Twin walls in anorthoclase are enriched in alkali and depleted in Ca and Al

Composition profiles have been measured in transformation twin walls of the albite type in anorthoclase. Regions close to the walls between two twin domains are enriched in K and Na and depleted in Ca and Al. Microanalysis experiments show high mobility of alkali cations close to twin boundaries. Twin walls are generated via the displacive phase transition C2/m-C (1) over bar in Al, Si disordered feldspar, and the chemical heterogeneity is a fingerprint for the subsequent diffusion of alkali and earth alkali ions during the cooling history of the sample.