Summary: Plagioclase feldspars in the peristerite range An₂-An₁₇ have been investigated by transmission electron-microscopy and electron diffraction. All except the more anorthite-rich specimens were found to be unmixed into albite and oligoclase lamellae, between a few hundred and several thousand Å thick and approximately parallel to (08¯1). A discussion is given of the part played by these lamellae when optical schiller is exhibited; and the effect of heat treatment on the lamellar structure and optical schiller is described.