Temperature-induced iron oxidation in bafertisite Ba₂Fe${}_4^{2+}$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 Mössbauer 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 ₂O₇)₂O₂(OH _1.29 O _0.65F_0.06)F₂. 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 Mössbauer 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.