The calcite-aragonite transition : mechanism and microstructures induced by the transformation stresses and strain

The calcite-aragonite transition has been experimentally investigated both under hydrostatic conditions and in the presence of deviatoric stresses. The calcite → aragonite experiments at high-pressure have been performed in a Griggs apparatus in the presence of deviatoric stresses whereas the aragonite → calcite experiments were conducted at room pressure (hydrostatic conditions) on a heating stage. For both kinds of experiments partially transformed samples were observed by transmission electron microscopy (T.E.M.). The same morphology of the interface between polymorphs and the existence of epitactic relationships among low-index crystallographic planes in both kinds of experiments suggest that whatever the experimental conditions and sense of reaction, the transition proceeds by the motion of highly-coherent interfaces. No evidence for a shear mechanism was found. The growth of calcite in aragonite at room pressure leads to a high dislocation density (1014 m-2) in calcite and to mechanical twins in aragonite. Recovery textures like subgrains or recrystallized grains are observed in the calcite grains. The defects observed in both phases are explained by the transformation stresses due to the irregular interface between polymorphs and to the volume change at the transition. The implications for transformation plasticity and for the stability of phases are briefly discussed.