M�ssbauer spectra of kyanite, aquamarine, and cordierite showing intervalence charge transfer

Abstract

The blue colors of several minerals and gems, including aquamarine (beryl, Be3Al2Si6O18) and cordierite (Al3(Mg, Fe)2Si5AlO18), have been attributed to charge transfer (CT) between adjacent Fe2+ and Fe3+ cations, while Fe2+→Ti4+ CT has been proposed for blue kyanites (Al2SiO5). Such assignments were based on chemical analyses and on polarization-dependent absorption bands measured in visible-region spectra. We have attempted to characterize the Fe cations in each of these minerals by Mossbauer spectroscopy (MS). In blue kyanites, significant amounts of both Fe2+ and Fe3+ were detected with MS, indicating that Fe2+→Fe3+ CT, Fe2+→Ti4+ CT, and Fe2+ and Fe3+ crystal field transitions each could contribute to the electronic spectra. In aquamarines, coexisting Fe2+ and Fe3+ ions were resolved by MS, supporting our assignment of the broad, relatively weak band at 16,100 cm−1 in E∥c spectra to Fe2+→Fe3+ CT between Fe cations replacing Al3+ ions 4.6A apart along c. A band at 17,500 cm−1 in E⊥c spectra of cordierite is generally assigned to Fe2+ (oct)→Fe3+ (tet) CT between cations only 2.74 A apart. However, no Fe3+ ions were detected in the MS at 293K of several blue cordierites showing the 17,500 cm−1 band and reported to contain Fe3+. A quadrupole doublet with parameters consistent with tetrahedral Fe3+ appears in 77K MS, but the Fe3+/Fe2+ ratios from MS are much smaller than values from chemical analysis. These results sound a cautionary note when correlating Mossbauer and chemically determined Fe3+/Fe2+ ratios for minerals exhibiting Fe2+→Fe3+ CT.