Structural vanadium and chromium in lateritic iron oxides: Genetic implications

Abstract

In following up a recent proof of a structural substitution of octahedral FeIII by VIII and CrIII in synthetic geothite, the same substitution was now recorded in geothites and hematites from lateritic crusts. Substitution up to 0.018 mole mole−1 V/V + Fe is shown by congruent dissolution of Fe and V and by the presence of Fe2+ equivalent to structural V in an acid digest. On heating the samples up to 800°C, homogenous distribution of V disappeared; i.e., V is ejected from the structure of iron oxides and oxidized to form crystalline FeVO4 (shown by XRD). As a consequence, congruent dissolution no longer takes place as V dissolved rapidly and independently of Fe. Chromium also dissolved congruently with Fe but was not ejected from the structure on heating up to 800°C. Thermodynamic considerations, taking the standard redox potential of the FeIIFeIII couple adsorbed to iron oxide into account, reveal that iron oxides, formed in a partly anoxic environment, may incorporate VIII. Thus, as in clay minerals, VIII may be used as a redox-indicator for a former anoxic environment.