Transmission Electron Microscope Study of Brazil Twins and Dislacations Experimentally Produced in Natural Quartz

Transmission electron microscopy has been used to examine single crystals of natural quartz which have been plastically deformed in compression at high temperature (500 to 700°C) and confining pressure (15 to 20 kbar) with high shear stress on (0001). In specimens deformed at 500°C, the only features found are closely spaced pairs of planar defects parallel to {0001}. Study of the diffraction contrast fringes reveals that each pair of defects bounds a Brazil twin band which has been produced by the high shear stress. At the higher temperature there are dislocations in the twin bands and other regions of the crystals with dislocations only. The dislocations are almost invariably either pure screw ( b = a 〈11 2 0〉) or of a particular mixed character (predominantly edge, but with a 30° screw component). Arrays of the mixed dislocations are observed which would produce stress fields consistent with the stress‐optical effects observed by optical microscopy.