Hematite crystal structure

The crystal structures of hematite and corundum have been determined through the use of Taue and spectral photographs, interpreted with the aid of the theory of space groups. The unit of structure is a rhombohedron with a = 55° 17 and a = 5.420 = = 0.010 A. for hematite, and with a = 55° 17 and a = 5.120 = = 0.010 A. for corundum. The space group underlying the atomic arrangement is D. ... 

Corundum-type Magnetic Oxide Surfaces. The substrate hematite with the corundum-type crystal structure is an antiferromagnet below 963 K. In the corundum-type structure of hematite, pairs of ferric ions are in a row spaced by single vacant sites along the <111> direction. The positions of ferric ions in each pair are shifted slightly upward or downward in the <111> direction. We denote these lattice positions as up and down sites (Au and A ), respectively. 

The structures of iron oxides have been determined principally by single crystal X-ray diffraction or neutron diffraction with supplementary information coming from infrared spectroscopy, electron diffraction and high resolution electron microscopy. A few years after the first successful application of X-ray diffraction to crystal structure determination, this technique was used to establish the major features of the structures of magnetite (Bragg, 1915 Nishikawa, 1915) and hematite (Bragg Bragg, 1918). 

TEM and differential X-ray line broadening (expressed by the ratio of the width at half height of the 104 relative to that of the 110 reflection) indicate that the thickness of the platy Al-hematite crystals decreases as Al/(Fe-t Al) increases (Schwertmann et al., 1977 Barron et al., 1984). It is this change in morphology, rather than the structural Al, which governs the IR spectra, in particular the shape factor and the absorp-... 

The commonest habits for hematite crystals are rhombohedral, platy and rounded (Fig. 4.19). The plates vary in thickness and can be round, hexagonal or of irregular shape. Under hydrothermal conditions, these three morphologies predominate successively as the temperature decreases (Rosier, 1983). The principal forms are given in Table 4.1. Hematite twins on the 001 and the 102 planes. The crystal structure of hematite has a less directional effect on crystal habit than does that of goethite and for this reason, the habit of hematite is readily modified. A variety of morphologies has been synthesized, but in most cases, the crystal faces that enclose the crystals have not been identified. 

Equally often, goethite and hematite have been used as model adsorbents because they have a well defined crystal structure, are widespread in nature and can be synthesized readily in the laboratory. 

Acta Cryst. B39 165-170 Pauling, L. Hendricks, S.B. (1925) The crystal structures of hematite and corundum. J. Am. Chem. Soc. 47 781-790 Pauling, L. (1929) The principles of determining the structure of complex ionic crystals. J. Amer. Chem. Soc. 51 289-296 Payne, J.E. Davis, J.A. Waite,T.D. (1996) Uranium adsorption on ferrihydrite — effect of phosphate und humic add. Radiochemica Acta 74 239-243... 

Table 3.2. shows that data collected from the literature for onset and flatband potentials for various pH generally are related. However, there are some discrepancies, especially for the onset potential of nanosized hematite in pH 13 solution. One may conclude that the onset and flat band potential are dependent on the crystal structure, surface morphology and electrolyte medium. The data listed in Table 3.2. are plotted and shown in Fig. 3.4. 

From these data it follows that when iron is precipitated in acid and neutral environments the first products should be X-ray-amorphous highly dispersed iron hydroxides, which in the course of time acquire the crystal structure of goethite or hematite. The mechanism of this process depends on kinetic factors (rate of oxidation of Fe " ), form of migration of the iron (ionic or colloidal), and acidity of the parent solution. In neutral environments ferrihydrite possibly is formed as an intermediate metastable phase, especially if the iron migrates in colloidal form or in the form of the Fe ion. The products of diagenesis of such a sediment may be both goethite (in the case of low Eh values typical of the Precambrian iron-ore process) and dispersed hematite (in the case of deposition of the oxide facies of BIF). 

Figure 27. Analysis of polarized EXAFS data, (a) EXAFS radial structure functions for irom oxide precipitates on quartz single crystal surfaces, r and m refer to the (1011 )and (1120) surface planes of quartz. The parallel and perpendicular refer to the polarization direction of the X-ray beam and thus the probe direction of the EXAFS scattering process, (b) Raw polarized EXAFS and fits for the same samples in (a), (c) Polarized stracture function simulations. Top radial stracture function for a single Fe atom within a 50-atom hematite crystal with [0001] orientation. Middle Same for 20-atom crystal. Bottom Weighted average of all Fe stracture func-tions in the 20-atom crystal. The analysis suggests highly textrrred hematite-like nanocrystals on the quartz surface but no epitaxial relationship. From Waychunas et al. (1999).

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