Universal stage measurements and transmission electron microscope observations of fractured plagioclase

At low to moderate temperatures of deformation, fracturing of plagioclase is common. The mechanism of fracturing is generally thought to be either a dislocation assisted process with fractures typically exhibiting some crystallographic regularity or a process of breaking along cleavage planes without the involvement of dislocations. In this study, naturally fractured plagioclase from granodiorites and a gabbro deformed at high strain rates are examined with the transmission electron microscope (TEM) to identify structures at that scale. In addition, fracture orientations are determined with the Universal stage.Some fractures observed in thin section occur parallel to (001) but many are not so simple but are confined to the [112], [1$\text{1}$2], [101], [$\text{1}$01] zones. At the TEM scale, dislocation walls or arrays are common in plagioclase. They also occupy the [101], [$\text{1}$01], [112], [1$\text{1}$2] zones. Microcracks form when dislocations are pinned in these arrays or when a free dislocation interacts with dislocations within a dislocation wall. In this way, large-scale fractures which develop inherit their crystallographic orientation from the dislocation wall.