Ionic diffusion in the quartz–β-eucryptite system is studied by DC transport measurements, SIMS and atomistic simulations. Transport data show a large transient increase in ionic current at the α–β phase transition of quartz (the Hedvall effect). The SIMS data indicate two diffusion processes, one involving rapid Li+ motion and the other involving penetration of Al and Li atoms into quartz at the phase transition. Atomistic simulations explain why the fine microstructure of twin domain walls in quartz near the transition does not hinder Li+ diffusion.