Mixed-valence minerals

Table 4.2. Intervalence charge transfer energies from optical spectra of mixed-valence minerals... 

Figure 4.17 Correlations between transition energies of Fe2+ — Fe3+ IVCT bands and metal-metal distances of several mixed-valence minerals (modified from Mattson Rossman, 1987a). Circles edge-shared octahedra squares face-shared octahedra cross calculated from molecular orbital energy level calculations ( 11.7.3 Sherman, 1987a). The key to the symbols is given in table 4.2, p. 117.

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. 

Bums, R. G. (1981) Intervalence transitions in mixed-valence minerals of iron and titanium. Ann. Rev. Earth Planet. Sci., 9, 345-83. 

Bums, R. G. (1991) Mixed valency minerals influences of crystal structures on optical and Mossbauer spectra. In Mixed Valence Systems Applications in Chemistry, Physics and Biology. (K. Prassides, ed. Plenum Press, New York), NATO ASI-C Series, Math. Phys. Sci., C343,175-200. 

In the previous chapter it was shown how measurements of polarized absorption spectra in the visible to near-infrared region can provide information on such crystal chemical problems as oxidation states of transition metal ions, coordination site symmetries and distortions, cation ordering and the origins of colour and pleochroism of minerals. Much attention was focused in chapter 4 on energies of intervalence charge transfer transitions appearing in electronic absorption spectra of mixed-valence minerals. 

Intervalence charge-transfer energies of various mixed valence minerals can be found in the review by Bums. A few of these minerals are listed in Table 2. 

Ilvaite is an especially interesting mixed valence mineral because it shows one iron(II) site. Fee, which retains a unique valence up to at least 9(X) K. Apparently the transfer of an electron from Fee across the shared face to FeA is difficult in comparison with transfer between the edge-shared FeA sites. The Mossbauer parameters for the Feg site given in Table 3.12 and the temperature dependence of its isomer shift (see Figure 3.41) indicate that it remains high spin iron(II). The situation for the FeA atoms in the double chain is quite different, since below about 300 K the Mossbauer spectrum... 

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