Characterization and Quantification of Aluminum-Substituted Hematite-Goethite Mixtures by X-ray diffraction, and Infrared and Mössbauer Spectroscopy
- D. D. Amarasiriwardena,
- L. H. Bowen and
- S. B. Weed
Mixtures of aluminous goethite and hematite with varying amounts of Al substitution up to about 25 mole % were analyzed by x-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), and Mössbauer spectroscopy. Crystalline dimensions decreased with Al substitution for both the hydrothermally-synthesized goethites and the hematites prepared from them by heating at 500 °C. Band positions in the infrared shifted to higher wave numbers with Al substitution for both the bending modes of goethite and the O-displacement modes of hematite. Mössbauer spectra obtained at 80 K down to 16 K were fitted with a distribution of hyperfine fields. Both the average and maximum-probability field decreased with Al substitution. The mixtures exhibited linear correlations of peak intensity with hematite concentration for all three techniques, although XRD and Mössbauer spectroscopy were more sensitive than FTIR for detection of small proportions (<50 g/kg) of hematite. Sensitivity of XRD decreased with Al substitution. The field distribution analysis was essential for obtaining quantitative ratios of hematite to goethite from Mössbauer spectra of mixtures where the hematite proportion was low.
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