TY - JOUR

T1 - Influence of F content on the composition of Al-rich synthetic phlogopite

T2 - Part I. New information on structure and phase-formation from 29Si, 1H, and 19F MAS NMR spectroscopies

AU - Fechtelkord, Michael

AU - Behrens, Harald

AU - Holtz, François

AU - Fyfe, Colin A.

AU - Groat, Lee A.

AU - Raudsepp, Mati

N1 - Copyright: Copyright 2017 Elsevier B.V., All rights reserved.

PY - 2003/1/1

Y1 - 2003/1/1

N2 - The influence of F content on the formation and stability of Al-rich phlogopite has been investigated. Samples with varying OH/F-ratios and nominal gel compositions of K(Mg3-xAlx) (Al1+xSi3-xO10(OH)y (F)2-y (0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8) were dtified using 29Si, 1H, and 19F MAS NMR spectroscopies, powder X-ray diffraction, electron-probe microanalysis, and scanning electron microscopy. The synthetic phlogopites were synthesized from sol-gels in cold-seal pressure vessels at 1073 K, 2 kbar. The main phase (phlogopite) and three other impurity phases [corundum (α-Al2O3), kalsilite (KAISiO4), and potassium aluminum hexafluoride (K3AlF6.0.5H2O)] were clearly identified by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters a0 and b0 decrease whereas c0 increases with increasing Al-content (x). The average crystal size of phlogopite is about 1-2 μm. The 29Si MAS NMR spectra show up to four resonances at approximately -91, -83, and -80 ppm, which can be assigned as Q3(n Al) signals with n = 0-3. The ideal Si/IV Al ratio calculated from the initial composition is always lower than that derived experimentally. Hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets, suggesting a stabilizing effect on the formation of Al-rich phlogopite. These conclusions are supported by 1H and 19F MAS NMR spectra. The 1H MAS NMR spectra show a water signal at 4.7 ppm. a signal due to Mg2AlOH at 1.8 ppm, and a signal due to Mg3OH at 0.7 ppm. The 19F MAS NMR spectra exhibit a signal for Mg2AlF at - 150 ppm, one due to AlF6 (K3AlF6·0.5H2O) at - 157 ppm, and one for Mg3F at - 174 ppm. Comparison of the 1H and 19F MAS NMR spectra for different Al-and F-contents reveals a non-statistical distribution of F- and OH-groups at the crystallographic sites in the octahedral sheets where F prefers sites coordinated by three Mg, and OH sites with Al in the next-nearest coordination sphere.

AB - The influence of F content on the formation and stability of Al-rich phlogopite has been investigated. Samples with varying OH/F-ratios and nominal gel compositions of K(Mg3-xAlx) (Al1+xSi3-xO10(OH)y (F)2-y (0.0 ≤ x ≤ 0.8 and 0.5 ≤ y ≤ 1.8) were dtified using 29Si, 1H, and 19F MAS NMR spectroscopies, powder X-ray diffraction, electron-probe microanalysis, and scanning electron microscopy. The synthetic phlogopites were synthesized from sol-gels in cold-seal pressure vessels at 1073 K, 2 kbar. The main phase (phlogopite) and three other impurity phases [corundum (α-Al2O3), kalsilite (KAISiO4), and potassium aluminum hexafluoride (K3AlF6.0.5H2O)] were clearly identified by powder X-ray diffraction and electron-probe microanalysis. For phlogopite, the unit-cell parameters a0 and b0 decrease whereas c0 increases with increasing Al-content (x). The average crystal size of phlogopite is about 1-2 μm. The 29Si MAS NMR spectra show up to four resonances at approximately -91, -83, and -80 ppm, which can be assigned as Q3(n Al) signals with n = 0-3. The ideal Si/IV Al ratio calculated from the initial composition is always lower than that derived experimentally. Hydroxyl-rich compositions indicate an increased Al-content in the tetrahedral sheets, suggesting a stabilizing effect on the formation of Al-rich phlogopite. These conclusions are supported by 1H and 19F MAS NMR spectra. The 1H MAS NMR spectra show a water signal at 4.7 ppm. a signal due to Mg2AlOH at 1.8 ppm, and a signal due to Mg3OH at 0.7 ppm. The 19F MAS NMR spectra exhibit a signal for Mg2AlF at - 150 ppm, one due to AlF6 (K3AlF6·0.5H2O) at - 157 ppm, and one for Mg3F at - 174 ppm. Comparison of the 1H and 19F MAS NMR spectra for different Al-and F-contents reveals a non-statistical distribution of F- and OH-groups at the crystallographic sites in the octahedral sheets where F prefers sites coordinated by three Mg, and OH sites with Al in the next-nearest coordination sphere.

UR - http://www.scopus.com/inward/record.url?scp=0037221419&partnerID=8YFLogxK

U2 - 10.2138/am-2003-0106

DO - 10.2138/am-2003-0106

M3 - Article

AN - SCOPUS:0037221419

VL - 88

SP - 47

EP - 53

JO - American Mineralogist

JF - American Mineralogist

SN - 0003-004X

IS - 1

ER -