Abstract Diffusion is the process by which atoms or ions or ionic species migrate within a medium in the absence of a bulk flow. Diffusion in solids has been a subject of interest in the fields of solid state science (physics, chemistry and metallurgy) for nearly a century, starting with the work of Einstein on the relationship between random atomic movement and diffusion process. The phenomenological study of diffusion began even 50 years earlier, with the empirical formulation of Fick on the relationship between the diffusion flux of a component and its concentration gradient. The subject of diffusion in the solids may be subdivided into volume, grain boundary and surface diffusion. The last topic has, however, received very little attention in the study of geological processes. The study of grain boundary diffusion is important to the understanding of many metamorphic processes including the problems of mass transport, fluid/rock interactions, thermal history and crystal growth. The interested reader is referred to Joestein (1991) for an excellent review of the subject of grain boundary diffusion and its applications to geological problems. Diffusion controlled processes within a mineral preserve important records of the thermal and physico-chemical history of the host rocks. Volume diffusion, that is diffusion through the crystal lattices, affects development of compositional zoning in minerals, ordering of atoms in nonequivalent crystallographic sites of a mineral, formation and coarsening of exsolution lamellae, and retention of isotopic characteristics in minerals that can serve as quantitative chronometers in their thermal and growth history.

Abstract One of the major goals of petrology is to retrieve values of the intensive parameters, such as pressure ( P ), temperature ( T ) and fluid composition, under which the major mineralogical properties of a rock were established, along with the time scales of evolution of the mineralogical properties. The basic approach in the retrieval of the intensive properties involves comparison of the mineralogical assemblages and the mineral compositions of the rock with the phase equilibrium constraints. The latter are calculated from the internally consistent thermochemical properties of the stoichiometric end members or determined in the laboratory on relatively simple systems, usually involving only the end-member phases, and then corrected for the effects of the compositional departures as observed in a specific natural assemblage. In addition, many mineral pairs ( e.g . garnet and biotite) respond to changes of P – T conditions, especially temperature, by continuous ion exchange reactions, and thus register the P – T condition or P – T history of the rock in their compositions. The ion-exchange reactions are also calibrated in the laboratory on relatively simple systems, but require corrections for the effects of additional components that enter into solid solution in the minerals in natural environments. The corrections for the compositional effects rely critically on the thermodynamic mixing properties of the components in the mineral solid solutions and fluid phase which are involved in a specific reaction ( e.g . Ganguly & Saxena, 1987 ). There has, thus, been a sustained effort over the last few decades on the determination of thermodynamic mixing properties of phases in geologically important systems.