Details

Autor(en) / Beteiligte

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

Titel

High-temperature behaviour of astrophyllite, K2NaFe72+Ti2(Si4O12)2O2(OH)4F: a combined X-ray diffraction and Mössbauer spectroscopic study

Ist Teil von

• Physics and chemistry of minerals, 2017, Vol.44 (8), p.595-613

Ort / Verlag

Berlin/Heidelberg: Springer Berlin Heidelberg

Erscheinungsjahr

2017

Quelle

SpringerLink

Beschreibungen/Notizen

• High-temperature X-ray powder-diffraction study of astrophyllite, K 2 NaFe 7 2+ Ti 2 (Si 4 O 12 ) 2 O 2 (OH) 4 F, 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 Fe 2+  + OH −  → Fe 3+  + O 2−  + ½H 2  ↑, and (2) defluorination according to the scheme Fe 2+  + F −  → Fe 3+  + O 2− . 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) Å 3 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) 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 M O 6 and D φ 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.