title: High-temperature behaviour of astrophyllite, K2NaFe72+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study
metadata.name: dc.date.accessioned
metadata.value: 2020-09-09T07:32:16Z
metadata.lang: -
metadata.name: dc.date.available
metadata.value: 2020-09-09T07:32:16Z
metadata.lang: -
metadata.name: dc.date.issued
metadata.value: 2017
metadata.lang: -
metadata.name: dc.identifier.uri
metadata.value: http://hdl.handle.net/11701/19308
metadata.lang: -
metadata.name: dc.description.abstract
metadata.value: 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.
metadata.lang: en_GB
metadata.name: dc.title
metadata.value: High-temperature behaviour of astrophyllite, K2NaFe72+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study
metadata.lang: en_GB
resources.filename: 6.pdf
resources.url: https://dspace.spbu.ru/bitstream/11701/19308/1/6.pdf
resources.format: Adobe PDF
resources.size: 2,28 MB