Characterization and quantification of aluminum-substituted hematite-goethite mixtures by X-ray diffraction, and infrared and Mossbauer spectroscopy
1988
Amarasiriwardena, D.D. | Bowen, L.H. | Weed, S.B.
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 Mossbauer spectroscopy. Crystalline dimensions decreased with Al substitution for both the hydrothermally-synthesized goethites and the hematites prepared from them by heating at 500 degrees 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. Mossbauer 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 Mossbauer spectroscopy were more sensitive than FTIR for detection of small proportions (less than 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 Mossbauer spectra of mixtures where the hematite proportion was low.
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