The time dependence of DC electrical conductivity in the c-axis direction of quartz can be accounted for by a transition in charge carriers from interstitial alkali impurities to interstitial H. The diffusive transport rates of Li, Na, and K are rapid parallel to c and have been shown to be responsible for the highly anisotropic electrical conductivity measured at short times. With increasing time, however, conductivities parallel to c decrease progressively to values that are roughly equal to those measured perpendicular to c. Comparison of these ultimate, nearly isotropic conductivities with those derived from recent measurements of H diffusion parallel and perpendicular to c suggests that H interstitials are the principal charge carriers at long times. The transient decrease in conductivities parallel to c is interpreted to result from depletion of initial alkali impurities, whereas the steady-state conductivities measured at long times may be sustained by the steady supply of H by the dissociation of atmospheric water vapor. The mobility of H along the c axis is anomalously low and at variance with the trend of increasing mobility with decreasing ionic radius exhibited by Cs, Rb, K, Na, and Li. Although the elastic lattice distortions required for H transport are insignificant in comparison with those required by the larger alkali impurities, the strong association of H interstitials with Al substitutions for Si may be responsible for the relatively low H mobilities.
