Use and Limitations of Second-Derivative Diffuse Reflectance Spectroscopy in the Visible to Near-Infrared Range to Identify and Quantify Fe Oxide Minerals in Soils

Abstract

Abstract--We measured the visible to near-infrared (IR) spectra of 176 synthetic and natural samples of Fe oxides, oxyhydroxides and an oxyhydroxysulfate (here collectively called "Fe oxides"), and of 56 soil samples ranging widely in goethite/hematite and goethite/lepidocrocite ratios. The positions of the second-derivative minima, corresponding to crystal-field bands, varied substantially within each group of the Fe oxide minerals. Because of overlapping band positions, goethite, maghemite and schwertmannite could not be discriminated. Using the positions of the 4Tl<----6AI, 4T2<----6AI, (4E;4AI)4---6A I and the electron pair transition (4T~ h-4Ti)<----(6Ai q-6ml) , at least 80% of the pure akaganeite, feroxyhite, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by discriminant functions. In soils containing mixtures of Fe oxides, however, only hematite and magnetite could be unequivocally discriminated from other Fe oxides. The characteristic features of hematite are the lower wavelengths of the 4"171 transition (848-906 nm) and the higher wavelengths of the electron pair transition (521-565 nm) as compared to the other Fe oxides (909-1022 nm and 479-499 nm, resp.). Magnetite could be identified by a unique band at 1500 nm due to Fe(II) to Fe(III) intervalence charge transfer. As the bands of goethite and hematite are well separated, the goethite/hematite ratio of soils not containing other Fe oxides could be reasonably predicted from the amplitude of the second-derivative bands. The detection limit of these 2 minerals in soils was below 5 g kg t, which is about 1 order of magnitude lower than the detection limit for routine X-ray diffraction (XRD) analysis. This low detection limit, and the little time and effort involved in the measurements, make second-derivative diffuse reflectance spectroscopy a practical means of routinely determining goethite and hematite contents in soils. The identification of other accessory Fe oxide minerals in soils is, however, very restricted.

Variations in the mass‐specific absorption coefficient of mineral particles suspended in water

“…The absorption properties in the spectral region from UV to near-infrared of mineral particles present in various geologic (clays, soils, and sediments) and aerosol samples have often been studied using an approach based on diffuse reflectance spectrophotometry (Hunt and Salisbury 1970;Strens and Wood 1979;Sherman and Waite 1985;Deaton and Balsam 1991;Scheinost et al 1998;Arimoto et al 2002). Measurements of diffuse reflectance spectra provide a rapid and sensitive technique for the qualitative characterization of the main spectral regions of absorption and the intensity and width of the absorption bands.…”

Separating two painting campaigns in Saul and David, attributed to Rembrandt, using macroscale reflectance and XRF imaging spectroscopies and microscale paint analysis

“…a Reflectance spectra of yellow ochre (blue color), red ochre (green color), and a mixture of red ochre and vermilion (red color). b Map showing the distribution of the reflectance spectra shown in a generally have a reflectance transition edge between 500 and 600 nm depending on their visible color, with broad absorption bands near 650 and ~ 875 nm for hematite, a red iron oxide, and ~ 950 nm for goethite, a yellow-tobrown iron oxyhydroxide [34,35]. The reflectance transition edge of the yellow ochre is ~ 545 nm, with a broad absorption centered at 950 nm similar to goethite, and that of the red ochre is ~ 585 nm with a broad absorption centered near 875 nm, indicative of hematite.…”

Variações espectrais em solos submetidos à aplicação de torta de filtro

“…Utilizando os dados espectrais, foram determinados os valores de primeira e segunda derivada como auxílio na interpretação dos dados (Scheinost et al, 1998). As curvas espectrais foram então interpretadas quanto às feições de absorção, formas e intensidades de reflectância.…”