Spectral analysis of Fe oxidation in the early stages of weathering and soil formation

Abstract

Chemical weathering involves the loss of bases and silica, as well as the oxidation of Fe. Using visible and nearinfrared spectroscopy, we investigated Fe oxidation at the beginning of weathering to form a soil. Rock (micaschist) and overlying soil (Leptosol) samples of the gravel, fine earth, coarse sand, fine sand, silt, and clay fractions (n = 69) were collected at four sites (S1-S4) of the Sierra Nevada Mountains (SE Spain). Micas, quartz, and chlorite were the main minerals in the rocks, which had 6–8 wt% Fe2O3. Continuum-removed spectra of the rock samples showed absorption bands near 380 and 480 nm due to octahedral Fe(III) electronic transitions, a multi-band absorption from 600 to 2100 dominated by strongly absorbing Fe(II) bands, as well as Al-OH and Fe- OH bands in the 2200–2500 nm region. The main change in the soil, progressively towards the smaller fraction size, was growing intensity of the bands at 380 and 480 nm as the multi-band absorption decreased, mainly near 900 nm (r = -0.78). This spectral change indicates a progressive release and oxidation of Fe from rock to the soil clay fraction. The second derivative of K/S in the visible range confirmed the decrease of Fe(II) (600 nm) and increase of Fe(III) as yellowish Fe oxides (420 and 480 nm) and reddish hematite (540 nm) from rock to clay. The reddish hematite resulting by weathering can also be differentiated from rock-forming hematite (520 and 560 nm). The consequent color change evaluated in the CIELAB space with the parameters hab and C*ab was an even more consistent index of the weathering than were the chemical indices. In short, spectral and color measurements prove useful for characterizing the early stages of weathering and soil formation when the oxidation is the dominant process.