Observations of Microcrystalline Plagioclase Spherulites With the Transmission Electron Microscope

Abstract

Two thin sections of macroscopic plagioclase spherulites of approximately 1 cm diameter found in a rhyolitic glass have been studied with the transmission electron microscope (TEM). Orientations of the thin sections were chosen to give views down and perpendicular to the major fiber axis. The crystalline fiber phase is high albite microtwinned on the (010) composition plane, elongated in the major growth direction, [001]. Fiber morphology is non-polygonal with an average fiber diameter of 2000 A perpendicular to c*. Fibers are separated by a non-crystalline residuum layer of approximately constant thickness (300–500 A). Microtwinning relationships, as well as selected area diffraction (SAD) patterns, reveal both crystallographic and non-crystallographic branching with the former unexpectedly dominant.