The effect of fluorine on viscosities in the system Na2O-Al2O3-SiO2: implications for phonolites, trachytes and rhyolites

Abstract

The effect of fluorine on melt viscosities of five compositions in the system Na2O-Al2O3-SiO2h as been investigateda t one atmospherea nd 1000-1600'Cb y concentric-cylinderviscometry. The compositions chosen were albite, jadeite and nepheline on the joinNaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and oneperaluminous. All melt viscosities were independent of shear rate over two orders ofmagnitude, indicating Newtonian behavior. All viscosity-temperature relationships wereArrhenian within error. Fluorine reduces the viscosities and activation energies of all meltsinvestigated. The viscosity-reducing power of fluorine increases with the SiO2 content ofmelts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate meltviscosities, probably due to depolymerization of these melts by replacement of Si-O-(Si,Al) bridges with Si-OH and Si-F bonds, respectively. Evidence from slag systems showsthat fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow ofphonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears thatfluorine contents of igneous rocks may be combined with water in calculation schemes fordetermining the viscosity of natural melts.