Ionic Diffusion in Quartz

Abstract

MORTLEY¹ has suggested that diffusion through quartz is limited to small univalent ions and occurs through lattice channels in the 〈 0001 〉 direction, and also that these channels (of radius 1 Å) act as an ionic sieve, allowing only the smaller ions to pass. Diffusion is certainly limited to the univalent ions, but it is not restricted to those ions with an ionic radius of 1 Å or less. Verhoogen² has reported the diffusion of potassium (ionic radius 1.33 Å), and I have diffused rubidium (ionic radius 1.49 Å) and caesium (ionic radius 1.65 Å), using an experimental procedure similar to that previously described³. An increase in the size of the ion decreases the conductivity, and it was found that the value for lithium diffusion at 500° C (2 × 10⁻⁶ Ω⁻¹ cm⁻¹) is two hundred times that for caesium. The activation energies increase, and the values obtained for alkali diffusion through basal sections of quartz were (kcalories): lithium, 18; sodium, 21; potassium, 28; rubidium, 30; and caesium, 33; each result being the average of many determinations. These values show a linear increase with an increase in ionic radius, and therefore the channels resist the movement of larger ions, but they do not prevent ionic motion.