Abstract
Fine intergrowth of diopside and pigeonite in a subcalcic diopside megacryst from Monastery kimberlite pipe has been studied by the lattice image technique of high resolution electron microscopy. The common plane of the intergrowth is approximately (001), and the average widths of diopside and pigeonite lamellae are about 100 and 50Å, respectively. The interfaces are coherent and sinusoidally modulated along the common a axis. The lattice fringes continuously curved along each c axis around the interfaces suggest a gradual compositional change in both phases.
The characteristics of the lattice images of the intergrowth indicate the following formation process of the diopside megacryst: the megacryst should have grown as a C2⁄c single crystal and spinodal decomposition into Ca-rich and Ca-poor regions with C2⁄c resulted in a lamellae texture, and then, the Capoor region, high pigeonite, was transformed to low P21⁄c pigeonite, forming the antiphase domains.
