Uptake and incorporation of Al, Cr, V, Zn and Mo in hematite: Competition, synergies and influence on structural properties

This study investigated multiple (di-, tri- and tetra-)incorporation of selected minor and trace elements (Al³⁺, Cr³⁺, V^3-5+, Zn²⁺, Mo⁶⁺ and As⁵⁺) into hematite. The purpose was to improve understanding of how hematite may control trace element mobility in the environment, and how physical and chemical properties of hematite are impacted by multi-element incorporation at x/Fe molar ratios of up to 10%. Simultaneous structural incorporation of Al±Cr±V±Zn into hematite was achieved, with both synergistic and antagonistic effects occurring between certain element combinations. Cr+Al had synergistic effects on their co-incorporation, while V negatively affected Al incorporation, and both V and Zn negatively affected Cr incorporation. In contrast, Mo was minimally associated with hematite, and As prevented hematite formation completely. X-ray diffraction indicated contraction and expansion of the hematite unit-cell upon substitution was related to the ionic radius of the substituting element in single-element samples, while V predominantly controlled the direction of deviation in multi-element samples. X-ray absorption near-edge structure spectroscopy indicated V was present as a mixture of V³⁺-V⁵⁺, with a higher average V oxidation state associated with multi-element samples. Results provide new insights into trace element geochemistry within hematite, and highlight the importance of multi-element studies to better understand natural and anthropogenic systems.