Theoretical calculations of the activation energies for direct interchange, interstitial and vacancy diffusion in f.c.c. copper compared to the empirical activation energy determined from self diffusion indicate the importance of the vacancy mechanism, a conclusion supported by calculated and experimental activation energies of diffusion in sodium chloride. Activation energies for diffusion of solutes in complex silicate minerals have been determined only in a few instances. Vapor pressure increases diffusion rates in ionic bonded minerals, e.g. at 400 degrees C and about 300 bars the diffusion rate of Na 22 in Yule marble is about 10 6 times the rate of 400 degrees C and 1 bar. The distances over which true solid diffusion can be significant are apparently relatively short.