Ca and Na positions in labradorite feldspar as derived from high-resolution electron microscopy and optical diffraction

Abstract

A model for the Ca/Na positions in the superstructure of labradorite in low structural state (i.e. having e-type satellite reflections) with approximately 50 mol% anorthite is proposed on the basis of high-resolution electron microscopic structure images and a comparison of the corresponding electron diffraction patterns with optical diffraction patterns obtained from masks made to represent projections of the structure models with the diameter of the holes representing the scattering factors of the 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 [5{\bar 1}1]. The AI/Si atoms influence the diffraction pattern only with regard to a few indices, thus the correspondence of optical and electron diffraction patterns is increased by including A1 and Si. The model agrees with the known average structure of labradorite (An53) but differs from models proposed previously for the diffraction satellites and the antiphase boundaries causing them.