X-RAY TOPOGRAPHIC STUDY OF EIFEL SANIDINE (VOLKESFELD)

Sanidine single crystals from Volkesfeld/Eifel (Germany) exhibit an un usually rapid Si, Al exchange on annealing at temperatures above 1023 K. X-ray topographic studies (Lang technique) before and after anneali ng show that large volumes (up to several cm3) of the crystals are hig hly perfect. There are few defects only (growth striations, growth-sec tor boundaries, grown-in dislocations), and Pendellosung fringes can b e observed. While the Si, Al-exchange rate is reduced by annealing, th e growth striations and Pendellosung fringes vanish, and for certain r eflections, e.g. (002), the diffracted intensity is increased due to r eduction of the ''dynamical'' extinction. These changes are illustrate d by selected X-ray topographs.