Optical absorption, Mössbauer, and FTIR spectroscopic studies of two blue bazzites

Two samples of bazzite, a very rare Sc analog of beryl, from Tørdal, Telemark, Norway and Kent, Central Kazakhstan were studied by electron microprobe, optical absorption, and Mössbauer spectroscopies; the latter sample was also studied by FTIR. Electron microprobe results show that the Norway bazzite is composed of two bazzites with slightly different FeO contents, viz. 5.66 and 5.43 wt%. The Kazakhstan sample consists of several varieties of bazzite displaying strong differences in iron, manganese, magnesium, and aluminum contents (in wt%): FeO from 2.02 to 6.73, MnO from 0.89 to 2.98, MgO from 0.37 to 1.86, and Al₂O₃ from 0.30 to 1.30. Mössbauer spectroscopy shows different degrees of iron oxidation. The Norway bazzite is completely Fe²⁺, while the Kazakhstan sample contains roughly equivalent Fe³⁺ and Fe²⁺ accommodated in the octahedral site. The difference in iron oxidation causes strong variations in the intensity of the broad optical absorption band around 13,850 cm^-1, which is assigned to Fe²⁺ → Fe³⁺ IVCT; as a result, there are strong differences in the intensity of blue color. Dichroism (E||c ≫ E⊥c) is much stronger in the Kazakhstan sample than in the Norway one. Intensities of the electronic spin-allowed bands of ^[6]Fe²⁺ at ~8900 and ~10,400 cm^-1 are somewhat higher in the latter than in the former. FTIR spectra of the sample from Kent show the presence of only water type II molecules with the H-H vector perpendicular to the c-axis, in contrast to more typical beryls that always show at least weak minor bands of H₂O I. This result shows that trapped water molecules in structural channels of studied bazzite occupy only sites next to or between six-membered rings centered by Na atoms. Definite structure can be observed in the vicinities of ν₂ and ν₃ peaks. Peaks at 1621 and 3663 cm^-1 are assigned to "doubly coordinated" H₂O (IId), whereas maximums at 1633 and 3643 cm^-1 likely represent "singly coordinated" H₂O (IIs). Interpretation of the third components in complex ν₂ and ν₃ bands needs further investigations.