Peristerite exsolution in metamorphic plagioclase from the Lepontine Alps:An analytical and transmission electron microscope study

Transmission and analytical electron microscopy were used to examine relati onships between microstructures and compositions in greenschist- and amphib olite-facies metamorphic plagioclase (albite and oligoclase) from the Lepon tine Alps (Switzerland and Italy). Two kinds of exsolution microstructures related to the peristerite miscibility gap (similar to An(1-25)) were obser ved: lamellae, in bulk compositions ranging from a few mole percent anorthi te to similar to An(10-15), and tweeds, in bulk compositions from almost pu re albite to similar to An(15-18). Lamellae are typically 15 to 35 nm thick . Individual lamellae in crystallographically homogeneous or tweedy areas c ommonly have highly irregular spacings or end at dislocations or subgrain b oundaries, suggesting formation by heterogeneous nucleation. Tweeds are cha racteristically diffuse, and probably formed by spinodal decomposition. Man y tweeds have one exsolution direction that is consistently sharper or coar ser than the other. Tweeds and lamellae may be intergrown in patches with i rregular, curving boundaries, some of which define narrow stripes several m icrometers long. Exsolution directions in these tweeds are approximately pa rallel and perpendicular to the lamellae. The perpendicular direction is us ually less diffuse or more regular than the parallel direction, and may con tinue between widely spaced individuals in patches of lamellae. Tweeds in t hese intergrowths appear to be slightly more sodic than adjacent areas with lamellae. Except near fractures and in areas with high dislocation densities, differe nces in microstructures within a single grain almost invariably reflect dif ferences in composition. Microstructural variability within single grains, and among different grains from the same hand sample, was so large that it was impossible to identify systematic relationships between microstructures and metamorphic grade.