Temperature-induced iron oxidation in bafertisite Ba2Fe42+Ti2(Si2O7)2O2(OH)2F2: X-ray diffraction and Mössbauer spectroscopy study

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 M¨ossbauer spectroscopy is Ba2.11(Fe2+ 2.70Fe3+ 0.17Mn1.09Zr0.04Na0.03) (Ti1 .96Nb0.07)(Si2O7)2O2(OH1.29 O0.65F0.06)F2. In situ high-temperature powder X-ray diffraction revealed abrupt shift ofreflections to the high-angle region and reduction of their intensity at T >525 ◦C. The M¨ossbauer spectroscopy studies indicated that the crystal structure of bafertisite contains Fe in octahedral sites as predominantly ferric ions with Fe3+/ Fe = 0.06, whereas bafertisite annealed at T = 600 ◦C has Fe in the same position with Fe3+/ Fe 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, Fe2+ 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