Crystal structure magnetite

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). 

This form of magnetism can be demonstrated by means of lodestone or magnetite (Fe304) which freely occurs in nature. A unit of this contains one Fe2+ ion, two Fe3+ions and four O2 ions. Its crystal structure is a cubic close packing of oxygen ions with an Fe 3+ ion in 1/8 of the tetrahedral cavities, an Fe 3+ ion in 1/4 of the octahedral cavities and an Fe2+ions in 1/4 of the octahedral cavities. Magnetic dipoles at tetrahedral sites line up antiparallel to the external field and dipoles in the octahedral cavities line up parallel to the field. 

Figure 4.18 The spinel crystal structure adopted by magnetite, Fe2+Fe3+204. Note the three-dimensional infinite chains of edge-shared [FeOe] octahedra extending along [110] directions, which accommode Fe2+ and Fe3+ ions separated by 297 pm. Fe3+ ions also occur in isolated tetrahedra linking the octahedral chains.

Opacity of mixed-valence minerals. The opacities of many end-member Fe2+-Fe3+ oxide and silicate minerals result from electron hopping between neighbouring cations when they are located in infinite chains or bands of edge-shared octahedra in the crystal structures. Opaque minerals such as magnetite, ilvaite, deerite, cronstedtite, riebeckite and laihunite owe their relatively high electrical conductivities to thermally activated electron delocalization, contributing to intermediate valence states of iron cations which may be detected by Mossbauer spectroscopy. 

Many of the mineralogically important transition-metal oxide phases contain more than one cation species, or more than one type of coordination site for the cations. Commonly, the cations are in more than one oxidation state. Examples include ilmenite (FeTiOj) and the family of minerals with the spinel-type crystal structure, including magnetite (Fe304), chromite... 

The crystal structure of silver nudybdate has been shown to be similar to that of. qjinels and magnetite. See Wyckoff. J. Amer. Chem. Soc., 1922, 44, 1994. 

Figure 6 Magnetite. The crystal structure of magnetite, a member of the spinel group of minerals. The [111] crystallographic direction is vertical to show the horizontal appearance of cubic-close-packed oxygen (O) atoms. Fe and Fe are in tetrahedral and in octahedral interstices (both dark colored) (after Gaines et al., 1997, p. 293).

Figure 1. The crystal structure of magnetite (a) expanded in [001] direction (b) overview of individual anion layers. Large open circles represent oxygen anions small open circles, tetrahedral cations small filled circles, octahedral cations and v s, vacant octahedral sites.

We can estimate saturation values of M and B by knowing the crystal structure and lattice parameter of the material and the orientation of the spin magnetic moments. We will illustrate this approach for magnetite. The magnetic moments of each type of ion are summarized in Table 33.7. The Fe ions... 

Attempted lattice energy calculations, as well as the experimental observation of different cation distributions in spinels containing ions of similar charge and size, suggest that the difference in electrostatic lattice energy between normal and inverse spinel is small in common 2-3 and 2-4 spinels. Other factors, such as the site preference of individual cations, will then determine the cation distribution. The crystal structure of magnetite is shown in Figure 9.2. 

---