On the nature of grown-in defects in topaz

A study has been made of crystals of topaz by optical and interference microscopy, X-ray topography, and scanning and transmission electron microscopy. Most of the samples show a nearly perfect region with dislocation density ~ 10² cm^-2 surrounded by a highly defective narrow rim containing dislocations (density ~ 10⁷ cm^-2 and higher), low-angle boundaries, networks and planar faults. The Burgers vectors of the dislocations observed in both regions of the crystal have been determined by means of diffraction contrast experiments and the nature of other defects is considered in detail. The relevance of the defects to the anomalous optical properties of the rim is discussed. A direct correlation between etch pits and dislocations in (001) sections of topaz is shown and evidence is presented that screw dislocations or even those with an appreciable screw component are not required for the formation of curl-bottomed pits. The origin of curl-bottomed pits is explained. It is suggested that the sudden increase in the density of dislocations in the rim is due to heterogeneous nucleation of dislocations by local internal stresses in growth layers which have been poisoned by increased impurity concentrations during growth.