-014The Structural Role of Lanthanum and Yttrium in Aluminosilicate Glasses: A 27 Al and 17 O MAS NMR Study.-High-resolution 17 O and 27 Al NMR data for a series of La-and Y-modified aluminosilicates show a significant increase in the concentrations of AlO 5 and AlO 6 groups caused by the smaller radii and higher field strengths of the modifier cation in the latter. The role of highly coordinated Al and the configurational complexity generated by the formation of such species is discussed.

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Abstract-Anhydrous nepheline, jadeite, and albite glasses doped with F as well as hydrous F-containing haplogranitic glasses were investigated using "F combined rotation and multiple-pulse spectroscopy; 19F + 29Si cross-polarization/magic angle spinning (MAS); and high-power r9F decoupled 29Si, 23Na, and 27A1 MAS nuclear magnetic resonance methods. Fluorine preferentially coordinates with Al to form octahedral AIF:-complexes in all glasses studied. In addition, F anions bridging two Al cations, units containing octahedral Al coordinated by both 0 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 alkalis in the glasses studied. 23Na spectra are identical for the samples and their F-free equivalents and the spectra do not change upon decoupling of 19F. 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,-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),AlF, 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 0 atoms available which form complexes with these cations. INTRODLJCPIONFLUORINE IS ONE OF THE MOST important volatile components of natural magmas, both in mantle-derived melts (e.g., lamproites) and in highly fractionated granitic magmas. Concentrations of several wt.% are reported in high-fluorine rhyolites (CONGDON and NASH, 1988) and in ongonites ( KOVALENKO, 1973). Similar to water, the presence of F strongly reduces the viscosity of silicate melts ( DINGWELL et al., 1985;DINGWELL, 1989) and increases diffusion coefficients ( DINGWELL, 1985). Even small amounts of F in the order of 1 wt.% will therefore significantly change the transport properties, mobility, and eruption behavior of such magmas. The correlation between high concentrations of certain trace elements (Sn, Zr, Nb, Ta, U, Th, etc.) and high F contents in granites and granitic pegmatites (STEMPROK, 1982) has stimulated the investigation of phase equilibria in F-containing granitic systems. MANNING ( 198 1) has shown that F strongly lowers both solidus and liquidus temperatures in the haplogranite system and causes a shift of the eutectic point towards more feldspar-rich...