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±36.82.3RTandLogD=-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.