Origin of Calcite Decomposition Nuclei

The thermal decomposition of natural calcite single crystals was found to occur at surface defects at which well-formed decomposition nuclei eventually developed. The occurrence of the nuclei was strongly dependent on the history of the specimen surface. An initial heat treatment in CO2 would anneal out many surface defects; however, longer annealing drastically increased the defect concentration by allowing impurity precipitation throughout the crystal. Closed dislocation loops appeared to surround the pockets of precipitation. Strong evidence is presented to suggest impurity-associated dislocations are primarily responsible for nucleation.