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 appli cation of kinetic constants extracted from the disordering experiments carr ied out on Johnstwon orthopyroxene leads to aberrant cooling rates without physical meaning. Crystals of this orthopyroxene were studied by several tr ansmission electron microscopy (TEM) techniques in order to elucidate the n ature of the exsolution products, their topology, chemistry and probable re lationship to kinetics. For comparison this study also included a terrestri al granulitic orthopyroxene with known "normal" disordering kinetics, and a Carich orthopyroxene of volcanic origin which was supposed to be free of e xsolution products due to fast quenching. The TEM results show that all stu died samples contain very fine clinopyroxene exsolution lamellae (several u nit-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 anomalo us kinetic behaviour.