRO Minerals, Bayview Avenue, Clayton, Victoria 3168, Australia
² Latrobe University, Albury-Wodonga Campus, Wodonga, Victoria 3689, Australia
³ ESRF, 38043 Grenoble Cedex, France

^* E-mail: ian.grey{at}csiro.au

X-ray spectromicroscopy has been applied to the characterization of weathered ilmenite sand samples from Australian localities. X-ray absorption near edge spectroscopy (XANES) studies were performed at the K-edges of the major elements Fe and Ti and minor impurity elements Mn and Cr. An extended suite of reference samples with crystallite sizes ranging from 1 nm to µm size were measured to establish if the absorption edge characteristics were influenced by crystal size effects. No changes were detected for oxides of Cr³⁺, Fe³⁺ or Ti⁴⁺, but the mixed Fe²⁺/Fe³⁺ oxide, Fe₃O₄, showed an edge shift to higher energies (by 1.5 eV) in a nanocrystalline sample. The XANES study of a composite ilmenite grain with an unweathered primary ilmenite core and a highly weathered rim showed that Fe was present as Fe²⁺ in the core and Fe³⁺ in the rim whereas Mn was present as Mn²⁺ in both core and rim. Chromium, which is incorporated into the grains during weathering, is present predominantly as Cr³⁺, although minor (~15%) Cr⁶⁺ also occurs in highly weathered grains. The absorption K-edges of Fe³⁺ and Mn²⁺ are shifted markedly (by 2–3 eV) to higher energies in titanate phases relative to the binary oxides Fe₂O₃ and MnO.

KEYWORDS: weathering, ilmenite, Australia, titanium