Mössbauer 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 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.