Microfracturing in relation to atomic structure of plagioclase from a deformed meta-anorthosite

Brittle deformation of Caledonian age affects the Harris (Scotland) meta-anorthosite and occurs as restricted areas with penetrative networks of shear fractures, frequently associated with pseudotachylite. Plagioclase is cut by both transcrystalline and intracrystalline fractures, the latter being of two types: those directly induced by the transcrystalline shear fractures and those which appear to be independent of them. Several orientations of intracrystalline fractures may occur in any one grain.Whereas the orientations of the transcrystalline fractures may be independent of the plagioclase lattice, intracrystalline fractures are clearly crystallographically controlled. The most common intracrystalline fractures follow the main cleavage planes, (001) in all cases, but also frequently (010), (110) and (110). Other fracture directions, often conjugate, are very common. They include (021) and others near (111)–(121) and (111)–(121) close to the 101] and the 112] and 112] zones. These latter planes are those which also occur as cleavages in experimentally shocked microcline and as slip planes and deformation bands in experimentally deformed feldspars.The easy slip and low cohesion in plagioclase can be explained in terms of periodic bond chains in the feldspar structure. The close agreement in orientation between the unusual cleavages developed in the meta-anorthosite and experimentally produced deformation bands in plagioclase suggests that fracture occurs along the deformation bands parallel to dislocation glide planes.