gsminmag Mineralogical Magazine Mineralogical Magazine 0026-461X Mineralogical Society 10.1180/minmag.1965.035.269.19 An Electron-Microscope Study of Peristerite Plagioclases Fleet S. G. Department of Mineralogy and Petrology, University of Cambridge Ribbe P. H. Crystallographic Laboratory, Cavendish Laboratory, University of Cambridge, and Department of the Geophysical Sciences, University of Chicago 3 1965 35 269 165 176 2 6 1964 Copyright © 1965, The Mineralogical Society 1965 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.