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Influence of F content on the composition of Al-rich synthetic phlogopite: Part II. Probing the structural arrangement of aluminum in tetrahedral and octahedral layers by ²⁷Al MQMAS and ¹H/¹⁹F-²⁷Al HETCOR and REDOR experiments

¹ Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, Universitätsstr. 150, 44780 Bochum, Germany
² Institut für Mineralogie, Universität Hannover, Welfengarten 1, 30167 Hannover, Germany
³ The University of British Columbia, Department of Chemistry, 2036 Main Mall, Vancouver, B.C. V6T 1Z1, Canada
⁴ The University of British Columbia, Department of Earth and Ocean Sciences, 6339 Stores Road, Vancouver, B.C. V6T 1Z4, Canada

Correspondence: ^* E-mail: Michael.Fechtelkord{at}ruhr-uni-bochum.de

The influence of F substitution on the local structure of Al in the tetrahedral and octahedral sheets of synthetic Al-rich phlogopite samples with nominal gel compositions of K(Mg_3–xAl_x)[Al_1+xSi_3–xO₁₀] (OH)_y(F)_2–y between 0.0 x 0.8 and 0.5 y 1.8, was studied by ²⁷Al MAS, MQMAS, {¹H/¹⁹F} ²⁷Al 2D CPMAS (HETCOR) and {¹H/¹⁹F} ²⁷Al REDOR solid-state NMR and by IR spectroscopy. Changes in intensity of the absorption bands in the OH-stretching region of the IR spectra clearly indicate the incorporation of octahedral Al. Signals from the different phases can be separated in the ²⁷Al MQMAS NMR spectra by generation of an isotropic dimension in F1. The ²⁷Al quadrupolar parameters of the four phases were estimated from ²⁷Al MAS NMR spectra obtained at 104.26 and 208.42 MHz. The quadrupolar coupling constant and isotropic chemical shift increases with increasing Al content for the ^IVAl site in phlogopite. The ^VIAl site shows a clear increase of the asymmetry parameter and C_Q with increasing F content. The estimated ²⁷Al signal areas show the lowest amount of impurity phases at high OH contents and a stabilization of ^VIAl sites by hydroxyl groups. The {¹H} ²⁷Al 2D CPMAS (HETCOR) NMR experiment at short contact times provides information about site neighborhoods of tetrahedral Al sites and Mg₃OH as well as Mg₂AlOH sites, whereas magnetization is only transferred to the octahedral Al sites from hydroxyl groups in Mg₂AlOH sites. The {¹⁹F} ²⁷Al 2D CPMAS (HETCOR) NMR spectrum is dominated by ^IVAl sites coupled to the Mg₃F complex in phlogopite. Resonances from Mg₂AlF complexes are not observed. Finally, the {¹H/¹⁹F} ²⁷Al REDOR experiments support the results of the 2D CPMAS (HETCOR) experiments.

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