X-ray imaging study of lattice defects related to diffusion of helium in quartz

Abstract

Production of He isotopes in surface rocks enables one to study their origin. Quartz is a widespread mineral; understanding He mobility in it will be valuable to helium studies in many environments. Measurements of the He diffusivity in quartz demonstrated that a perfect crystalline quartz lattice was virtually impermeable for He. In this paper, we show that lattice dislocations serve as the high diffusivity paths for helium in quartz. Synthetic crystals were grown to contain a certain amount of helium, which was then degassed under heating. Synchrotron imaging was applied to characterize and map grown-in defects. Samples for desorption were prepared from the chosen growth zones where the average dislocation densities were evaluated. From the highly dislocated samples helium appeared to release more rapidly and to have a lower activation energy. The gain in diffusivity was interpreted due to a smaller amount of energy required to permit the passage of a He atom through an increased 'doorway' provided by a dislocation pipe.