The effect of F on the glass transition behavior of albite, diopside, and four other silicate melts has been investigated using scanning calorimetry. The addition of F to all silicate melts investigated results in a strong, nonlinear decrease of the glass transition temperature (Tg, as recorded by the peak temperatures of heat capacity). The decreases observed extrapolate consistently to published fluoride glass transition temperatures. The largest Tg decrease is observed for albite-F2O−1 melts (ΔT ≅ 250 °C at 6 wt% F). The effect of F is similar to that previously observed for H2O (Taniguchi, 1981).
Physical properties of low-temperature silicate liquids are a valuable constraint on low-temperature petrogenetic processes in granite and pegmatite petrogenesis. Low-temperature viscosities can be estimated from the glass transition data. These data are combined with previously published high-temperature, concentric-cylinder viscosity data to obtain a much more complete description of the temperature dependence of viscosity for these melts.
The present data, obtained on supercooled liquids close to the glass transition, are of special significance because it is at the glass transition that silicate glass structures are frozen. A separate multinuclear NMR study of glasses quenched from these experiments has shown that the predominant coordination of F in albite glass is octahedral to Al. The coordination state of F does not appear to be concentration dependent, and thus the structural origin of the nonlinear Tg decrease does not arise from such a mechanism.