STRUCTURAL VANADIUM AND CHROMIUM IN LATERITIC IRON-OXIDES - GENETIC-IMPLICATIONS

In following up a recent proof of a structural substitution of octahed ral Fe-III by V-III and Cr-III in synthetic goethite, the same substit ution was now recorded in goethites and hematites from lateritic crust s. Substitution up to 0.018 mole mole(-1) V/V + Fe is shown by congrue nt dissolution of Fe and V and by the presence of Fe2+ equivalent to s tructural V in an acid digest. On heating the samples up to 800 degree s 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 d issolved congruently with Fe but was not ejected from the structure on heating up to 800 degrees C. Thermodynamic considerations, taking the standard redox potential of the Fe-II/Fe-III couple adsorbed to iron oxide into account, reveal that iron oxides, formed in a partly anoxic environment, may incorporate V-III. Thus, as in clay minerals, V-III may be used as a redox-indicator for a former anoxic environment.