An electron-microscope investigation of a moonstone

Abstract

Abstract Unmixing in a moonstone has been directly observed, using an electron microscope. The mineral consists of alternating, irregular sheets of orthoclase and low albite, approximately parallel to () and of widely varying thicknesses. The albite sheets consist of finely twinned lamellae parallel to (010)—-the composition face of the albite twin law—-and exactly four albite unit cells wide. They often repeat regularly over a sufficiently large distance to constitute a superstructure which gives sharp subsidiary reflections in the electron diffraction patterns. The white schiller of the moonstone is discussed on the basis of these structural details.