• journal_title：American Mineralogist
• Contributor：Michael Fechtelkord ; Harald Behrens ; François Holtz ; Jeremy L. Bretherton ; Colin A. Fyfe ; Lee A. Groat ; Mati Raudsepp
• Publisher：Mineralogical Society of America
• Date：2003-
• Format：text/html
• Language：en
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：88
• issue：7
• firstpage：1046
• section：Articles

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.