Identification and quantification of iron oxides by diffuse reflectance spectroscopy with Praying Mantis accessory and integration sphere

Soil color is one of the main attributes used in the Brazilian System of Soil Classification and is related to the presence and concentration of iron oxides. This work aimed to identify and quantify the iron oxides (hematite, goethite and maghemite) present in soils and synthetic minerals and compare the diffuse reflectance spectroscopy (DRS) data equipped with the Integration Sphere (DRS-IS) or the Praying Mantis (DRS-PM) accessories with reference values obtained by XRD. Four samples of synthetic iron oxides (hematite, goethite, maghemite, magnetite) and two samples of aluminosilicates (kaolinite and smectite) were selected along with the A and Bw Horizons from 6 Oxisols of the State of Parana-Brazil. The soils were characterized on their physical and mineralogical attributes and organic carbon content. All minerals, soil samples, soils clay fractions treated and not treated with the boiling 5 M NaOH procedure and with 1.8 M H2SO4 were analyzed using the integration sphere, the Praying Mantis accessory and color determined in the Munsell scale using a Minolta CR 400 spectroradiometer. The DRS-PM, by means of the second derivative of Kubelka-Munk function was very promising for identification and quantification of hematite for presenting results close to those obtained by XRD. Otherwise, the quantification of goethite was overestimated in both DRS's measurements due to the presence of organic matter. It was not possible to identify and quantify the presence of maghemite in soils and mixture minerals samples using both DRSs accessories due to the superposition of secondary bands of goethite, hematite, and the magnetite on the maghemite spectrum. The presence of maghemite in the soil material induces an overestimation of hematite content by DRS. The estimation of the Hm/(Hm + Gt) and Gt/(Gt + Hm) ratios by both accessories presented good correlation with data obtained by XRD.

Automated summary

This study aimed to identify and quantify iron oxides in soils using diffuse reflectance spectroscopy (DRS) and compare the results with X-ray diffraction (XRD). The DRS-PM technique showed promising results for hematite identification and quantification, but overestimated the quantification of goethite. Both DRS accessories were unable to accurately identify and quantify maghemite due to overlapping bands with other minerals.