High-temperature behaviour of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study

Abstract

Å 3 for the HT (annealed) modification of astrophyllite. The oxidation of iron is confirmed: (1) by the presence of an exothermic effect at 584 °C in the DTA/TG curves in an Ar-O atmosphere and its absence in an Ar-Ar atmosphere and (2) by ex situ Mössbauer spectroscopy that showed the oxidation of Fe 2+ to Fe 3+ in the samples heated to 700 °C. Deprotonation was detected by the evolution of IR spectra in the region 3600-3000 cm −1 for astrophyllite and its HT modification. Defluorination was detected by the presence of F in the electron microprobe analysis of unheated astrophyllite and the absence of F in the analysis of unpolished heated astrophyllite. The significant difference between astrophyllite and its HT modification is in the reduction of the M-O interatomic distances after heating to 500 °C and the distortion indices of the MO 6 and Dφ 6 octahedra. Thermal behaviour of astrophyllite in the 25-475 °C temperature range can be described as a volume thermal expansion with maximal coefficient of thermal expansion in the direction perpendicular to the plane of the HOH layers. In contrast, the HT phase experiences a strong contraction in the 600-775 °C temperature range, again in the direction perpendicular to the plane of the HOH layers.

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

“…The temperature-induced iron oxidation was recently reported for astrophyllite, ideally K 2 NaFe 2+ 7 Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F [19]. The crystal structures of astrophyllite and bafertisite are related and consist of the HOH blocks with alkali and alkali-earth cations in the interlayer.…”

“…6b) at 100 • C (6.6 • C −1 ) is replaced by contraction at 500 • C (−3.4 • C −1 ). Most likely, this effect indicates the reduction of thickness of the octahedral layer (t oct ) due to the shortening of Fe-O interatomic distances and flattening of MO 6 octahedra due to the iron oxidation [19]. The same is outlined by reduction of the d-value (Fig.…”

“…When compared to astrophyllite [19], the high-temperature behavior of bafertisite is similar in general features: (i) the temperature of iron oxidation (450 • C for bafertisite and 500 • C for astrophyllite); (ii) abrupt shift of some reflections to the high-angle region in the powder XRD pattern; (iii) broad exothermic effect (537 • C for bafetisite and 584 • C for astrophyllite) and (iv) evolution of Mössbauer spectra toward the increase of the Fe 3+ / Fe ratio after annealing (Table 3). However, the significant difference is in the crystallinity of the high-temperature phase.…”

“…The oxidation of 5Fe 2+ in astrophyllite is charge-compensated by: (i) deprotonation of 4(OH) − → 4O 2− and (ii) defluorination and deprotonation of the X P D site via the (F,OH) − →O 2− substitution (Fig. 6d) [19] it can be seen from its chemical formula, 1.29(OH) − cannot compensate for the oxidation of 2.70Fe 2+ . Moreover, defluorination in bafertisite is most probably impossible, due to the absence of the direct linkage of Tiϕ 6 octahedra in comparison to astrophyllite.…”

“…-[19] 2 -μ is an angle between crystallographic axes and eigenvectors (µ a1 -between α a and α 11 ; µ b2 -between α b and α 22 and µ c3 -between α c and α 33 )…”