Computer Science
Scientific paper
Feb 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992gecoa..56..701s&link_type=abstract
Geochimica et Cosmochimica Acta, vol. 56, Issue 2, pp.701-707
Computer Science
14
Scientific paper
Anhydrous nepheline, jadeite, and albite glasses doped with F as well as hydrous F-containing haplogranitic glasses were investigated using 19 F combined rotation and multiple-pulse spectroscopy; 19 F 29 Si cross-polarization/magic angle spinning (MAS); and high-power 19 F decoupled 29 Si, 23 Na, and 27 Al MAS nuclear magnetic resonance methods. Fluorine preferentially coordinates with Al to form octahedral AlF 6 3- complexes in all glasses studied. In addition, F anions bridging two Al cations, units containing 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 19 F 29 Si CP/MAS spectra. There is no evidence for any significant coordination of F with alkalis in the glasses studied. 23 Na spectra are identical for the samples and their F-free equivalents and the spectra do not change upon decoupling of 19 F. The speciation of F in the hydrous and anhydrous 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 concentration 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 AlO 4 -units due to complexing with F, which causes depolymerization of the melt. The same mechanism can account for the shift of the eutectic point in the haplogranite system to more feldspar-rich compositions with increasing F content, and for the peraluminous composition of most F-rich granites. Liquid immiscibility in F-rich granitic melts might be caused by formation of (Na,K) 3 AlF 6 units in the melt with little or no interaction with the silicate component. The presence of F in granitic melts might increase the solubility of high field strength cations by making nonbridging O atoms available which form complexes with these cations.
Dingwell Donald B.
Keppler Hans
Knöller W.
Merwin L.
Schaller T.
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