Vanadium (IV) in a Multimineral Lateritic Saprolite: A Thermoanalytical and Spectroscopic Study

Abstract

Soil environments as multimineral systems are difficult to investigate by spectroscopic methods without extensive chemical pretreatment. A new approach to obviate this problem was applied to a saprolite sample from a laterite profile in southern Mali (West Africa). Spectroscopic methods (Fourier-transform infrared [FTIR] and electron spin responance [ESR]), along with calorimetry (differential thermal analysis [DTA] and thermogravimetric analysis [TGA/DTG], x-ray diffraction (XRD), and scanning electron microscopy (SEM), were used to characterize the bulk sample and its sand and silt + clay size fractions. The multimineral system consists of kaolinite, mica, and quartz. Comparison of FTIR with XRD and calorimetric data showed that the kaolinite decomposes at ≅ 520 °C, whereas the mica dehydroxylates at ≅ 600 to 780 °C, followed by structural disintegration between 810 and 980 °C. Electron spin resonance spectra of the multimineral system at room temperature showed the presence of defect free radicals, as well as Fe(III) and V(IV) as VO2+ in substituted sites in the structure of the layer silicates. Iron(III) and the free radicals occur in both kaolinite and mica. The persistence of hyperfine splitting in the V(IV) ESR signal up to 900°C indicated that VO2+ is located in octahedral sites of the mica structure. The drop of the hyperfine coupling constant from 18.6 to 17.5 mT on heating between 450 and 500 °C was interpreted in terms of a change of the ligand field around V(IV) caused by dehydroxylation at the octahedral site. The newly coordinated V(IV) site was stable up to 900 °C and disintegrated simultaneously with the bulk mica structure.