Coordination symmetry of iron and cobalt in staurolite

3 Coordination symmetry of iron and cobalt in staurolite The crystal field spectra of Fe2+ ions surrounded by oxygen in regular octahedral sites normally contain absorption bands centred near 1,000 nm or 10,000 cm-1 (see fig. 3.2). By changing from octahedral to tetrahedral coordination, absorption bands for tetrahedral Fe2+ ions would, according to eq. (2.7), be predicted to occur at (% x 10,000) or 4,444 cm-1 (2,250 nm) if the iron-oxygen distances remain identical in the two coordinations. Tetrahedrally coordinated Fe2+ ions in spinel, MgAl204, for example, produce an absorption band near 4,830 cm 1 (2,070 nm) ( 5.3.3). 

The polarized spectra of lusakite, the cobaltian staurolite from Zambia (Cech et al., 1981), which are illustrated in fig. 4.7 together with (inset) the spectra of the hexahydrated Co2+ and tetrahedral Co2+ cations in spinel, show that Co2+ ions also occur in tetrahedral coordination in the staurolite structure. The profiles and intensities of the triple-peak absorption band between 500 nm and 650 nm are characteristic of tetrahedral Co2+ ions. 

Absorption spectra of cobalt(II) in octahedral and tetrahedral coordination in aqueous solution (...) and spinel (—), respectively. 

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