Anisotropy of hydrogen diffusion in tourmaline

Hydrogen deuterium exchange in tourmaline single crystals of elbaite composition from Nepal has been studied at 1 atm and at temperatures between 973 and 1073 K. H/D ratios were determined after each annealing experiment using micro FTIR-spectroscopy. Diffusion coefficients (10?16 10?15 m2 s?1) were determined by fitting the data using a 3D numerical simulation. The rate of diffusion is two to three times faster along the c direction than along directions parallel to the basal plane. The diffusion laws are, respectively: LogD=-9.8±1.0-106.3±36.82.3RT and LogD=-12.2±0.5-66.8±19.62.3RT, with D in m2 s?1, T in Kelvin and the activation energy in kJ mol?1. The diffusion rates are one order of magnitude faster, and have slightly lower activation energies, than those found by previous authors for schorlite (125 kJ mol?1). The closure temperature of hydrogen isotope exchange in the tourmaline-water system is lower for elbaite than for schorlite.