Amorphization in natural omphacite and its implications

High-resolution transmission electron microscopic data have revealed the existence of nanoscale intragranular amorphous phase in omphacite of eclogite from the Dabieshan UHP metamorphic belt, Eastern China. The results of profiles in the omphacite across the amorphous phase and micro-fractured regions indicate variations in compositions and OH band intensity, respectively. This implies that the occurrence of amorphization in the omphacite appears to be pressure induced and controlled by kinetics. OH may diffuse outward and further react with the atom at M2 site during more distorted plastic deformation, which occurs as the omphacite crystalline structure collapses to a denser amorphous phase. The reported propensity to amorphization and the strong influence on the outcome of the initial structure and of the pressurization protocol indicate the presence of metastability and large kinetic barriers.