Comparing FTIR spectra of goethite, hematite, and ferrihydrite and testing a semi-quantitative approach

Abstract Iron-(oxyhydr)-oxides like goethite, hematite and ferrihydrite affect soil properties like sorption capacity and can be distinguished by their properties like color, crystal structure (XRD), and spectroscopic data like FTIR. During dissolution–precipitation processes the iron (oxyhydr)-oxides can convert from one into the other. Some studies used Fourier-transform infrared spectroscopy (FTIR) to analyze hematite and goethite. The objective of this study was to identify absorption bands in FTIR of synthetic (oxyhydr)-oxides that allow to distinguish hematite, goethite, and ferrihydrite and to compare the FTIR of iron (oxyhydr)-oxides synthesized in the laboratory with soil born ones. Additionally, we test a semi-quantitative approach to estimate goethite/hematite weight ratios from FTIR of goethite-hematite mixtures. Comparing the FTIR spectra indicate that all studied synthetic iron (oxyhydr)-oxides show absorption bands located at wavenumber (WN) region from 1180 to 400 cm − 1 typical for each of them. Especially goethite shows characteristic absorption bands in FTIR. Hence, FTIR might be a suitable method to distinguish goethite from the other iron (oxyhydr)-oxides. Our analysis also suggests that binary mixtures of goethite and hematite can be semi-quantitatively quantified using characteristic absorption bands from FTIR. However, when analyzing soil samples such identification may become complicated by overlapping bands caused by other minerals and/or soil organic matter.