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.
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