ISSN:
1432-2021
Source:
Springer Online Journal Archives 1860-2000
Topics:
Chemistry and Pharmacology
,
Geosciences
,
Physics
Notes:
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 Mössbauer 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 4.6Å 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 Å 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 Mössbauer and chemically determined Fe3+/Fe2+ ratios for minerals exhibiting Fe2+→Fe3+ CT.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00307569
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