Fe2+-Ti4+ charge-transfer in garnets from mantle eclogites

Single crystal absorption spectra of synthetic Ti, Fe-substituted pyropes

“…The fact that the FeTi CT-band in our synthetic pyropes fits in all aspects to the corresponding band found (Platonov et al 1991) shows, that the interpretation is correct. It should be pointed out that the slope dc(d(Xve*XTi) is nearly the same for the synthetic garnets discussed here and in the natural garnets (Platonov et al 1991), when the a-values of the latter are determined from baseline-corrected spectra (Table 4).…”

“…e.g. Burns et al 1980, Abs-Wurmbach et al 1985 and that bands possibly originating from Jahn-Teller split 5T2g ~ 5Eg transitions of Ti 3 § near 22000 cm-1 and 16500 cm -I, overlap in andradite-schorlomite series with other exitations to form a complex spectrum with increasing absorbance towards higher energies, In addition, Fe 2 + Ti 4 § charge-transfer was identified in the same spectral region for natural (Ca, Mg)-bearing high-pressure garnets (Platonov et al 1991).…”

“…It should be pointed out that the slope dc(d(Xve*XTi) is nearly the same for the synthetic garnets discussed here and in the natural garnets (Platonov et al 1991), when the a-values of the latter are determined from baseline-corrected spectra (Table 4). The presence of Ti 3 + bands in the spectra of Fe, Tibearing garnets grown under low oxygen fugacities is questionable.…”

“…So far, charge-transfer (CT) bands have been identified in spectra of high-pressure garnets from xenoliths in kimberlites (Taran et al 1978) and found to originate from electronic exchange between dodecahedral Fe 2+ and octahedral Ti 4+ (Platonov et al 1991). Furthermore, it was shown from experimental work at high pressures and water fugacities, in combination with FTIR-microscope spectrometry, that Ti 4+ may substitute for A13 + [6] in pyropes with concomitant replacement of one (OH)-, in (OH)~-defect clusters, by 0 2-, providing charge balance (Khomenko et al 1993a).…”

“…Also, it was hoped to experimentally model the broad and intense Fe2+Ti 4+ chargetransfer band at 23000 cm -1, identified in spectra of (Mg, Ca)-bearing garnets from mantle eclogites (Platonov et al 1991).…”