A combined IR and XRD study of natural well crystalline goethites (α-FeOOH)

Abstract

Goethite (α-FeOOH) is one of the most abundant minerals on the Earth surface, occurring in temperate, tropical and equatorial climates. Fe in goethite can be substituted by many cations such as Al, Ni for instance. A large amount of research has been conducted on the effect of varying elemental compositions (mainly Al-content) on the spectral features of goethites with most of the studies based on materials synthesized with different elemental ratios. The different elemental ratios, however, may not only affect the composition of the products but also their crystallinity and/or particle size and shape. Both parameters are known to affect results of both X-ray diffraction (XRD) and infrared spectroscopy (IR). These methods are predominantely used to characterize goethites. In the present study, therefore, a significant set of natural goethites was considered in order to investigate the effect of elemental composition on XRD and IR results. The focus was on crystallised samples which had a limited chemical variability but artefacts caused by the presence of admixtures could be excluded in most cases. First of all Rietveld refinement was optimized based on varying different parameters. A fairly good correlation of Rietveld derived crystallite sizes and specific surface area determined by N2-adsorption (SSA) was found which proves the importance of considering the crystallite size parameters for Rietveld refinement and at the same times proves the quality of it. Using IR spectroscopy yet published relations of band position and Al-content could be confirmed despite the fact that the range of Al-contents was small. However, the band position of the Fe–O stretching, previously used as proxy for crystallinity assessment, was found to be least variable hence contradicting yet published results. Controversial results were also published for the effect of the Al-content on the position of the asymmetric FeOH stretching band at 450 cm−1. The goethites investigated in the present study indicate that the crystallite size determines the band position rather than the Al-content which is at least valid for the limited range of Al-contents. The results of the present study indicate that using synthetic sample sets bears the problem that more than one parameter might show systematic differences (e.g. crystallite size in a set of chemically varied goethites). The paper, therefore, provides IR reference data based on a set of natural well crystallised goethites.