Epidote polarized spectra

Figure 4.9 Polarized absorption spectra of epidote...a spectrum ---------P spectrum ...

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,... 

Variations of extinction coefficients and spectrum profiles with changes in chemical composition of a mineral provide information on cation ordering in the structure. Examples involving Al3+-Mn3+ ordering in epidotes and andalusites are discussed in 4.4.2 and 4.5, and Mn2+-Fe2+ ordering in olivine is illustrated in fig. 4.8. Compositional variations of intensities of absorption bands in polarized spectra of orthopyroxenes described in 5.5.4. (fig. 5.15) have yielded Fe2+/M2 site populations (Goldman and Rossman, 1979), while similar trends in the crystal field spectra of synthetic Mg-Ni olivines described in 5.4.2.4 (fig. 5.12) have yielded site occupancy ratios of Ni2+ ions in the olivineMl and M2 sites (Hu etal., 1990). 

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