Chemically induced grain boundary migration in calcite: temperature dependence,phenomenology, and possible applications to geologic systems

Chemically induced grain boundary migration (CIGM) is a solid-state reaction mechanism which involves grain boundary migration. Initially straight grain boundaries in calcite bicrystals exhibit CIGM when exposed to a SrCO₃ or BaCO₃-rich melt at temperatures of 680–840° C in a 1-bar CO₂ atmosphere. Under these conditions, CIGM is apparently a much more efficient way to form solid-solutions than lattice diffusion. The maximum observed rate of migration, 1×10^–9 m/s, occurred at about 760° C. Below that temperature, migration was thermally activated, but above it, the process was thermally inhibited. Some portions of the boundary swept through a given crystal volume more than once. In the (Ca, Sr) CO₃ system, the solute concentrations incorporated during a single pass of the boundary were far less than values expected at chemical equilibrium, and departed further from equilibrium at higher temperatures. CIGM may be geologically important when the distances characteristic of grain boundary migration and diffusion are much larger than those characteristic of lattice diffusion. The great similarity between results in ionic calcite and metals systems suggests CIGM is a very general process which may be expected to occur in a large number of geologic systems.