Opacity of mixed-valence oxides and silicates

Laporte and spin-multiplicity selection rules ( 3.7) and have intensities 103 to 104 times higher than those of crystal field transitions (table 3.6), their absorption edges may extend well into the visible region and overlap crystal field spin-allowed and spin-forbidden peaks. 

For cations most frequently encountered in terrestrial and lunar minerals, OMCT energies involving transition metal ions are calculated or observed to decrease in the order 

With rising temperatures and pressures, absorption edges of OMCT bands show red-shifts, absorbing increasing amounts of the visible region in spectra measured at elevated temperatures and high pressures (Bums, 1982). The importance of such effects in radiative heat transfer and electrical conductivity in the Earth s Mantle is discussed in chapter 9 ( 9.10). 

One consequence of thermally activated electron delocalization behaviour is that techniques such as Mossbauer spectroscopy, which might be expected to distinguish between discrete Fe2+ and Fe3+ valences, instead often detect Fe cations in intermediate oxidation states. Such cation species originate when 

Intermediate between these two extremes are minerals classified as Class II compounds in which the two sites are similar but distinguishable that is, both are octahedral sites, but with slightly different metal-oxygen distances, ligand orientation or bond-type. Examples include the amphibole Ml, M2 and M3 sites (figs 4.14 and 5.18), the mica trans-Ml and c -M2 sites (fig. 5.21) and babingtonite (Bums and Dyar, 1991). Such materials still exhibit properties of cations with discrete valences, but they have low energy IVCT bands and may be semiconductors. 

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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. 

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