Isomorphous replacement and superlattice structures in the plagioclase felspars

Abstract

From X-ray oscillation and rotation photographs of a limited number of plagioclase felspars it seems probable that the structure type depends upon the chemical composition in the following way. From pure soda felspar to at least 22% lime felspar there extends an isomorphous series with thealbitestructure, based on a unit cell with linear dimensions 8 x 13 x 7A approximately and containing four molecules of NaAlSi3O8or the equivalent when (CaAl) replaces (NaSi). There is probably a similar range of isomorphism, near pure lime felspar CaAl2Si2O8, with theanorthitestructure which has axes 8 x 13 x (2 x 7)A, but is of course very similar to the albite structure. The characteristic features of the photographs of the intermediate plagioclases can be explained, with the help of an optical model, in terms of a structure in which sheets with the albite structure and sheets with the anorthite structure alternate, but more experimental data are needed for the complete elucidation of the problem.