REASSIGNMENT OF THE OPTICAL-ABSORPTION BANDS IN BIOTITES

The electronic absorption spectra of three biotites with largely differing Fe2+/Fe3+ ratios were studied before and after thermal dehydration and oxidation of divalent iron. Three absorption bands near 17,100, 20,500 and 24,100 cm-1 and an absorption edge at slightly higher energies are assigned to trivalent iron present in clusters of strongly interacting ions. The presence of additional broad absorption bands due to intervalence transfer between Fe2+ and Fe3+ or Ti4+ in this region cannot be excluded for biotites with high Fe2+ concentrations. Three bands at lower energies show a satisfactory correlation with concentration of divalent iron and decrease in the same proportions with oxidation. We therefore assign them to split components of the spin-allowed ligand field transition of Fe2+ at the M1 and M2 sites. This contradicts the assignment of one of these bands to an intervalence charge transfer between Fe2+ and Fe3+ by previous authors. It is shown that there is no indisputable evidence against our assignment. © 1979 Springer-Verlag.