Fluorine speciation as a function of composition in peralkaline and peraluminous Na2O–CaO–Al2O3–SiO2 glasses: A multinuclear NMR study

Abstract

The incorporation mechanisms of fluorine (F) into peralkaline and peraluminous Na2O–CaO aluminosilicate glasses with ∼65mol% SiO2 (model system for phonolites) were investigated by magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy. In 19F MAS NMR spectra of the fluorine-bearing peralkaline glasses at least five F sites could be distinguished, while only three of these sites could be found in the corresponding peraluminous glasses, which shows that there are more F incorporation mechanisms in peralkaline than in peraluminous glasses. In the peralkaline glasses containing up to 6.2mol% F the following F environments were identified: F–Ca(n) at ∼−113ppm, Si–F–Na(n) or Al–F–Ca(n) at ∼−146ppm, Al–F–Al at ∼−168ppm, Al–F–Na(n) at ∼−188ppm and F–Na(n) at ∼−225ppm (“n” indicates that the number of atoms is variable or uncertain). F–Ca(n) is the most abundant site which is surprising as Ca is the least common cation in the glasses. The fraction of F–Ca(n) sites increases from 42% to 53% as the F content increases from 1.2 to 6.2mol%. The addition of up to 16.5mol% (5.3wt%) water strongly affects F speciation in peralkaline glasses and results in a decrease in the fraction of F–Al sites compared to F–Ca(n) sites. It seems that hydroxyl groups (OH) and F occupy similar Al environments and that F cannot compete with OH.In the peraluminous glasses containing up to 18.3mol% F only three F environments Si–F–Na(n) or Al–F–Ca(n) at ∼−149ppm, Al–F–Al at ∼−170ppm and Al–F–Na(n) at ∼−190ppm are observed. Al–F–Na(n) is the most abundant site with a fraction of 54–61%. The F speciation also changes with the F concentration, with a minimum in Al–F–Na(n) sites between 3.5 and 9.7mol% F.Fluorine has only a small effect on the 23Na and 29Si MAS NMR spectra. 27Al MAS NMR spectra of the peralkaline glasses show only four-coordinated Al while in the peraluminous glasses ∼5% of the Al was found to be five-coordinated. The amount of five-coordinate Al does not change with increasing F content, but the environment of the five-coordinate Al becomes more symmetric with increasing F.