Chemical diffusion of fluorine in phlogopite

Abstract

Fluorine is a minor yet important component of volatiles in silicate minerals, and affects greatly the physicochemical properties of the host materials. The diffusivity of fluorine provides critical information for quantitatively understanding its many effects, but very few studies have been documented. Here we evaluated the orientation-related chemical diffusion 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 °C, and durations were typically 25–264 h. Concentration profiles of fluorine in recovered samples 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 the direction ⊥(110) and slow along the direction ⊥(001), with the direction ⊥(010) falling between 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 species in phlogopite and fluorine in other minerals suggests a strong enhancement of the ionic diffusion along the interlayer direction, thus the diffusivity anisotropy, by the presence of molecular H2O in the system. The theoretically calculated electrical conductivity of phlogopite, by applying the Nernst-Einstein relation and our determined diffusion data, is remarkably lower than that experimentally measured for the phlogopite with the same fluorine content and with conduction dominated by fluorine. This indicates that the self diffusion of fluorine in phlogopite is much faster than its chemical diffusion, and/or that the mobility of fluorine is largely different between the diffusion and the conductivity experiments. The diffusivity data provide crucial constraints on the closure temperature of fluorine in phlogopite, which is critical for the kinetic analyses of preserved fluorine zonation in natural phlogopites.