Spectra of the different Fe oxides

1) As the electron orbitals of the isolated Fe coordination polyhedra must be converted to the multielectron states found in the actual oxides, 

Based on these results, and in spite of considerable variation in the band positions of samples of the same Fe oxide, at least 80 % of the pure akaganeite, feroxyhyte, ferrihydrite, hematite and lepidocrocite samples could be correctly classified by Scheinost et al. (1998) by discriminant functions based on the above four bands (Fig.7.3 right) Magnetite could be identified by its band at 1500 nm, but for goethite. 

The band positions of Fe oxides are also influenced by the substitution for Fe by other cations in the structure, as indicated partly by their colour. Scheinost et al. (1999) noticed a linear shift in the position of the Ai Ti transition from 943 to 985 nm and that of the Ai T2 transition from 653 to 671 nm for 47 synthetic goethites whose Al-substitution (Al/(Al-i-Fe) ranged between 0 and 0.33 mol mol (R = 0.92 for both). Mn -substituted goethites showed bands arising from Mn near 454 and 596 nm. The overall reflectivity in the visible range decreased as structural Mn increased from 0 to 0.20 mol mol (Vempati et al., 1995). The same effect has been observed for V -substituted goethites (Schwertmann Pfab, 1994). The position of the EPT band of Mn -substituted hematite shifted to 545 nm and that of the Ai T2 transition to 700 nm (Vempati et al., 1995). The position of the same transition shifted from ca. 600 to 592 nm as the Al-substitution in hematite rose from 0 to 0.125 mol mol (Kosmas et al., 1986). Crystal size and crystal shape also have an effect on diffuse reflectance, as shown for hematite (see Fig. 6.12). As the crystals become smaller, reflectance increases and needles also reflect more than cubes, i. e. the colour becomes more vivid. 

Both Mbssbauer spectroscopy and magnetometry are based on the magnetic behaviour of (essentially) iron in a crystal structure, but operate on different dimensional scales. Whereas Mbssbauer spectroscopy yields information about charge and coordination, magnetometric methods are more sensitive to the type of magnetic coupling and to the magnetic domain status of particles. 

The Mbssbauer effect involves resonant absorption of y-radiation by nuclei in solid iron oxides. Transitions between the I = Y2 the I = 72 nuclear energy levels induce resonant absorption (Fig. 7.4). A Mbssbauer spectrum is a plot of the transmission of the rays versus the velocity of their source movement of the source ( Co for iron compounds) ensures that the nuclear environments of the absorber and the source will match at certain velocities (i.e. energies) and hence absorption takes place. In the absence of a magnetite field the Mbssbauer spectrum consists of one (if the absorbing atoms are at a site of cubic symmetry) or two (symmetry distorted from cubic) absorption maxima. When a static magnetic field acts on the resonant nuclei, this splits the nuclear spin of the ground state into two and those of the ex- 

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Figure 7.3-left shows diffuse reflection spectra of the different Fe oxides and Table 7.5 lists their transitions. Fig. 7-3 right summarizes the range of the crystal field band... 

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