Epidote cation ordering

In aluminosilicates, too, including epidote and the Al2SiOs polymorphs andalusite, kyanite and sillimanite, Al3+ ions occupy several coordination sites. Again, trivalent cations such as Cr3+, V3+, Mn3+ and Fe3+ may show cation ordering and be relatively enriched in one specific Al3+ site in a crystal structure. 

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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