Dingwell, Donald B.; Scarfe, Christopher M.; Cronin, David J.
The effect of fluorine on viscosities in the system Na2O-Al2O3-SiO2: implications for phonolites, trachytes and rhyolites.
In: American Mineralogist, Vol. 70, No. 1-2: pp. 80-87
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-cylinder
viscometry. The compositions chosen were albite, jadeite and nepheline on the join
NaAlOlSiO2 and two others of the join at 75 mole percent SiO2, one peralkaline and one
peraluminous. All melt viscosities were independent of shear rate over two orders of
magnitude, indicating Newtonian behavior. All viscosity-temperature relationships were
Arrhenian within error. Fluorine reduces the viscosities and activation energies of all melts
investigated. The viscosity-reducing power of fluorine increases with the SiO2 content of
melts on the join NaAlO2-SiO2 and is a maximum at Na/Al (molar) = I for melts containing
75 mole percent SiO2. Fluorine and water have similar effects on aluminosilicate melt
viscosities, 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 shows
that fluorine also reduces the viscosity of depolymerized silicate melts. The viscous flow of
phonolites, trachytes and rhyolites will be strongly afected by fluorine. It appears that
fluorine contents of igneous rocks may be combined with water in calculation schemes for
determining the viscosity of natural melts.