A TEM study of Ca-rich orthopyroxenes with exsolution products: Implications for Mg-Fe ordering process

Orthopyroxene from the Johnstown diogenite contains exsolution products. It shows anomalous kinetics for disordering Fe-Mg distribution, and the application of kinetic constants extracted from the disordering experiments carried out on Johnstown orthopyroxene leads to aberrant cooling rates without physical meaning. Crystals of this orthopyroxene were studied by several transmission electron microscopy (TEM) techniques in order to elucidate the nature of the exsolution products, their topology, chemistry and probable relationship to kinetics. For comparison this study also included a terrestrial granulitic orthopyroxene with known 'normal' disordering kinetics, and a Ca-rich orthopyroxene of volcanic origin which was supposed to be free of exsolution products due to fast quenching. The TEM results show that all studied samples contain very fine clinopyroxene exsolution lamellae (several unit-cell wide). Additionally, only Johnstown orthopyroxenes contain single unit-cell wide Guinier-Preston zones. These represent a chemically distinct pyroxene and their distribution in Johnstown is pervasive. They are still present after annealing experiments at the temperatures of kinetic studies. This high density of defects is proposed to be responsible for the anomalous kinetic behaviour.