Fluorine in silicate glasses: a multinuclear nuclear magnetic resonance study.
Anhyd. nepheline, jadeite, and albite glasses doped with F as well as hydrous F-contg. haplogranitic glasses were investigated using 19F combined rotation and multiple-pulse spectroscopy; 19F -> 29Si cross-polarization/magic angle spinning (CP/MAS); and high-power 19F decoupled 29Si, 23Na, and 27Al MAS NMR methods. The F preferentially coordinates with Al to form octahedral AlF63- complexes in all glasses studied. In addn., F anions bridging 2 Al cations, units contg. octahedral Al coordinated by both O and F, or tetrahedral Al-F complexes might be present. The presence of Si-F bonds cannot be entirely ruled out but appears unlikely on the basis of the 19F -> 29Si CP/MAS spectra. There is no evidence for any significant coordination of F with alkali metals in the glasses studied. 23Na spectra are identical for the samples and their F-free equiv. and the spectra do not change upon decoupling of 19F. The speciation of F in the hydrous and anhyd. glasses appears to be very similar. Over the range of F contents studied (up to 5 wt.%), there seems to be hardly any dependence of F speciation on the concn. of F in the samples. The spectroscopic results explain the decrease of the viscosity of silicate melts with increasing F content by removal of Al from bridging AlO4-units due to complexing with F, which causes depolymn. of the melt. The same mechanism can account for the shift of the eutectic point in the haplogranite system to more feldspar-rich compns. with increasing F content, and for the peraluminous compn. of most F-rich granites. Liq. immiscibility in F-rich granitic melts might be caused by formation of (Na,K)3AlF6 units in the melt with little or no interaction with the silicate component. The presence of F in granitic melts might increase the soly. of high field strength cations by making nonbridging O atoms available which form complexes with these cations.
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