Diffusion-induced grain-boundary migration in calcite

Grain boundaries in synthetic calcite bicrystals and slabs of Yule marble exposed to SrCO₃ at temperatures between 650 and 800 °C undergo diffusion-induced grain-boundary migration (DIGM). As the boundary migrates through the crystal, it produces a (Ca, Sr)CO₃ solid solution. DIGM is known to occur in metals at temperatures as low as one-third the melting temperature, where lattice diffusion rates are insignificant compared to grain-boundary diffusion rates. The similarity between microstructures in calcite and metals systems is striking. In both systems, grain boundaries migrate to increase their total area and reduce their radius of curvature, migration occurs in the absence of significant storud internal strain energy, and boundary migration is accompanied by changes in the chemical composition of the solid.

Although a widely accepted theory for DIGM is lacking, it is clear that the process involves the formation of a compositionally distinct phase, which grows by material transport along an advancing grain or interphase boundary. Grain-boundary migration during DIGM must be driven at least in part by the reduction of chemical free energy associated with the chemical composition change that takes place behind the advancing boundary. Although the reaction mechanisms by which multicomponent, multiphase systems in the earth reach equilibrium are likely to be numerous and diverse, it is possible that some mechanisms are analogous to the DIGM as observed in Laboratory experiments on calcite bicrystals.