Temperature-induced iron oxidation in bafertisite Ba2Fe 4 2 + ${}_{4}^{2+}$ Ti2(Si2O7)2O2(OH)2F2: X-ray diffraction and Mössbauer spectroscopy study

Abstract

The high-temperature behavior of bafertisite was studied by combination of techniques in order to characterize the temperature-induced iron oxidation associated with deprotonation of an octahedral layer. The chemical formula of bafertisite from Darai-Pioz alkaline complex (Tajikistan) determined by electron-microprobe analyzes and Mossbauer spectroscopy is Ba 2.11(Fe ${}_{2.70}^{2+}$ Fe ${}_{0.17}^{3+}$ Mn 1.09Zr 0.04Na 0.03) (Ti 1.96Nb 0.07)(Si 2 O 7)2 O 2(OH 1.29 O 0.65 F 0.06)F 2. In situ high-temperature powder X-ray diffraction revealed abrupt shift of reflections to the high-angle region and reduction of their intensity at T > 525 ∘C. The Mossbauer spectroscopy studies indicated that the crystal structure of bafertisite contains Fe in octahedral sites as predominantly ferric ions with Fe 3+/ ΣFe = 0.06, whereas bafertisite annealed at T = 600 ∘C has Fe in the same position with Fe 3+/ ΣFe up to 0.39. The differential scanning calorimetry and thermogravimetric analyzes reveal the occurrence of a broad exothermic effect at T ∼ 537 ∘C associated with the mass loss corresponding to deprotonation. Since in the studied sample of bafertisite, Fe 2+ apfu strongly prevails over OH apfu, the stoichiometric (charged-balanced) high-temperature oxidized modification cannot be obtained. In the paper, the high-temperature behavior of bafertisite is discussed and compared to that of astrophyllite.