Chemically induced migration in low and high angle calcite grain boundaries

Chemically induced grain boundary migration (CIGM) occurs in the CaCO₃-SrCO₃ system at 680°C, producing microstructures similar to those observed in metals systems. Forty calcite bicrystals with varying misorientation were examined. CIGM was usually observed along the entire length of high angle boundaries, and along very small lengths of boundary with misorientation magnitudes (θ_m) as low as 0.7° ± 0.35°. Percentage of the boundary which migrated, migration rate, node spacing, and Sr concentration correlate with θ_m below ≈ 14–18°. Sr concentrations were heterogenous and well beneath values expected for chemical equilibrium, and were not measurable by electron microprobe or secondary ion mass spectroscopy when θ_m was below 3°. At θ_m ≈ 14–18° there may be a change in behavior associated with the transition to high angle boundary structure. Extrinsic factors probably complicate the experiments, but the observations suggest CIGM in calcite may be regarded as a two-stage process of nucleation and growth. Existing theories for CIGM do not explain many of the results.