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

Elena S. Zhitova, Sergey V. Krivovichev, Frank C. Hawthorne, Maria G. Krzhizhanovskaya, Andrey A. Zolotarev, Yassir A. Abdu, Viktor N. Yakovenchuk, Yakov A. Pakhomovsky, Alexey G. Goncharov

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3 Citations (Scopus)

Abstract

High-temperature X-ray powder-diffraction study of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F, and investigation of the samples annealed at 600 and 700 °C, reveal the occurrence of a phase transformation due to the thermal iron oxidation coupled with (1) deprotonation according to the scheme Fe2+ + OH → Fe3+ + O2− + ½H2 ↑, and (2) defluorination according to the scheme Fe2+ + F → Fe3+ + O2−. The phase transformation occurs at 500 °C, it is irreversible and without symmetry changes. The mineral decomposes at 775 °C. Both astrophyllite and its high-temperature dehydroxylated (HT) modification are triclinic, P-1. The unit-cell parameters are a = 5.3752(1), b = 11.8956(3), c = 11.6554(3) Å, α = 113.157(3), β = 94.531(2), γ = 103.112(2)º, V = 655.47(3) Å3 for unheated astrophyllite, and a = 5.3287(4), b = 11.790(1), c = 11.4332(9) Å, α = 112.530(8), β = 94.539(6), γ = 103.683(7)º, V = 633.01(9) Å3 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 Fe2+ to Fe3+ 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 MO6 and Dφ6 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.

Original languageEnglish
Pages (from-to)595-613
Number of pages19
JournalPhysics and Chemistry of Minerals
Volume44
Issue number8
DOIs
Publication statusPublished - 1 Sep 2017

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Scopus subject areas

  • Materials Science(all)
  • Geochemistry and Petrology

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