Acentric coordination site

Figure 3.10 Partial energy level diagram for the Fe3+ or Mn2+ ions with 3tfi configurations in high-spin states in an octahedral crystal field. Only sextet and quartet spectroscopic terms and crystal field states are shown. Note that the same energy level diagram applies to the cations in tetrahedral crystal fields (with g subscripts omitted from the state symbols for the acentric coordination site).

The three polarized spectra of piemontite shown in fig. 4.3 are distinctive with absorption bands of different intensities occurring in each spectrum at approximately 450 nm (22,200 cm-1), 550 mn (18,200 cm-1) and 830 nm (12,000 cm-1) (Bums and Strens, 1967 Smith et al., 1982 Kersten et al., 1987). The molar extinction coefficients of these bands are one order of magnitude higher than those in absorption spectra of other Mn(III) compounds, indicating that Mn3+ ions are situated in an acentric coordination site in the epidote structure. Moreover, extinction coefficients of corresponding bands in the polarized spectra of a suite of manganiferous epidotes vary with composition,... 

Figure 3.20 Vibrational modes, u, to u7, of atoms in a square planar site such as the [FeOJ coordination environment in gillespite (from Hitchman, 1985). Note the acentric a2u, p2u ar>d eu modes that facilitate electronic transitions within Fe2+ ions by vibronic coupling.

The epidote structure illustrated in fig. 4.4 contains three positions of six-fold coordination. The M3 coordination polyhedron is acentric with irregular metal-oxygen and oxygen-oxygen distances due to compression of the site along one... 

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