TY - JOUR

T1 - High-temperature behaviour of astrophyllite, K2NaFe7 2+Ti2(Si4O12)2O2(OH)4F

T2 - a combined X-ray diffraction and Mössbauer spectroscopic study

AU - Zhitova, Elena S.

AU - Krivovichev, Sergey V.

AU - Hawthorne, Frank C.

AU - Krzhizhanovskaya, Maria G.

AU - Zolotarev, Andrey A.

AU - Abdu, Yassir A.

AU - Yakovenchuk, Viktor N.

AU - Pakhomovsky, Yakov A.

AU - Goncharov, Alexey G.

PY - 2017/9/1

Y1 - 2017/9/1

N2 - 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.

AB - 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.

KW - Astrophyllite

KW - Crystal structure

KW - Defluorination

KW - Dehydroxylation

KW - Mössbauer spectra

KW - Thermal iron oxidation

UR - http://www.scopus.com/inward/record.url?scp=85016428579&partnerID=8YFLogxK

U2 - 10.1007/s00269-017-0886-1

DO - 10.1007/s00269-017-0886-1

M3 - Article

AN - SCOPUS:85016428579

VL - 44

SP - 595

EP - 613

JO - Physics and Chemistry of Minerals

JF - Physics and Chemistry of Minerals

SN - 0342-1791

IS - 8

ER -