Fluorine is a minor yet important component of volatiles in silicate minerals, and affectsgreatly the physicochemical properties of the host materials. The diffusivity of fluorineprovides critical information for quantitatively understanding its many effects, but veryfew studies have been documented. Here we evaluated the orientation-related chemicaldiffusion of fluorine in phlogopite, by conducting experiments on single crystal samples.Runs were carried out by the diffusion couple technique at 1 GPa and 700-800 ºC, anddurations were typically 25-264 hours. Concentration profiles of fluorine in recoveredsamples and chemical compositions of minerals were analyzed by electron microprobe,and sample water content was determined by Fourier-transform infrared spectroscopy.The results show that, under the experimental conditions, the fluorine diffusivity is on the order of 10-19 to 10-17 m2/s and is slightly anisotropic. The diffusion is fast along thedirection ⊥(110) and slow along the direction ⊥(001), with the direction ⊥(010) fallingbetween them, and the activation energy is 176 to 246 kJ/mol along the three directions.The comparison of the diffusivity data of fluorine with those reported for other speciesin phlogopite and fluorine in other minerals suggests a strong enhancement of the ionicdiffusion along the interlayer direction, thus the diffusivity anisotropy, by the presenceof molecular H2O in the system. The theoretically calculated electrical conductivity ofphlogopite, by applying the Nernst-Einstein relation and our determined diffusion data,is remarkably lower than that experimentally measured for the phlogopite with the samefluorine content and with conduction dominated by fluorine. This indicates that the selfdiffusion of fluorine in phlogopite is much faster than its chemical diffusion, and/or thatthe mobility of fluorine is largely different between the diffusion and the conductivityexperiments. The diffusivity data provide crucial constraints on the closure temperatureof fluorine in phlogopite, which is critical for the kinetic analyses of preserved fluorinezonation in natural phlogopites.303