The structure of grain boundaries in granite-origin ultramylonite studied by high-resolution electron microscopy

The structure of grain boundaries in a granite-origin ultramylonite, compos ed mainly of fine-grained feldspar and quartz, was studied by high-resoluti on electron microscopy (HREM). At most of the boundaries, not only between the same minerals but also between different minerals, lattice fringes in a djacent grains meet at the interface with no other appreciable phases. In t hese boundaries, some of the straight segments correspond to a low-index pl ane of one of the connected grains. Boundaries containing voids, with a sph eroidal shape elongated along the boundaries, were observed only between qu artz grains. It is suggested that these boundaries were formed by healing o f microcracks. The structural width of major boundaries, deduced from latti ce-fringe imaging, is less than about 0.5 nm.