Diffusion in Feldspars, Liquids and Glasses. Viscosity

Abstract

Chemical diffusion is a mass-transport phenomenon which may occur within solids, liquids, or gases and is governed by non-equilibrium thermodynamics (Fisher and Lasaga 1981). It is the transport of matter in response to a “driving force” which may be a concentration gradient (or more correctly, a chemical-potential gradient, as for “uphill” diffusion during spinodal decomposition, Chapters 19 and 21), a temperature gradient, or some other operative gradient (Philibert 1985). In liquids only surface and volume diffusion can occur, whereas in solids diffusion may occur either through the crystal (volume diffusion) or on the surface, on grain boundaries, and through dislocations (nonvolume diffusion). Nonvolume (extrinsic) diffusion is important at low temperatures and in deformed or fine-grained rocks, whereas volume (intrinsic) diffusion is important at high temperatures. Volume diffusion mainly requires the presence of point defects, the number of which per unit volume increases with temperature. Migration of an atom into a defect normally produces a new defect so that further diffusion can occur. Ultimately, at low temperatures, where new defects are not spontaneously produced (unlike at high temperatures), diffusion is reduced by migration of defects to sinks.