Fluorine in silicate glasses: A multinuclear nuclear magnetic resonance study

Abstract

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...

Contrasting partition behavior of F and Cl during hydrous mantle melting: implications for Cl/F signature in arc magmas

“…Gaetani et al 2003). Previous work on F and Cl solution behavior in aluminosilicate melts (Mysen and Virgo 1985;Kohn et al 1991;Schaller et al 1992;Zeng and Stebbins 2000;Mysen et al 2004;Zimova and Webb 2006;Baasner et al 2013) have shown that F preferentially bonds with tetrahedrally coordinated cations such as Al and Si. Although in more complex melts such as basalt, F can also preferentially bond with Mg (Kiczenski et al 2004;Filiberto et al 2012) and form more complex bonds as Al-F-Ca or Al-F-Na (Zeng and Stebbins 2000;Mysen et al, 2004); Si-F and Al-F are always found.…”

Can crystallization of olivine tholeiite give rise to potassic rhyolites?—an experimental investigation

“…F interacts strongly with silicate melts (Lange 1994) by breaking framework bonds and forming Al-F (Kohn et al 1991;Liu and Nekvasil 2001;Schaller et al 1992;Zeng and Stebbins 2000) and Si-F complexes (Liu and Nekvasil 2002;Zeng and Stebbins 2000) in the melt. F suppresses the solidus temperature below that of the halogen-free water-saturated solidus, thereby allowing the production of greater volumetric abundances of these silicic compositions.…”

An experimental study of tin partition between melt and aqueous fluid in F/Cl-coexisting magma

“…Moreover, the viscosity and liquidus of silicate melt decrease with increasing F contents in melt as a result of removal of more Al from bridging AlO 4 -units due to complexing with F, which caused depolymerization and lower liquidus of the melt. The presence of F in granitic melts will lead to the increase of the solubility of high field strength cations by making nonbridging oxygen atoms complex with these cations [54,55] . Fluorine is preferentially distributed in melt and the distribution coefficient of F is generally less than 1 between aqueous fluid and silicate melt at high pressure and temperature, so that F should be enriched in this kind of melts through granitic crystallization and differentiation [46,56,57] .…”