Phase relationships in the system kalsilite-forsterite-quartz with fluorine added by direct substitution for oxygen were examined at 28 kb. A large liquidus field for fluorphlogopite exists with approx. 4 wt% F added to the system and the thermal stability of phlogopite is increased by ∼300° C relative to the water saturated system. Fluorine expands the phase volume of enstatite relative to forsterite so that the peritectic point PHL+EN+FO+L, a model for melting of a phlogopite harzburgite, lies in the silica-undersaturated field. Experimental phlogopites have excess Si which correlates with F content and are Al-deficient. The high Si contents indicate solid solution with an end member intermediate between tri- and di-octahedral micas. Glasses with compositions analogous to partial melts from phlogopite harzburgite were examined by infrared spectroscopy in the mid- and far-IR regions. Results show that fluorine polymerises the melt by bonding with all the network modifying cations K, Mg and Al. At higher F contents, but still less than 1 wt%, tetrahedral KAlO2-groups are complexed by fluorine and removed from the aluminosilicate network simultaneously polymerising and increasing the Si/(Si+Al) ratio of the network. However, when HF rather than F is present, the overall effect will be to depolymerise melts due to the effect of OH released by dissolution of HF. The presence of abundant Si-F bonds is considered unlikely even in silica-rich magmas: the viscosity decrease characteristic of fluorine-bearing melts can be attributed to the formation of fluoride complexes.