Influence of dislocations on the thermal decomposition of calcium carbonate

By deforming single crystals of calcite at high temperatures and subsequently allowing decomposition to continue at very low rates and to minute extents (using vacuum microgravimetry), it has been established, by optical microscopy, that preferential decomposition is marked at dislocations that glide in {100} and {2} planes, but not so pronounced at those that glide in {11} planes. Decomposition is also favoured along twin boundaries. From the shapes of decomposition nuclei, it is concluded that {1/},{11/}, and {100} planes are of some significance in the preferred growth of the product, CaO, within the parent lattice. Preliminary kinetic measurements indicate that the activation energies computed from initial rates of decomposition may be lower than those derived from constant rates (at higher fractions decomposed). This is tentatively interpreted to mean that the intrinsic energy of a dislocation contributes to the energy of activation.