Abstract
A model for the Ca/Na positions in the superstructure of labradorite feldspar with approximately 50 mol % anorthite is proposed on the basis of high resolution electron microscopic structure images and a comparison of the electron diffraction patterns with optical diffraction patterns obtained from masks representing the projected Ca and Na atoms. Good agreement in corresponding positions is found between these optical diffraction patterns and the electron diffraction patterns for projections along the four different axes [010], [311], [131] and [511] The model proposed is the only one which does not contradict the images taken along different crystal directions and the average structure of labradorite (An 55) analyzed by X-ray diffraction.
